III. Design of the ATST Advanced Technology Solar Telescope Construction Phase Proposal

DESIGN OF THE ATST

1. SYSTEMS OVERVIEW

The Advanced Technology Solar Telescope (ATST) is an all-reflecting, four-meter, off-axis Gregorian telescope housed in a co-rotating dome. It delivers a 300×300-arcsec field of view (unvignetted over a 240 arcsec square) to a Gregorian and two coudé observing stations. Energy outside of this field is rejected from the system by a heat stop located at prime focus, allowing manageable thermal loading on the optical elements that follow. The telescope also includes an integrated adaptive optics system designed to provide diffraction-limited images to the focal-plane instruments at the coudé observing stations. Our design meets all science requirements stated in the Science Requirements Document (SRD, ATST Document #SPEC-0001).

A persistent systems approach is essential to the success of a telescope like ATST. Systems engineering works with project management, the scientific staff, and the other engineers to accomplish various activities. In this chapter the emphasis will be on design requirements flow-down, error budgets, and performance predictions. It will conclude with a top-level description of the telescope design that serves as an outline and general background material for the subsequent detailed design descriptions that follow. Other aspects of systems engineering are discussed in Part IV of this proposal, Management of the ATST Construction, Integration, and Testing (see Chapter 6).

1.1 THE FLOW-DOWN PROCESS Systems engineering has been responsible for flowing the science requirements – as specified by the scientific community – down to design requirements on the telescope. For example, the science that ATST will perform requires a sharp image. Systems engineering must first list all of the telescope and instrument subsystems that have the potential to cause the image to blur. These will include the quality of the optical components (mirror figures and polish quality), telescope mount vibrations, and thermal distortion of the air above the telescope dome, to name just a few. While the science requirement is expressed in terms of the size of a point-source image, this constraint must be converted to a mirror-polish specification, mount stiffness, and maximum allowed temperature variation on the dome skin to be useful when designing the telescope and specifying the manufacturing tolerances of its components.

The process of flowing science requirements down to design requirements began with the ATST SRD. That document established the top-level science requirements based on the solar community’s vision and proposed mission of the telescope. These requirements lead directly to a set of critical science use cases listed in the SRD, selected because they place the most stringent technical requirements on the telescope and instrumentation. These use cases lead, in turn, to specific performance requirements placed on the telescope and instrumentation. All of this has been spelled out in the SRD.

It has been the task of the engineering team to produce a design that meets the top-level telescope and instrument requirements, and hence the science requirements. The process followed to get to an initial design was highly iterative, involving a baseline concept that was proposed, tested against the top-level requirements and error budgets, and modified until cost and performance requirements were met. The following paragraphs highlight a few aspects of this process. The balance of this part of the proposal describes the resulting design. . 1.1.1 Science Use Cases The Science Requirements Document describes 18 science use cases. They establish minimum performance requirements on a variety of important parameters and operational modes that must be

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supported by ATST to meet the science requirements. The detailed justification of each requirement is included in the SRD. Table 1.1 lists the use cases that demand the highest performance in each area:

Table 1.1. Science Use Cases. 1. Spatial Resolution Half of the science use cases require observations at or near to the diffraction limit at visible wavelengths. These include • Interaction of strong and weak magnetic fields • Flux emergence and disappearance • Dynamics of kilogauss flux tubes • Internal structure of flux tubes / irradiance variations • Magnetoconvection in sunspots • Generation of acoustic oscillations • Temperature and velocity of the photosphere and chromosphere • Prominence formation and eruption • Solar Flares 2. Field of View Most use cases require a field of view of several arcmin. The coronal use cases established the most stringent requirement, desiring 3 to 5 arcmin: • Prominence formation and eruption • Coronal magnetic fields • Coronal plasmoid search • Coronal velocity and density in active region loops • Coronal intensity fluctuation spectrum 3. Wavelength Coverage Three use cases require observations in the thermal IR (12 µm): • Turbulent/Weak fields • Dynamo processes in deep layers of the convection zone • Solar Flares Four require observations at or near to the atmospheric UV cutoff at 300 nm: • Dynamics of kilogauss flux tubes • Internal structure of flux tubes / Irradiance variations • Turbulent/Weak fields • Hanle effect diagnostics Many use cases require or strongly desire simultaneous observations over broad wavelength ranges. 4. Spectral Resolution Many use cases require spectral resolution of 1 pm (picometer) or less. The most stringent use case requires 0.42 pm at 500 nm: • Dynamics of kilogauss flux tubes 5. Polarimetric Sensitivity The most stringent use cases require polarimetric sensitivity of 10-5: and Accuracy • Turbulent / Weak fields • Hanle Effect Diagnostics 6. Scattered Light The coronal observations are the most demanding in terms of sky and instrumental scattered light near the limb of the sun, requiring excellent coronal sky conditions, and low instrumental scatter: • Coronal magnetic fields • Coronal plasmoid search • Coronal velocity and density in active region loops • Coronal intensity fluctuation spectrum On-disk observations of large sunspots also place requirements on in-field scattering: • Magnetoconvection in sunspots 7. Observing modes The most demanding use cases involve active-region evolution, which require simultaneous observations with multiple instruments in both the visible and the thermal infrared: • Dynamo processes in deep layers of the convection zone • Solar flares

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1.1.2 Telescope Requirements The use cases described above and other scientific considerations lead to several direct top-level requirements on the telescope and site that are justified in detail in the Science Requirement Document:

Resolution: • ATST must have a minimum four-meter aperture. • ATST shall include high order adaptive optics capable of deriving information from solar granulation and other solar structure. Photon flux and sensitivity: • ATST shall provide a minimum collecting area of 12 m2 Polarization sensitivity and accuracy: • The polarization sensitivity must be 10-5 with polarization accuracy of 5×10-4 Scattered light: • The scattered light from the telescope and instrumentation from angles greater than 10 arcsec shall be 1% or less • The total instrumental scatter due to dust and mirror microroughness must be less than 25×10-6 at 1.1 solar radii (1.6 arcmin from the limb of the sun) Field of view: • The ATST shall provide a minimum field of view of 3 arcmin, with a goal of 5 arcmin. Wavelength Coverage: • The ATST shall cover the wavelength range from 0.30 to 28 µm Flexibility: • The ATST must accommodate simultaneous multi-wavelength observations at visible and IR wavelengths. • It must be possible to carry out simultaneous observations with different instruments. • Image rotation introduced by the telescope must be counteracted by de-rotation, preferably without additional reflections. • Maximum scientific productivity requires easy and fast (less than 30 minute) switching between facility instruments. Lifetime: • The ATST is expected to be the major solar ground based facility for a minimum of two decades. The useful lifetime of ATST is expected to exceed 40 years. Adaptability: • The ATST shall be designed with a minimum of limitations for future use and in a way that allows future upgrades and the addition of new instruments. Availability: • Scheduled engineering and maintenance should not exceed 10-15%. • Of the remaining time, ATST should set a goal of achieving telescope reliability that allows observing during 97-98% of the available clear time (similar to the best nighttime telescopes). Location: • ATST should be located at the best affordable site in terms of seeing, sky clarity and sunshine hours. This will maximize the telescope performance and minimize the cost of adaptive optics.

The science use cases also place several important derived requirements on the telescope:

Pointing and Tracking: • Absolute (blind) pointing shall be accurate to better than 5 arcsec. • Offset pointing shall be accurate to better than 0.5 arcsec. • Open-loop tracking stability must be better than 0.5 arcsec for one hour.

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Active Optics: • Active control of the primary mirror figure will be required to achieve the necessary resolution. The active-optics system must be run in an open-loop mode during coronal observations when real-time wavefront information is unavailable.

1.1.3 Design Requirements Many design requirements were derived directly from the science requirements; others were determined from error allocations within several systems error budgets (see Section 1.1.4). The job of flowing science requirements down to design requirements is often made easier because existing telescopes have met similar requirements, and the engineering specifications of those systems are known and available. Table 1.2 lists important subsystems that are constrained by a specific telescope requirement. Note that the demand for high-resolution observations yields the greatest number of design requirements. The error budget discussion that follows helps to show how some of these constraints were derived in detail. The table also shows the current “compliance” status, noting briefly what features of our design allow us to meet the more challenging requirements.

Table 1.2. Flow-down to Subsystems, and Compliance Status Telescope Subsystem Flow Down Design Compliance and Requirement Strategy Resolution Constrains telescope mount drives, control, and In Compliance – Challenging thermal systems; coudé rotator and drive systems; • 4-m aperture pier design; M1 aperture size, figure, support • Diffraction-limited optical system and thermal control; active optics design performance; heat stop thermal control; M2 figure, • Active thermal control of mount, and thermal control; feed optics figures and components thermal control; adaptive optics performance; • Heat rejection at prime focus guiding systems; tip-tilt performance; optical • Rigorous error budgeting alignment; polarimetry optics; Gregorian and coudé applied to many subsystems optics; instrument lab thermal control; science instrumentation; telescope control software; enclosure thermal system (both skin temperature requirements and ventilation requirements); support facility location and construction methods. Photon flux and Constrains M1 aperture area, M1 ancillary In Compliance – Straightforward sensitivity equipment (mirror washing), and all mirror coating • 4-m aperture specifications, telescope and instrument optical designs (number of reflections). Polarization Constrains mirror coatings, polarimetry analysis In Compliance – Challenging sensitivity and and calibration performance, and science • Pre-Gregorian Modulation accuracy instrumentation. • Pre-Gregorian Calibration • Optical designs • Charge-caching cameras Scattered light Optical design, mirror polishing specifications, M1 In Compliance – Challenging mirror cover design, M1 ancillary equipment (mirror • Off-axis configuration cleaning and washing), occulting system, mirror • Gregorian station coatings, baffles and stops, enclosure thermal • In-situ cleaning/washing system (specifically the interior ventilation system • Active ventilation filtration. and its relation to dust control), and coating and • Prime-focus occulting cleaning facilities. Field of view Constrains heat stop dimensions, occulting system, In Compliance – Straightforward feed mirror dimensions, mirror cell dimensions, • Optical design acquisition and guiding system, and instrument • Mechanical design designs.

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Table 1.2. Flow-down to Subsystems, and Compliance Status (continued) Wavelength Constrains mirror coatings, imaging system In Compliance – Straightforward coverage optical materials, science instrumentation, and • All-reflecting design restricts the general use of transmissive windows • Laminar-air coudé station for thermal control of the light path. isolation Flexibility Constrains coudé rotator and drives, polarimetry In Compliance analysis and calibration, coudé stations, • Software design instrument control system, science instruments, • Modular components telescope control system, data handling system, • Facility instrument concepts and the observatory control system. • Coudé station layout Lifetime The ATST lifetime requirement affects all designs, In Compliance requiring a high level of robustness or • Coudé station design replacement and reconfiguration strategies. • Modular component design Adaptability The adaptability requirement drives many of the In Compliance features of the feed optics, the Gregorian and • Software designs coudé observing stations, the science • Modular component designs instruments, the instrument control system, • Facility instrument concepts telescope control system, and observatory control • Coudé station design system. Availability The Availability requirement affects many systems In Compliance that could impede observing efficiency. In • In-situ mirror cleaning and particular, it drives the of mirror cleaning and washing washing facilities, acquisition and guiding, • Control software polarimetry analysis and calibration, Gregorian • Facility instrument concepts and coudé platform configurability, optical • Coudé layout enclosure ventilation system, mirror cleaning and coating facility. Location The location of ATST constrains the details of the In Compliance – Requires pier design, in-situ M1 mirror cleaning and flexibility in current design washing (depending on local dust levels), mirror • Design generally meets thermal control systems, dome thermal control requirements at all sites still (based on wind and temperature extremes), under consideration coudé room thermal control, site infrastructure, • Site specific details can be buildings, facility equipment, and coating and deferred without impacting cleaning facilities (and their broader availability). schedule Pointing and Constrains telescope mount, drive system, In Compliance – Straightforward tracking telescope control software. • Mount Design • Pointing Kernel selection Active Optics Constrains M1 Mirror, M1 support structure, M1 In Compliance – Challenging at controller, wavefront sensor, telescope control high zenith distances system, and AO (adaptive optics) system. • Thin meniscus mirror • 120 axial supports • 24 lateral supports • Dedicated wavefront sensor

There are some technical risks associated with the more challenging requirements. These and the project’s plans for risk mitigation are outlined in the management discussion (see Part IV of this proposal, Chapter 8, Risk Assessment and Management).

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1.1.4 Error Budgets Error budgeting is fundamentally a systems-level issue. A given error budget will typically be distributed across many disparate subsystems. These have been designed by different engineers and will be fabricated by different vendors (Systems Error Budget Plan, ATST Document #SPEC-0009). Error budgeting is a useful tool at all levels of design since it represents a means to negotiate design trades in the broadest possible context. This process is central to the mission of systems engineering.

The highest priority error budgets developed to support the flow-down process involved delivered image quality. Three different image quality error budgets were derived from science use cases:

Diffraction limited observations at 500 nm The ATST shall provide diffraction-limited observations (at the detector plane) with high Strehl (S>0.6 required, S>0.7 goal) during good seeing conditions (r0 (500 nm) > 15 cm) and S > 0.3 during median seeing (r0 (500 nm) = 10 cm) at visible and infrared wavelengths. This science requirement applies to most of the science use cases listed under Spatial Resolution in the table above. It presumes that closed-loop wavefront correction is operating, including active optics, fast tip-tilt correction, and high-order adaptive optics.

Seeing limited on-disk observations at 1.6 µm

[For] excellent seeing conditions, (r0 at 1.6 micron ≥ 100 cm)… Minimum requirement: 50% Encircled Energy Diameter < 0. 15 arcsec. This requirement is derived from science use cases performing on-disk observations, but it presumes excellent seeing conditions when good images can be obtained over a wider field of view than can typically be obtained with single-conjugate AO (adaptive optics). Hence, the aO (active optics controlling M1 figure) and tip-tilt loops are closed, but the high-order AO system is not in use.

Seeing limited coronal observations at 1.0 µm

Off-pointing up to 1.5 solar radii, wavelength 1 micron, excellent seeing conditions: r0 (1 micron) ≥ 50 cm, FWHM seeing limited PSF 0.4 arcsec. The minimum resolution required for coronal magnetometry is 2 arcsec. The Telescope shall deliver the following image quality: 50% Encircled Energy Diameter < 0 .7 arcsec 85% Encircled Energy Diameter < 2 arcsec This requirement applies to most coronal observations. “Off-pointing” implies that there is no granulation present within the 5-arcmin field of view, so no wavefront information is available. The AO loop (including tip/tilt) is open. Similarly, the aO loop is open, and the primary mirror figure must be corrected based on constant or repeatable errors via a look-up table or function fit.

These three cases are of particular interest because they span the range of possibilities for wavefront correction, as shown in Table 1.3. Complete error budgets with Monte Carlo simulations show that ATST meets the science requirements for each of these cases. An example is shown in Section 1.1.5.

Table 1.3. Wavefront correction cases. Active Optics Loop Tip-Tilt Loop High-order AO loop Diffraction limited Closed Closed Closed observations Seeing limited on- Closed Closed Open disk observations Seeing limited Open Open Open coronal observations (Look-up table)

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The fundamental difference between these three observing modes is the range of spatial frequencies that can be corrected in each case. It is useful to look at the problem from the perspective of the power spectral density (PSD) of spatial frequency errors in the wavefront delivered to the focal plane. For example, if optical surface polishing errors are analyzed in this way, it is found that they obey a power law distribution over a very broad range (five orders of magnitude) of spatial frequencies extending from dimensions near to the full aperture all the way down into the realm of surface microroughness. Throughout this range the slope of the PSD on a log-log plot is roughly –2. This is shown schematically in Figure 1.1.

Figure 1.1. Periods and spatial frequencies are relative to the four-meter entrance pupil.

When the active optics loop is closed allowing figure errors to be compensated, the power at the lowest spatial frequencies is reduced considerably. Switching on the adaptive optics system causes further improvement at higher frequencies. This analysis suggests that it is useful to allocate errors separately within each of four frequency regimes. Table 1.4 defines the four frequency regimes.

Table 1.4. Frequency regimes. Definition Spatial Period (mm) Description Low 4000 ! 800 Active Optics Influence Range (10 actuators across the 4-m primary)

Intermediate 1 800 ! 200 Adaptive Optics Influence Range (100 mm sub aperture on primary)

Intermediate 2 200 !4 Uncorrectable figure errors High 4! down Microroughness

With these definitions in place, it is possible to use the same error tree for all three observing cases, and modify the error allocation according to the active controls available.

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A specific example of how these conventions are used to flow down to design requirements is shown in Table 1.5.

Table 1.5. Flow down example. M2 Errors apportioned by frequency band Low Int. 1 Int. 2 High Total Correction applied 140 30 8 2 144 None (Manufacturing Spec.) 70 30 8 2 77 aO loop open (with look-up table) 2 30 8 2 31 aO loop closed 0 2 8 2 8 AO loop closed

Taken in combination, the three delivered image quality error budgets place limiting constraints on many telescope subsystems, and hence form the basis of their design requirements.

1.1.5 Performance Predictions The error budgets maintained for ATST are used in two different modes. The first mode represents snapshots in time, assuming specific observing conditions. The image-quality science requirements, for example, specify seeing conditions that are good or excellent. We make assumptions about other free parameters, like ambient temperature, wind speed, and zenith distance, and these values are entered as constants. Normally we must adopt “worst case” values when the science requirement does not include these details. Details are contained in the System Error Budget Plan, ATST Document #SPEC-0009.

The second mode used in the error budgets brings additional information into the error calculation. This includes distributions of expected parameter values that affect the image quality. For example, wind- speed statistics are available for the candidate sites. With these distributions in place, Monte Carlo simulations can be performed to randomly select wind speeds weighted by the probability functions (histograms). By looking at thousands of system manifestations, it is possible to predict the fraction of the time that the telescope system will deliver images of a given quality.

Wind speed is a particularly interesting case to study because of the diverse ways in which wind affects the final image. As wind speed increases the flushing it provides to the enclosure and mirrors will improve seeing by sweeping away the warm, turbulent boundary layer. The vents in the dome will allow some fraction of this flow to flush the telescope mirrors and mount structure, again improving self- induced seeing. In all of these cases more wind is better. Wind can also degrade performance, however. The pressure of the wind on the thin M1 mirror will cause it to deform, resulting in poorer images. The wind also excites vibrations in the telescope mount assembly. For these cases the higher the wind-speed, the worse the performance.

The Monte Carlo simulations built into Telescope Delivered Image Quality w site stats Haleakala the error budget spreadsheets allow all of 1.0 these effects to be analyzed with 0.9 underlying wind-speed probability 0.8 distributions, and any other error 0.7 0.6 parameters for which probability 0.5 Site Seeing distributions exist or can be estimated. 0.4

Probability Bottom Up 0.3 Figure 1.2 shows the results of such an Telescope & Instrument analysis for the seeing-limited error 0.2 budget using both seeing and wind- 0.1 0.0 velocity probability distributions for 0.0 0.8 1.6 2.4 3.2 Haleakala. The green curve shows that 50% Encircled Energy (arc sec) ATST will meet the 0.15 arcsec Figure 1.2. Telescope delivered image quality with site statistics.

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requirement most of the time. The blue curve includes seeing statistics for the site, and the red includes telescope effects. These give a more accurate prediction of how often we will meet the requirements. Similar positive results are produced for the adaptive optics and coronal cases for all sites (ATST Document #SPEC-0009).

1.2 DESIGN OVERVIEW The design for ATST that has emerged from the requirements flow down is described in detail in the chapters that follow. We describe each subsystem in some detail starting with the requirements place upon it, followed by a general description of the design. The top-level organization of these subsystems is as follows:

• Telescope Assembly • Enclosure • Wavefront Correction Systems • Support Facilities and Buildings • Instrument Systems • Remote Operations Building • High Level Controls and Software

While most aspects of these subsystems can be discussed in isolation, several important features of the ATST design span several subsystems: the optical design and its overall performance, thermal control, and special features of the design that facilitate coronal observations. Each of these will benefit from a brief systems-level description. The individual components are discussed in more detail later.

1.2.1 Optical System Design The ATST optical design has two features that distinguish it from nighttime telescopes with similar aperture size: it is off axis and Gregorian. Both of these features are included in the design in direct response to the science requirements, and have many practical advantages as well.

The basic idea of the off-axis configuration is shown in Figure 1.3. M1, the primary mirror, is a four-meter section of a 12-meter parent M2 parabola. The parent is shown as a wire-frame Prime structure across the bottom of the figure, but Focus only the solid shaded part on the left is used in ATST. Similarly M2, the secondary mirror, is a 0.62-meter section of a two-meter parent ellipsoid. Only the solid shaded section on the top right is used. The red beam filling the primary represents a point source at zenith.

The underlying Gregorian design has the feature that an image is formed in front of M2, Gregorian offering an opportunity to reject most of the Focus M1 energy in the concentrated beam before introducing it onto M2 and the optics that follow. There is an 80-mm diameter image of Figure 1.3. Basic off-axis configuration. the sun formed at prime focus. Only a 13-mm diameter circular section of this image (five arcmin) is passed through the heat stop and into the balance of the optical train. The rest of the energy is reflected away or absorbed by a liquid-cooled heat stop. As a result, the irradiance incident upon M2 is roughly the same as that on M1 (i.e., the same as any object lying in direct sunlight). For a reflective coating that is 90% efficient (typical of aluminum coatings) only about 30 watts of power is absorbed by M2, which is manageable.

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The stray-light performance of such a telescope is improved by the lack of diffraction around M2 or a spider structure supporting it. This is critical to coronal observations close to the limb of the sun (see Section 1.2.3 below). Also, the beam is unobstructed by M2, so the full four-meter aperture is available. Both the heat stop and the secondary mirror are outside of the beam incident on M1, reducing the effect of any residual heat plume on seeing performance. The off-axis design also simplifies the delivery of cooling and other utilities to the heat stop, limb occulter, and M2 since these services can be provided without crossing the beam. Studies carried out by the ATST project and commercial vendors during the design and development phase have shown that the off-axis optical elements do not represent any significant technical challenge or unmanageable risk.

The Gregorian focal plane is the first science observing station. This station will be used primarily for infrared coronal spectro-polarimetry. While the Gregorian image quality is not diffraction limited over the full 3-arcmin field of view, it meets the science requirements for the observations to be performed there.

The other mirrors in the ATST optical train are feed optics that deliver an image to the coudé stations (Figure 1.4). These can be divided into three groups based on function:

Transfer Optics – Four mirrors are required to transfer the beam over the mechanical altitude and azimuth axes (Figure 1.4, top right insert). These mirrors are designated M3 through M6. M3 and M6 are flat folding mirrors. M4 is an off-axis parabola that serves to collimate the diverging beam. M5 is a nominally flat fold mirror that also serves as the deformable mirror for the high-order adaptive optics system. M6 also provides fast tip-tilt correction. Beam Reducing Optics – Three additional mirrors, M7 through M9, form an afocal system that approximately halves the diameter of the collimated beam and renders it horizontal before introducing it onto the coudé instruments. Imaging systems –Several mirror and lens options are provided as part of the instrument lab facility that focus the collimated beam onto the focal-plane instrumentation. The resulting images are diffraction- limited over the required 3-arcmin field of view. 1.2.2 Thermal Control The ATST design addresses the need to control so-called “self induced seeing,” which is seeing that results from the presence of the telescope and telescope enclosure. This component of seeing tends to contain higher spatial frequencies than atmospheric wavefront distortions, so the adaptive optics system is less effective at correcting it when present. We have paid careful attention to this problem, particularly because observations will be carried out during daylight hours when the sun can heat exposed components to well above ambient air temperatures.

One of the common methods of controlling seeing at existing solar telescopes involves evacuating the optical column within the optical system. This scheme is not possible with ATST because of the requirement to observe simultaneously over a broad range of wavelengths, thus precluding use of an entrance window at the top of the column. Instead ATST will use the approach of actively controlling the temperature of insolated components such as telescope mirrors and the enclosure’s outer skin. Our various studies and experiments have shown that if the temperature of these components can be maintained close to or slightly below the ambient air temperature, self-induced seeing can be controlled within the error-budget allocations. The details of this process are discussed item by item in the following chapters.

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M5 M6

Altitude Axis

M3

Azimuth M4 Axis

Azimuth Axis Altitude Axis

M8

M9

M7

Figure 1.4. Feed optics layouts. Another aspect of the general thermal-control strategy involves active or passive flushing of surfaces within or near to the optical beam with non-turbulent air. This is provided either by natural winds that enter the enclosure through passive ventilation gates, or forced ventilation provided by air-handling units that can be operated during periods of little or no wind. This system will be discussed in more detail below in the enclosure description (Chapter 6).

1.2.3 Coronal Capabilities The case for including coronal capabilities in the ATST design has already been made in the science discussions elsewhere in this proposal (See Part II: Science Goals.) In summary, while space-based instruments can generally deliver very sharp images of the sun, it is the four-meter aperture of ATST that will make it a unique and powerful tool for coronal science. An aperture of this size will allow scientists to do high-resolution coronal spectroscopy – and hence polarimetry – to probe the magnetic properties of off-limb coronal features.

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As we noted in the error budget discussion in Section 1.1.4 above, the requirement on image quality for coronal use cases is relaxed considerably relative to the diffraction-limited observations. The diffraction limit for a four-meter telescope operating at 1 µm is 0.06 arcsec, while the SRD specifies 0.7 arcsec (both in terms of 50% encircled energy diameter) for coronal use cases. The emphasis here shifts to observing relatively faint features that are very close (as close as 5 arcsec) to the bright limb of the sun. In addition to the requirement to build ATST at a site with low levels of atmospheric scatter, several features of the design presented in the following chapters are included specifically to address this unique need: • A Gregorian platform is included, providing a faster beam with a minimum number of reflections (described in Section 4.1.2). • The Gregorian module of the Near IR Spectro-polarimeter is housed in a cryostat to reduce thermal background levels (described in Section 4.2.3). • An active occulting system is provided to block light from the photosphere while observing the corona (described in Section 2.3.2). • Many aspects of the optical and mechanical design of the telescope (including the off-axis configuration discussed above) are driven by the desire to keep stray light to a minimum.

This last bullet point addressing stray-light control is critical to the success of ATST. The science requirements for coronal observations at the Gregorian focus place strict and challenging constraints on the stray-light performance. Detailed stray-light analysis on the ATST design confirms what experience has shown with existing coronal instruments operated at excellent sites: when observing close to the limb of the sun, instrumental scattered light is dominated by scatter off the reflecting surfaces in the beam. When mirror surfaces are clean, the underlying microroughness imparted by the polishing process limits the performance. By specifying an RMS microroughness of 2 nm, the science requirements can be comfortably met. This level of polish is routinely achieved using modern polishing techniques, so this requirement presents no manufacturing challenge.

It is found, however, that close attention must be paid to keeping the surfaces clean during coronal observations. If only 0.01% of the mirror surface is covered by dust, the dust contribution to instrumental scatter overtakes mirror microroughness to become the dominant factor. The length of time necessary to reach or exceed the 25×10-6 at 1.1 solar radii requirement will vary considerably depending on site conditions, but even under average conditions this contamination level can be reached in one day of uncontrolled exposure. The ATST has adopted several strategies to mitigate and control dust contamination and its effects:

Occulting of the sun’s disk at prime focus. The ATST design includes a reflecting occulter that rejects the sun’s photosphere at the prime-focus image, passing only the corona. (See “Active Occulter Insert,” in Section 2.3.2 below.) For the coronal instruments operating at the Gregorian focus, this eliminates dust on M2 as a significant contributor since the bright on-disk radiation never reaches that mirror. It also reduces stray light due to diffraction around the Lyot stop located just down stream of M2, again because no direct power from the sun’s photosphere ever reaches that point in the optical system. In-situ cleaning and washing system. This system – part of the M1 mirror mount assembly – incorporates a CO2 snow cleaning capability that is efficient and convenient for use as often as required during coronal observations, and a wet washing station that can be used without removing the mirror from its cell. (See “Cleaning and Washing System” in Section 2.2.2 below.) Closed vent gates. As noted above, coronal observations have much-relaxed image-quality requirements compared with on-disk diffraction-limited observations. Hence, the open vent gates and active flushing

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that must take place during high-resolution observations is unnecessary, allowing better short-term control of dust infiltration. Block Scheduling. Whenever possible, coronal observations will be block scheduled to take maximum advantage of freshly coated or recently cleaned optical surfaces and good observing conditions (dark sky conditions and low dust levels).

2. TELESCOPE ASSEMBLY The Telescope Assembly is shown in Figure 2.1. It is comprised of (1) the Telescope Mount Assembly; (2) the M1 Assembly; (3) the Heat Stop Assembly; (4) the M2 Assembly; (5) the Feed Optics; (6) the Baffles and Stops; and (7) the System Interconnects. These are described in detail in this chapter.

Wherever possible, the design of the Telescope Assembly and its subcomponents were based on previous successful large telescope systems, including the structural layout, the servo and control systems, the pier concept, and many of the mechanical subsystems. Where it was Figure 2.1. Telescope Assembly. impossible to emulate existing telescope designs, we have verified the ATST Telescope Assembly design by a variety of proven methods. For example, full static and dynamic finite element (FE) studies have been performed on the overall structure and pier. Transient thermal analyses, computational fluid dynamics (CFD) studies, and various other calculations have been performed as well to support the design.

In addition to these analyses, a variety of potential telescope fabricators and vendors were involved in design evaluation studies from early in the project. The purpose of these evaluations was to review the subassembly designs, suggest technical improvements, and provide fabrication cost estimates. The results helped refine the overall system design, and concentrated the project’s efforts on reducing technical risk and improving overall telescope performance. Industry involvement of this type is especially useful in addressing manufacturing concerns and logistics early in the design process.

2.1 TELESCOPE MOUNT ASSEMBLY The Telescope Mount Assembly (TMA) provides structural support for the major optics and instruments of the ATST observatory. It includes a variety of mechanical subassemblies, bearings, controllers, drives, and equipment that are used to point, track, and slew these optics and instruments during science observations. The TMA is comprised of five major components: (1) the Mount and Drive System; (2) the Coudé Rotator and Drive System; (3) the Pier; (4) the Ancillary Mechanical Systems; and (5) the Mount Control System. These five items are described in detail, below.

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2.1.1 Telescope Mount Assembly Design Requirements The Telescope Mount Assembly serves a number of important roles and functions during science observations. Of these, five are considered to be top-level, or most important to the performance of ATST. These top-level functional requirements are as follows:

Optics Mounting: The TMA provides precise and stiff mounting interfaces for the M1 through M9 mirror assemblies, heat stop, occulter assembly, polarimetry optics, acquisition and guidance system, and the telescope alignment system. The nominal positions and allowable deflections for all of these mounted assemblies are derived from the Static and Dynamic Optical Alignment specifications, which are derived from the delivered image quality error budgets. The worst-case (i.e., most stringent) error budget terms were then used to design the structural members and mounting points of the Telescope Mount Assembly. A series of detailed FE analyses was performed to optimize and validate the design and mounting interfaces.

Gregorian Instrument Interface: The TMA provides a precise and stiff mounting interface for the Gregorian instrument rotator. The specifications of this interface were derived from the requirements of the Gregorian instrument suite, including such factors as geometric size, overall mass, required mounting stiffnesses, thermal considerations, stray light requirements, and a variety of handling and operational concerns.

Coudé Lab Instrument Interface: The TMA provides a precise and stiff mounting platform for all the coudé-lab instruments. Structural designs and stiffnesses were flowed down from the respective error budget provisions into the design. The overall layout and detailed design of the Coudé Rotator was validated in terms of these requirements by way of FE analyses.

Pointing, Tracking, and Slewing: The TMA provides for accurate and repeatable pointing, tracking and slewing of the ATST optics and instruments over their required full ranges of travel. The specifications for pointing, tracking, and slewing are based on a combination of direct flow-down from the SRD and from derivations of the delivered image quality error budgets (e.g., drive jitter).

Throughput, Thermal, Stray Light: The TMA provides for an unobstructed optical path from the sun to the Gregorian instrument station and to the various Coudé Rotator instrument stations. It does this without imparting excess thermal input into the beam (i.e., degrading seeing) or adding deleterious stray light into the science light paths. A number of thermal and stray-light analyses were performed to verify that the TMA layout met these requirements.

In addition to these top-level requirements, there are a number of second-level functions that the TMA provides. For example, the TMA is designed to allow for periodic removal of the major optics for servicing operations (e.g., M1 stripping and recoating). The TMA also provides a variety of features and safety systems designed to protect personnel and the telescope from damage (e.g., failsafe M1 cover; GIS interface, etc.). The complete specifications and design for the Telescope Mount Assembly, including all the top-level and second-level requirements are outlined in the TMA Design Requirements Document (ATST Document #SPEC-0010).

2.1.2 Telescope Mount Assembly Design Description Mount and Drive System: The Mount is comprised of two major structural elements: the Optics Support Structure (OSS), which rotates about the altitude axis, and the Mount Base, which rotates about the azimuth axis. These are shown in Figure 2.2. The OSS provides mounting interfaces for the M1 Assembly, the heat stop, the M2 Assembly, feed optics M3 and M4, and the Gregorian rotator and instruments. The Mount Base provides interfaces for the M5 and M6 adaptive feed optics.

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Figure 2.2. Telescope Mount Assembly, showing the OSS and Mount Base.

The structural design of the Mount employs large steel weldments that have been stress relieved prior to final machining. This technique has been very successful on many other recent telescope projects, such as Gemini, SOAR, and WIYN. Detailed FE analyses were used to verify the basic design. The Mount structure is configured with minimal bolted joints. This is critical to minimizing non-repeatable errors that can affect telescope performance. The overall Mount layout provides high stiffness (i.e., minimize static and dynamic flexure), is resistant to vibrations (e.g., wind-induced resonances), and allows for direct load paths from the supported optic assemblies down into the structure, the bearings, and ultimately the concrete pier and the ground. The major subcomponents of the Mount are described as follows.

Optics Support Structure: The OSS is a performance-based design, configured to accommodate the large bending loads of the off-axis optical layout. The M1 Assembly alone weighs more than 12,000 kg, with its center of gravity cantilevered four meters horizontally from the altitude axis. This structural challenge is met by a novel two-piece layout of the OSS.

The bottom portion of the OSS carries the large off-axis loads via an arrangement of large square and rectangular steel tubes that are optimized for bending loads. This system maximizes stiffness by making the best use of the relatively stiff section properties (i.e., large area-moment of inertias) of the rectilinear shapes. The upper portion of the OSS, in contrast, utilizes round steel tubing members to reduce weight. It also results in minimal thermal mass, and reduces airflow obstructions on the upper portion, which improves thermal flushing and minimizes the cross-sectional areas and coefficients of drag of the upper OSS.

The upper portion of the OSS is joined to the lower via non-slip bolted joints. The two-piece configuration eases manufacturing and helps facilitate transportation from the fabricator to the ATST observatory site. When bolted together, the complete assembly is partially self-compensating for relative M1-to-M2-to-altitude axis displacements as the OSS rotates from zenith to horizon. It also is extremely stiff, allowing only minimal deflections and rotations of the heat stop assembly, M1-M2 optic assemblies, and the Gregorian instruments. The M1 Assembly is installed into the OSS from underneath via a specialized handling/lifting cart.

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Mount Base: The Mount Base is comprised of three major structural elements: the base pedestal and the two mount columns. The overall design is based on a monocoque layout using steel plate construction with internal stiffening ribs and shear panels. The two mount columns are affixed to the pedestal with slip-free bolted joints. The Mount Base arrangement provides a direct load path from the altitude bearings down through the structure and into the azimuth bearings.

The adaptive optic assemblies (M5 and M6; deformable and fast tip-tilt mirrors, respectively) are supported on a reinforced vertical column that rises from the top of the pedestal up to the altitude axis. The static and dynamic structural performance of the Mount Base has been verified with FE analyses.

Bearings and Azimuth Track: The Mount altitude bearings are large bore tapered roller bearings, mounted in preloaded pairs on the top of each mount column. They are sized for high radial stiffness, and to accommodate differential thermal expansions between the OSS and the Mount Base. The bearings are high accuracy ABEC class-type units, with run-out characteristics specified by the optical tolerances (i.e., dictated primarily by M4 and M5 requirements).

The azimuth bearings incorporate a hybrid design of hydrostatic and rolling element bearings. Axial support (vertical loading) is provided by a conventional hydrostatic oil bearing system (HBS). Four individual load-carrying HBS pads are mounted underneath the mount base. These bear vertically downward against the top surface of a large-diameter azimuth track. Pressurized oil from a remote pumping plant provides pad lift-off, while maintaining excellent stiffness and extremely low running friction. A rolling element bearing is used to provide radial support and location to the Mount Base. The position of the entire Mount can be precisely positioned such that the Mount azimuth axis of rotation coincides with the Coudé Rotator azimuth axis. This is a requirement derived from the operational tolerances required between the M6 and M7 optic assemblies.

The azimuth track is based on standard large telescope track systems. It incorporates a segmented design for ease of manufacture and transport to the site. The fact that the radial positioning of the mount is controlled by the separate radial centering bearing means that roundness tolerance issues normally associated with large telescope azimuth tracks are not significant here.

Mount Drive System: The mount drive system is the assembly of drives, encoders and controllers that allow the OSS and Mount to move during telescope operations. Both altitude and azimuth rotations are provided by this system. The design of the mount drive system is dictated primarily by the pointing, tracking, and slewing requirements of ATST. The top-level specifications are as follows:

Blind pointing < 5 arcsec Offset-pointing < 0.5 arcsec Tracking stability < 0.5 arcsec/hr Tracking rate = solar rate Slew speed = +3º/sec.

The Mount is driven in altitude and azimuth by friction drive units. On altitude, the friction interface is on the altitude drive disks; on the azimuth axis it is on a separate drive ring affixed to the outside of the azimuth track. The drive motors are brushless DC units, commanded by industry-standard digital servo- controller/amplifiers. Position feedback on both axes is via tape encoders, using multiple read-heads. Fail- safe brakes are mounted on both axes, and shock-absorbing over-travel stops on both axes limit the range of motion to 0-90 degrees in altitude, and 270 degrees in azimuth.

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Mount Thermal Control: The telescope mount has a large amount of surface area, much of it above the level of the primary mirror. It is therefore important that the mount temperature track the ambient air temperature closely to avoid self-induced seeing. The mount temperature is controlled primarily by shading it from direct solar radiation with the enclosure. In the absence of solar heat loads, the main source of temperature differences is the thermal inertia of the mount. As the ambient temperature rises in the morning and falls in the late afternoon, the mount temperature will lag the ambient air temperature by an amount that depends upon the wind speed. Wind passing over the mount helps to reduce the lag time. In addition, provision is made for drawing ambient air through the interior of structural members to reduce the thermal equilibration time.

Gregorian Rotator and Drive System: The Gregorian Rotator is the mechanical assembly mounted to the bottom of the OSS. Its axis of rotation is aligned with the gut ray that is delivered from the M2 assembly. The Rotator provides mounting interfaces for the Gregorian instrumentation of ATST. It can accommodate a single instrument of 1000 kg, and up to 2 m long by 600 mm in diameter. The range of travel of the Rotator is ± 270 degrees.

Figure 2.3. Coudé Rotator. Coudé Rotator and Rotator Drive System: The Coudé Rotator is the large, two-level steel assembly inside the Pier that rotates about an azimuth axis coincident with the Mount (Figure 2.3). It provides mounting interfaces for the M7-M9 beam-reducing mirrors and all of the coudé-level instrumentation of ATST. The Rotator can accommodate the simultaneous installation of up to eight instruments of 3000 kg each. The flooring system is designed to allow personnel to work around the instruments during normal observations. There is a 1m-wide annular, non-rotating safe zone around the outside of both the upper and lower levels.

The Coudé Rotator is fabricated from standard structural steel members (e.g., wide-flange I-beams and plate steel flooring). Hard-points are built into floor members for the larger instruments and optics benches. All structural members are sized to maximize stiffness (i.e., minimize deflections due to instrument and personnel loads) and to ensure precise and repeatable operations. A detailed FE model was constructed to analyze and verify the Rotator design, including static stresses, deflections, and dynamic performance. The Rotator bearing system is identical to that of the Mount azimuth bearing system.

Coudé Rotator Drive System: The Coudé Rotator drive system is identical to that of the Mount. Requirements were derived primarily from the worst-case pointing, tracking, and slewing requirements:

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Blind pointing < 5 arcsec; Offset-pointing < 0.5 arcsec; Tracking stability < 0.5 arcsec/hr; Tracking rate = nominally solar rate (dependent upon site); Slew speed = +3º/sec.

Pier: The Pier is the large reinforced concrete cylinder upon which the Mount and Coudé Rotator are supported. The Pier also provides intermediate flooring and access for the telescope mechanical equipment, the azimuth tracks, and for supporting the Mount and Coudé Rotator azimuth cable wraps. The Pier is designed for maximum stiffness and to efficiently transfer the axial and radial loads from the Mount and Rotator down into the soil. The foundation and soil interface of the Pier is dependent on the observatory site soil properties (i.e., geotechnical and seismic requirements).

Ancillary Mechanical Systems: M1 Cover Assembly: The M1 Cover provides three key functions on the TMA:

1. Impact damage: The M1 Cover protects the primary mirror from damage whenever the telescope is not observing the sun. Dropped tools and other types of impact damage are a significant danger over the 40-year lifespan of an observatory the size and complexity of ATST. The M1 Cover is designed to survive an impact load of a falling 2.5 kg weight released from a height of 15 m, and to carry the weight of up to three workers simultaneously walking on top of it. 2. Contamination control: The M1 Cover keeps dust and other airborne particulates from collecting on the mirror at night and during non-operational periods. A slight overpressure of dry air is maintained underneath the cover, to reduce the infiltration of contaminants. 3. Thermal safety system: The M1 Cover serves as an integral part of the ATST thermal safety system. The Cover is designed to rapidly close in an emergency event. This closure results in an interruption of the light path from the sun to M1, and thereby causes a safe removal of focused light from reaching the Heat Stop.

The M1 Cover design, shown in Figure 2.4, is based on a traditional folding panel system common to many modern large telescopes. Aluminum honeycomb panels, joined with full-length hinges, ride on guide rails. The M1 Cover is opened via an electric drive, and then held in that position via an electromagnetic clutch. In the event of a power failure, the clutch disengages, and large fail-safe springs passively close the cover in approximately 15 seconds. Rotary dampers are used to smooth and control the spring closure of the cover.

The M1 Cover is designed to operate in Figure 2.4. M1 Cover any orientations of the OSS. When open, the cover folds back into a low-profile package inboard of the mirror, to minimize air flow obstructions.

Cable Wraps: Powered cable wraps are employed on all three axes of the TMA (Mount Altitude, Mount Azimuth, and Coudé Rotator Azimuth; see Figure 2.5) to manage the system utilities throughout their ranges of travel. These utilities include AC and DC power lines, copper and fiber signal and data lines,

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Figure 2.5. Azimuth and Altitude Cable Wraps. coolant supply and return hoses, compressed air and nitrogen lines, and various communication links. Powered wraps minimize non-repeatable torque inputs that can affect the pointing and tracking performance telescope by isolating utility cable stiction and slip from being input into the telescope structure. The wraps are mechanically isolated from the telescope, and they are continuously slaved to follow the respective TMA axis rotation.

Sensor Arrays: The Telescope Mount Assembly is outfitted with an array of functional, safety, and diagnostic sensors that are distributed on and around the system. These sensors are used to continuously monitor the performance and health of the TMA, and to provide feedback to the Telescope Control System (TCS), Observatory Control System (OCS), and Global Interlock System (GIS) systems. The sensor arrays include (1) Thermal Sensors; (2) Vibration Sensors; (3) Wind Velocity Sensors; and (4) Bearing Health Sensors.

Mount Control System: The Mount Control System (MCS) provides control software for the telescope mount assembly. It is an integral part of the ATST telescope mount. The MCS operates all associated sub- assemblies, including azimuth and altitude drives, coudé rotator, Gregorian rotator, and thermal management. It is controlled by the TCS for all operations except low-level engineering activities and safety interlock situations. The MCS is directly connected to the GIS to perform safety operations.

2.2 M1 ASSEMBLY The M1 Assembly (Figure 2.6) is the heart of the ATST telescope; it contains the primary four-meter diameter off- axis mirror (M1) that is the first element in a chain of optics that collects and focuses the solar energy into high- resolution images. The assembly consists of the four- meter diameter M1, the axial and lateral support system for M1, the M1 cell, the M1 thermal control system, M1 Figure 2.6. M1 Assembly. cleaning and washing system and M1 control system. The M1 assembly defines the position of M1 and maintains its optical figure under the operating conditions of

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changing gravity load, thermal conditions and wind loading. The M1 support system also has active optics capability, to slowly adjust the figure of M1 to compensate for a wide range of effects, including changes in zenith angle and thermal conditions.

2.2.1 M1 Design Requirements The SRD defines the aperture of the telescope as four meters. The M1 is consequently sized at 4.24 meters diameter to yield a 4.0-meter clear aperture after the necessary baffling of the outer edge and accommodation of the inclination angle of M1 with respect to the incoming solar beam. The optical quality of M1 is critical to maintaining the required solar image quality; a surface figure of 32 nm rms must be maintained over the operational limits of 0° to 80° zenith angle (changing gravity vector), thermal conditions and wind loading. A comprehensive Design Requirements Document has been developed to address these requirements (ATST Document #SPEC-0007).

2.2.2 M1 Design Description

Ø12100 PARENT PARABOLOID

ATST PRIMARY MIRROR (4100)

AAØ4237 PARENT SURFACE A PARABOLOID 100

SURFACE B AXIS A, GEOMETRICAL AXIS OF GEOMETRICAL AXIS OF PARENT PARABOLOID ATST PRIMARY MIRROR BLANK

SECTION A-A

ALL DIMENSIONS IN MM. 4000

Figure 2.7. M1 Mirror. M1 Mirror: The M1 Mirror is a 4.24-meter diameter constant thickness meniscus, approximately 100 mm in thickness. The physical configuration is an off-axis paraboloid with a focal length of 8000 mm, effectively part of a 12.1-meter diameter f/0.67 parent paraboloid as shown in Figure 2.7.

The material for M1 is ultra-low expansion fused silica or glass-ceramic; this choice was driven by the large temperature gradients from the front to the back of M1 due to solar loading and thermal control. Two materials are available at the four-meter scale for an M1 blank: ULE from Corning, Inc., New York and Zerodur from Schott Glasswerke of Germany. Both materials have a long and well-established history of use in astronomical and solar telescopes.

M1 Support System: The function of the M1 Support System is to support the weight of the M1 and maintain its nominal surface figure over the operational zenith angles and thermal environments of the telescope; it also defines the position and orientation of M1. In addition, the support system makes small changes to the surface figure of M1 by applying active optics correction forces through the axial support actuators.

The support system is composed of an array of axial supports and an array of lateral supports. The axial support system consists of 120 discrete support actuators arranged in five concentric rings on the back of the mirror. The body of each axial support actuator is attached to the M1 cell and a support rod from each

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actuator is attached to a load spreader bonded to the back of M1. The lateral support system consists of 24 discrete 120 AXIAL support actuators arranged around the periphery of M1. The SUPPORTS body of each lateral actuator is attached to a bracket at the outside edge of the M1 cell and a support link from each lateral actuator is attached to an Invar pad bonded on the outer edge of M1. Figure 2.8 shows the arrangement of axial and lateral support points and arrows that represent the force vector applied by each support.

24 LATERAL Finite element analysis was used to optimize the axial and SUPPORTS lateral support systems. First, the radial location and force value of the five rings of axial support actuators was determined. The goal in this process was to reduce the deflections of the optical surface of M1 caused by gravity at a zenith pointing position to an acceptable level that meets the error budget requirements for image quality. During the Figure 2.8. Axial and Lateral Support Point optimization, several constraints were imposed to simplify Locations the mechanisms and reduce cost:

The force applied by all the axial actuators in any given ring is equal (this clearly follows from the rotational symmetry of M1). Only two nominal force values were allowed for the five rings.

Since the back of the meniscus mirror is smooth and continuous, there were no constraints on the radial locations of the five rings. The optimization was successful in yielding a surface figure accuracy of 18 nm rms with a force value of 180 N for ring 1 and 320 N for rings 2 through 5. These force values are for the passive support of M1; small changes up or down from the nominal value will be made for active optics correction. Figure 2.9 shows a plot of the optical surface deformation of M1 at zenith pointing on the 120 axial supports. The small “bumps” due to print-thru of each axial support pad will be smoothed off during the polishing of M1.

A similar optimization routine was carried out for the lateral support system. In this case M1 is at the position for a telescope zenith angle of 80º, oriented in a near vertical, slightly over-hanging configuration. The 24 lateral support points on the perimeter of M1 are equally spaced 15º apart Figure 2.9. M1 Optical Surface Deformation on Axial and symmetrical about the vertical axis as shown Supports. in Figure 2.8. This arrangement was chosen to allow adequate clearance between actuators. During the optimization process, the force level and vector orientation of each support was varied. However, to simplify and reduce cost, force levels were limited to a small number of values.

M1 Cell: The M1 cell is a steel structure, with a structural plate on the rear and a honeycomb rib structure attached to the rear plate (see Figure 2.10). The cell will support the axial and lateral support mechanisms and the air distribution system for the thermal control of M1, and interface to the OSS. The

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axial support mechanisms extend through holes in the rear plate of the cell and are serviceable from the rear of the cell.

The design of the M1 cell is proven, having been used on many existing nighttime telescopes such as ESO NTT, WIYN and Gemini.

Thermal Control System: The M1 receives the largest amount of solar heat load of any optic and also contributes the most to localized “seeing” that can degrade the quality of the solar image. The purpose of the M1 thermal control system is to remove the solar energy that is absorbed by M1 and to maintain the M1 optical surface temperature as close to ambient as possible. Both Figure 2.10. M1 Cell Configuration analysis and empirical observation has shown that maintaining the optical surface of M1 at or slightly below ambient temperature, combined with wind flushing of the surface will minimize local seeing to acceptable levels.

The M1 thermal control system is an array of air jets, or tubes, located behind the rear surface of M1 that direct conditioned air against the rear surface. Several hundred air jets are fed by a network of larger distribution tubes; the array of air jets and distribution tubes are divided into six zones with each zone fed by its own fan and liquid/air heat exchanger (see Figure 2.11).

Considerable analysis has been performed to determine the effectiveness of the thermal control system under different conditions. There is a significant time lag between when a change in cooling air temperature is applied to the backside of the mirror and when this change is seen on the front optical surface due to the thickness and relatively poor thermal conductivity of the M1 substrate. However, the diurnal change in temperature over the observation period is predictable and daily temperature models will be developed and used as base curves to drive the mirror temperature. This approach, combined with Figure 2.11. M1 Air Jet Distribution System wind flushing across the optical surface of M1, allows the temperature of the optical surface to be maintained within 1 to 3 ºC below the ambient air temperature.

Cleaning and Washing System: The M1 Assembly will have the capability of cleaning the M1 on a daily basis and in-situ washing of M1 on a periodic basis. A CO2 dispersal device will be attached to the M1 cover for cleaning of M1 at the beginning of each day (Figure 2.12). This will be done with the telescope in a near horizon pointing position; particulates and other matter removed from the surface of M1 during the cleaning operation will be collected by a vacuum trough and removed from the area.

During in-situ washing, the telescope is moved to a near horizon pointing position and a collection trough is positioned at the lower edge of the mirror to collect cleaning effluent (Figure 2.13). The optical surface

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of M1 is washed, rinsed and subsequently dried with an air knife that is mounted on the telescope mount next to the M1 cell.

Figure 2.13. Edge seal and collection trough for M1 Figure 2.12. CO2 dispersal as M1 cover opens. washing.

M1 Control System: The primary tasks of the M1 Control System are to control the application of active forces to M1 and to control the M1 thermal management system. The system accepts input mirror figure information at up to 10 Hz and blends and averages this figure information at up to 0.1 Hz. It also controls the temperature of the front side of M1 and the aperture stop to within a pre-determined range around ambient temperature. The system will also store and apply a 24-hour thermal profile estimation to be used in the thermal control of M1. The M1 Control System also provides status information at up to 10 Hz and interfaces to the TCS, GIS and OCS. Hexapod 2.3 HEAT STOP ASSEMBLY

The main purpose for arranging the ATST M2 optics in a Gregorian configuration is to reject energy at prime focus. This “heat stop” prevents unwanted light and heat from proceeding to subsequent optics. The Heat Stop Assembly (HSA) consists of a prime focus reflector assembly and all its supporting equipment, including coolant HSA loop, passive safety systems, plume removal system, and control systems (Figure 2.14). Figure 2.14. Prime focus area showing a cutaway of the Heat Stop Assembly (circled) in the context of the M2 Assembly at In many observing scenarios (on-disk and the top of the OSS. The beam path is yellow. off-disk coronal), the HSA is simply required to block the occulted field (OF) and pass the FOV. Observations very near the solar limb, however, require the prime focus occulter to quickly and actively track the solar limb.

2.3.1 Heat Stop Design Requirements The HSA provides five top-level functions. They are as follows: Block OF: The HSA blocks (reflects) solar disk light at prime focus over an area sufficient to allow off- pointing as much as 2.5 solar radii (SRD requirement, approximately 82 arcmin). Pass FOV: The HSA allows a 5 arcmin FOV to pass to M2.

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Track Solar Limb: During near limb coronal observations the HSA occults limb light while actively compensating for telescope shake and atmospheric seeing. It must permit coronal observations as close as 5 arcsec of the solar limb. Remove Irradiance Load: The HSA removes the prime focus irradiance load of up to 2.5 MW/m2 from the optical path. Minimize Self-Induced Seeing: The HSA introduces no more seeing than the error budgets allow. Experiments and scaling laws for small hot objects near M2 indicate insensitivity for seeing-limited observations (e.g., Beckers and Melnick, 1994, and Zago, 1997). A reasonable bottom line requirement is that surface temperature must be kept within some 10 ˚C of the ambient air temperature. In addition to the five top-level requirements, there are a number of second-level requirements such as easy periodic removal of major subsystems for servicing and replacement, and safety systems to protect personnel and the telescope from damage.

The complete specifications and design for the HSA are outlined in the Heat Stop Design Requirements Document (ATST Document #SPEC-0003).

2.3.2 Heat Stop Design Description The HSA consists of a reflector assembly, an active occulter insert, a coolant loop, a plume control system, a beam dump, and control and interlock systems.

Reflector Assembly: The Reflector Assembly is the heart of the HSA (Figure 2.15). The assembly is designed to remove high heat flux with minimum temperature rise in a compact package. The reflector plate lies nearest M1 and sees the solar image at prime focus, nearly 2.5 MW/m2 irradiance at noon. The reflector is tilted 19.5˚ from the prime focal plane and directs the OF out Figure 2.15. Section view of Reflector Assembly, Insert, Plume Control System, and Safety Shield. of the optical path. The reflector is made of a highly conductive, high strength alloy of copper, coated with AlMgF2 to provide high reflectivity. The reflector is cooled from behind by an array of liquid jets.

Outside the reflector assembly lies the safety shield, a ceramic ring mounted on the M1 side of the HSA to provide passive irradiance protection in the event of a tracking or power failure.

Active Occulter Insert: The Active Occulter Insert (AOI) is a small conical device that fits into the conical interior of the Reflector Assembly (Figure 2.16). The AOI provides a solar limb shaped occulting edge in the center of the FOV for observations very near the limb (as close as 5 arcsec). The AOI rotates to follow the solar limb and senses and tracks the solar limb at rates Figure 2.16. The Active Occulter Insert

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of several tens of Hz. The poor quality of the prime focus image away from the center of the field of view will require that the radius of the occulting edge be somewhat greater than that of the sun’s image.

Plume Control System: The Plume Control System (PCS) creates a flow of air across the Reflector Plate that sweeps away buoyant flows. The PCS consists of a blower, a getter, supply fans, and ductwork.

Coolant Loop: The Coolant Loop supplies the Reflector Assembly with coolant of the proper temperature, pressure, and flow rate. The Coolant Loop consists of a heat exchanger that transfers energy to the primary coolant supplied by System Services, a secondary coolant that is both highly effective at transferring heat and compatible with optics and mirror coatings, pumps that circulate the coolant, an accumulator that stores a sufficient volume of coolant to supply the HSA during emergency conditions, pressure relief valves, and an array of functional, safety, and diagnostic sensors.

Beam Dump: The Beam Dump absorbs the irradiance reflected from the Reflector Plate. The irradiance at the Beam Dump is spread out over a large area cooled by liquid from System Services.

Control and Interlock Systems: The Heat Stop Control System (HSCS) is responsible for the control and coordination of the HSA, including the pump speed, system pressure, coolant temperature, and the sundry sensors. The HSCS provides all software interfaces for these components to the TCS, OCS, and GIS. The HSA Local Interlock Controller provides an independent safety override.

2.4 M2 ASSEMBLY The M2 Assembly (Figure 2.17) contains the 62 cm diameter off-axis mirror (M2) that is the second element in a chain of optics that collects and focuses the solar energy into high-resolution images. The assembly consists of M2, the M2 positioning system composed of a hexapod and fast tip-tilt mechanism, the M2 thermal control system and the M2 control system. The M2 positioning system defines the position of M2 and maintains its position under the operating conditions of changing gravity load, thermal conditions and wind loading. The M2 positioning system also provides fast tip-tilt motion to compensate for some aspects of atmospheric seeing.

2.4.1 M2 Design Requirements The M2 is sized at 62 cm diameter to yield a 5-arcmin unvignetted field after the necessary increase in diameter Figure 2.17. M2 Assembly. to eliminate outer edge effects. The optical quality of M2 is critical to maintaining the required solar image quality; a surface figure of 32 nm rms must be maintained over the operational limits of 0° to 80° zenith angle (changing gravity vector), thermal conditions and wind loading. A comprehensive Design Requirements Document (ATST Document #SPEC-0008) has been developed to address these requirements.

Interface requirements for the M2 Assembly include interfaces to the OSS, the M2 lifter used to install and remove M2 from the assembly, the TCS and the required utility services.

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2.4.2 M2 Design Description The M2 is a 62-cm diameter structured mirror, approximately 75-mm in thickness at the center. It consists of a continuous facesheet with a triangular rib pattern on the backside (Figure 2.18). Bosses are provided at three areas to allow attachment of the mounting flexures. The optical configuration is a concave off-axis paraboloid with a focal length of 2081 mm.

The baseline material for M2 is silicon carbide. This choice was driven by the requirement for extremely low mass and high stiffness to achieve the desired fast tip-tilt motion; in addition, the excellent thermal conductivity of silicon carbide minimizes optical surface deformations under the solar load. Finite element thermal and structural analysis was employed to evaluate several potential materials and it was shown that silicon carbide has the best optical and thermal performance under the ATST operating Figure 2.18. M2 Configuration conditions. Figure 2.19 shows a global figure change of only 40 nm P-V for a silicon carbide M2 during the peak solar load.

There are many different processes for fabricating silicon carbide mirror substrates, but special attention has been given to CVD (chemical vapor deposition), reaction bonding and sintering since each of these processes has a demonstrated capability of producing a blank of the required size. ATST personnel have been in contact with major silicon carbide manufacturers during the design and development phase to determine the most feasible, lowest risk and cost effective methods of M2 blank fabrication.

Figure 2.19. Global figure change for SiC substrate during peak solar load - 40 nm P-V

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Positioning System: The function of the M2 positioning system is to support the weight of M2 and define its position and orientation over the operational zenith angles and thermal environments of the telescope. In addition, the positioning system provides fast tip-tilt motion of M2.

The M2 positioning system is composed of a commercial off-the-shelf hexapod, a fast tip-tilt mechanism and a three-point flexure support that connects M2 to the fast tip-tilt mechanism. The hexapod provides six degree of freedom movement to allow x-y positioning, focus, tip-tilt and rotational orientation of M2 with respect to M1. The fast tip-tilt mechanism provides rigid body motion of M2 at a rate of up to 10 Hz and an amplitude of 5 arcsec to counteract certain types of atmospheric seeing.

Thermal Control System: Similar to M1, M2 receives a significant amount of solar heat load and also contributes to the localized “seeing” that can degrade the quality of the solar image. The purpose of the M2 thermal control system is to remove this solar energy that is absorbed by the optical surface of M2 and to maintain the M2 optical surface temperature as close to ambient as possible.

The M2 thermal control system is an array of air jets, or tubes located behind the rear of M2 that direct conditioned air into each triangular pocket of the structured mirror (Figure 2.20). Approximately 140 air jets are utilized, fed by a manifold system that provides conditioned air from a fan and liquid/air heat exchanger.

Ancillary Equipment: The ancillary equipment consists of the necessary utilities services for the M2 assembly. This includes the conditioned coolant for the M2 thermal control system.

Control System: The M2 Control System monitors and controls the M2 positioning system, the M2 thermal system and the M2 fast tip-tilt system. Control of the positioning system involves taking wavefront correction input and making the necessary changes in the position of M2 by moving the appropriate actuators on the hexapod. Look-up tables will also be utilized to compensate for slow and predictable changes in the position of M2 with respect to M1 due to deflections in the optical support structure over changing zenith angles. Figure 2.20. Thermal control air jet nozzles directed toward 2.5 FEED OPTICS back of M2. A train of smaller reflective optical components, both flat and powered, are used to transfer the solar beam from the Gregorian focus of the moving telescope assembly to the stationary observing floor, then down to the coudé observing rooms. Their specific functions were discussed above in Section 1.2.1, and illustrated in Figure 1.4. Their optical properties are summarized in Table 2.1.

As with M1 and M2, all of these mirrors receive significant solar heat loads, and active cooling and thermal control will be necessary at various levels to maintain their optical surface temperatures at or near

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ambient. Mirror substrates will be silicon carbide or zero-expansion glass material depending on heat load and sensitivity to surface-figure change. The deformable mirror (M5) and the fast tip tilt mirror (M6) are discussed in more detail in the adaptive optics discussion in Chapter 4. Table 2.1. Feed Optics Optical Properties. Mirror Function Diameter Radius of curvature Conic constant M3 Fold Mirror 158 mm – – M4 Collimator 350 mm -5616.72 -0.996925 M5 Deformable Mirror 221 mm – – M6 Fast tip-tilt 340 mm – – M7 Beam Reduction 814 mm -20000 mm -1 M8 Fold Mirror 434 mm – – M9 Beam Reduction 270 mm -10000 mm -1

2.6 BAFFLES AND STOPS Baffles and stops are used at various places throughout the optical train to block stray light. These components are most critical during coronal observations at the Gregorian Station when stray light from the disk of the sun has the potential to degrade coronal observations made at small limb distances. The science requirement calls for less than 25×10-6 (relative to the on-disk irradiance) at 1.1 solar radii (1.6 arcmin from the limb of the sun).

During the design and development phase of the ATST project, several detailed stray-light analyses were performed to determine sources of stray light. It was found that the off-axis design tends to reduce the number of secondary stray-light paths in the optical system. There will be a Lyot stop placed at the first pupil image located approximately 1.3 meters below the secondary mirror. By slightly under-sizing this stop, light diffracting at the entrance aperture can be greatly reduced prior to reaching the Gregorian focal plane without significantly reducing the throughput of the system. The telescope will also have a tubular baffle extending from a position just above the Gregorian focus toward the secondary mirror. This structure provides an analogous function to the lower “stove-pipe” baffle found in most Cassegrain telescopes.

2.7 SYSTEM INTERCONNECTS System Interconnects are sets of pipes, hoses, cables, and other means of conveying utilities (e.g., power, chilled fluids, vacuum), network services, and the system time base from their source to any subsystem that requires them. Most of the interconnections are routed through the altitude, azimuth, and coudé cable wraps, which were described in Section 2.1.2.

2.8 SYSTEM ALIGNMENT System alignment components are provided to facilitate initial rough alignment of the telescope, realignment after optical elements are removed and reinstalled as part of operational maintenance procedures, and realignment at other times as required.

Figure 2.21. Initial axis alignment.

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The off-axis optical system, while unusual, does not pose any insurmountable challenges for the alignment system. Initial mechanical alignment of optical component centers relative to mechanical rotation axes will be accomplished with targets and small alignment telescopes (Figure 2.21 above). This will facilitate initial system alignment at all orientations of altitude, azimuth, and coudé rotation angles to within about 1 mm. Similar methods will also be used to co-align pupils, removing shifts as a function of pointing position.

The next alignment steps will be accomplished looking at point-source (stellar) images in the various focal planes. Gross M1 deformations and alignment errors producing coma and other low-order aberrations apparent in the point-spread function will be removed based on guidance provided by the optical alignment sensitivity analysis at various field points. Finally, wavefront analysis will be used while observing bright stars – again depending on the sensitivity analysis for guidance – to complete alignment of the telescope mirrors and initial calibration of the active optics system.

2.9 ACQUISITION AND GUIDING The Acquisition and Guiding system provides a full-disk image of the sun that can be used by the observer to select and acquire solar features. It is a small auxiliary telescope mounted to the M1 mirror- cell structure that will yield one-arcsec resolution images through a relatively narrow-band H-alpha filter. Other alternative filters (such as one centered on the calcium K-line) may also be provided.

In addition to the acquisition function, this system also furnishes the most basic level of guiding feedback during observations (limb guiding) to the tracking system, and includes an offset stage with sufficient travel to allow limb guiding during off-disk coronal observations.

3. WAVEFRONT CORRECTION

The Wavefront Correction system includes the Adaptive Optics system and the wavefront-sensing elements of the active optics system.

3.1 ADAPTIVE OPTICS Adaptive optics (AO) is a critical technology that is essential in achieving the science goals of the ATST. AO will enable diffraction-limited imaging to resolve the fundamental scales in spectroscopic and polarimetric observations of solar fine structure, which generally require long exposures. Compared to nighttime AO, solar AO faces a number of different challenges, and solar AO systems are in some aspects technically more challenging than nighttime AO. The main challenges are the inferior daytime seeing, the fact that solar astronomers mostly observe at visible wavelengths – although infrared observations are becoming increasingly important – and the solar wavefront sensor that has to work on low-contrast, extended, time-varying objects such as solar granulation.

3.1.1 Adaptive Optics Design Requirements The requirements for diffraction-limited observations are discussed in detail in the SRD, and listed in above in Section 1.1.4. In summary the requirements are:

The ATST shall provide diffraction-limited observations (at the detector plane) with high Strehl (S > 0.6 (goal S > 0.7) during good seeing conditions (r0 (500 nm) > 15 cm); S > 0.3 during median seeing – r0 (500 nm) = 10 cm – at visible and infrared wavelengths.

1. The wavefront sensor must be able to lock on granulation and other solar structure, such as pores and umbral and penumbral structure.

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2. Time sequences of consistent image quality are required for achieving many of the science goals. Spectral or spatial scans often suffer from varying image quality during the scan. The AO system shall provide consistent image quality during varying seeing conditions (time scales of seconds) often encountered during the day-time 3. The AO system shall correct residual (not corrected by active optics) optical aberrations and self induced and atmospheric seeing to the performance levels specified in the SRD. Mirror seeing or internal seeing in general must be avoided and any “residual” local seeing components must be correctable by adaptive optics. 4. The AO system shall be robust enough to perform during transparency fluctuations typically encountered in thin cirrus clouds. The detailed AO systems parameters and specifications that flow from these requirements depend heavily on the site characteristics, such as median r0, range of temporal fluctuations of r0, Greenwood frequency, and isoplanatic patch size. The final decision on where ATST will be sited has not been made at this point. The site survey effort has resulted in a down-selection to three potential sites that meet the ATST requirements.

For the purpose of defining the baseline AO system we use the average of the median r0 values at two sites, measured at the telescope height. The average r0 is about 10 cm. The bandwidth requirements for the AO system were derived from subaperture tilt spectra measured at the Big Bear Solar Observatory (BBSO) site and the Sac Peak site using a wavefront sensor (WFS) with 10-cm subaperture. The power spectra were used to estimate the Greenwood frequencies over a range of seeing conditions. A detailed performance error budget analysis was performed to define the baseline AO system using the average median r0 of 10 cm and the average Greenwood frequency (~32 Hz). These design parameters will be adjusted once the ATST site has been selected and its relevant characteristics are known.

3.1.2 Adaptive Optics Design Description Building on a vast amount of experience with developing solar AO systems we describe in the following a “base- line” AO system for ATST. The system design is modeled very closely after the very successful “high- order AO system” operated at the Dunn Solar Telescope (DST). The system is based on a correlating Shack Hartmann wavefront sensor and uses a parallel processing approach using commercially available digital signal processors (DSPs). Figure 3.1 shows a functional block diagram of the ATST AO system. Figure 3.1. AO Block Diagram Optics

AO is integrated into the telescope optical train as noted above in Section 1.2.1. The advantage of this approach is the ability to easily feed all coudé lab instrumentation with an AO corrected beam. Locating the deformable mirror (DM) in this position achieves this goal in an efficient way, i.e., with a minimum number of reflections. The WFS will be located close to the instrument(s). A beam splitter is used to send <4% of the light to the WFS. The lenslet array is conjugated to the telescope entrance aperture (Figure 3.2).

Deformable Mirror: The deformable mirror (M5) will be a continuous phase sheet DM with 1313 actuators. The mirror is 221 mm in diameter and the actuators are on a 40×40 square grid. Each actuator will have a stroke of at least 5 µm. The actuators are located in the corners of the subapertures (Fried

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Figure 3.2. Optical Layout

geometry) yielding a total of 1232 useful subapertures. One of the leading DM manufacturers (Xinetics Inc.) considers the ATST DM a straightforward extension of their 941-actuator off-the-shelf DM system.

Thermal control—The solar irradiance imposed on the front surface of the DM is approximately 10 kW/m2. To maintain the DM front surface at acceptably cool temperatures (< 5 ˚C above ambient temperature), the stock DM structure will be modified to allow convective cooling of the faceplate. Coolant (air or helium) enters the rear of the DM and impinges normally on the faceplate backside before exiting the DM radially. In addition, the faceplate will be sealed to prevent coolant leaks, and a broadband high reflectivity coating is used on the faceplate. External to the DM, the coolant is recirculated through a heat exchanger that transfers thermal energy to System Services facility coolant (ethylene glycol or similar). We are currently working with Xinetics Inc. on developing detailed thermal models and designing the modifications necessary to the DM to include the thermal control system.

Tip-Tilt Mirror: The tip-tilt mirror (M6) will have a range ± 1 mrad in both tip and tilt. The mirror will be made from light-weighted material (SiC). The mirror is elliptical in shape with dimensions 328×270 mm. The controller will have position feed back from the mirror and a closed-loop bandwidth of at least 100 Hz.

Thermal control: The peak solar irradiance imposed on the front surface of the tip-tilt mirror is approximately 9 kW/m2. To maintain the front surface of M6 at acceptably cool temperatures (approximately 3 ˚C above ambient temperature), an air jet array cools the rear of the SiC substrate. A flexible boot encloses the air jet array and prevents the cooling air from passing into the optical beam. In addition, M6 is coated with a high reflectivity coating to reduce the absorbed solar heat flux. The M6 cooling air is recirculated through a heat exchanger that transfers thermal energy to System Interconnect facility coolant (ethylene glycol or similar).

Wavefront Sensor: The WFS is a correlating Shack-Hartmann WFS similar to the one successfully used for the DST and BBSO AO systems. The principle of the WFS is shown in Figure 3.3. The telescope

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aperture is sampled by an array of lenslets, which forms an array of images of the object (e.g., granulation, sunspots). Cross- correlations between subaperture-images and a selected subaperture-image, which serves as reference, are computed. These cross correlations are shown in Figure 3.3, upper right. By locating the maximum of the cross-correlation we determine the displacement of the images with respect to the reference, thereby measuring the local wavefront tilts. The pixel resolution in the SH-WFS images and the 2-D cross- correlation, respectively, is typically 0.5 arcsec. The FOV is about 10×10 arcsec. Image displacements are computed to Figure 3.3: Principle of correlating Shack-Hartmann wavefront subpixel precision by fitting a parabola to sensor the correlation peak using and interpolating between pixels. A tilt map is shown in the lower right corner of Figure 3.3. We use a modal reconstructor to derive the actuator drive signals.

Wavefront sensor camera: The extended field-of-view WFS uses between 16×16 and 20×20 pixels per subaperture image. This means that for the nominal 40 subapertures across the 4-meter aperture a WFS camera with about 800×800 pixels running at a rate of at least 2000 frames pre second is needed. A number of options exist at this point in time. The Vision Research Phantom V9.0 camera with streaming output is a 1600×1200 pixel camera but only 816×816 pixels will be read out and the camera is expected to run at rates of up to about 2,900 frames/second. A second option we are investigating is based on a custom sensor developed by the Advanced Sensor Group, Sarnoff Corporation (James Janesick). Based on a modification of an existing design the CMOS sensor would have 800×800 pixel and sixty-four (64) parallel read-out ports running at 20 MHz each. A camera would need to be developed around this sensor. We have extensive experience with this approach since the WFS camera for the high order AO system was custom developed around a CMOS sensor for the project in close collaboration with industry (BAJA Technologies). We note that the relatively high read noise of CMOS devices is not a problem for the solar WFS, which operates with broad-band light and photons are therefore plentiful.

Real-time Controller: The Correlator and Reconstructor will consist of 64 Analog Devices Tiger Sharc DSPs running at 500 MHz. The DSPs will be off-the-shelf cPCI boards, each with eight DSPs from Bittware, Inc. An earlier generation of these processor boards has been successfully used for both the low- and high-order AO systems operated at the DST and BBSO. In spite of the approximately ten-fold increase in subapertures compared to the high-order DST/BBSO AO systems the increase in size and complexity of the ATST AO processing unit is very moderate since there has been a more than ten-fold increase in processing power of the new generation of DSPs. The processing unit consists of 64 Tiger Sharc DSPs compared to the 40 DSPs (Hammerhead) used in the high order AO systems.

AO Control System: The AO Control system will reside in a single board PC in the cPCI chassis with the Reconstructor DSPs. Calibration routines will include: camera flat field, dark field, wavefront sensor correction, deformable mirror flattening, and mirror gain.

The correlation and reconstruction real time software running in the DSPs will be inherited from the low order and high order systems running successfully at the DST and Big Bear telescopes. The performance critical core of the software will be written in assembly language, as before, for minimal latency. The

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instruction set for the new generation DSPs is slightly different but the conversion effort is expected to be moderate. The code will be modified where differences in architecture will result in higher bandwidth. Using the existing working code and algorithms will significantly reduce the time and risk of completing the software.

We will attempt to use existing software or build on existing experience from nighttime AO systems in areas where solar AO and nighttime AO systems are similar, if not identical. For example, off-loading fixed aberrations from the AO to the active optics system of the telescope is such an area.

3.2 ACTIVE OPTICS The Active Optics (aO) system includes a dedicated wavefront sensor, wavefront-sensor camera, and the aO controller. It provides feedback to the M1 axial support system that allows the optical figure of M1 to be maintained to the desired level.

3.2.1 Active Optics Design Requirements As noted in the M1 design requirements (Section 2.2.1 above) a surface figure of 32 nm rms must be maintained over the operational limits of 0° to 80° zenith angle (changing gravity vector), thermal conditions and wind loading.

3.2.2 Active Optics Wavefront Sensor The drive signals for the active optics system will be derived in two ways:

The adaptive optics system delivers temporal averages of Zernike coefficients and off-loads the lowest order aberration modes to the active optics system, which decides what corrective elements to adjust (e.g., secondary, primary). The adaptive optics system at the DST, which serves as the prototype for the ATST AO system, already delivers time averaged Zernike coefficients. However, since the DST optics are not active only “slow” pointing errors are corrected in this case.

A separate aO wavefront sensor is needed for a) Gregorian focus, b) the seeing limited observations specified in the SRD, and c) to provide a tool for telescope alignment. We will implement a low order (24 subapertures) extended FOV correlating Shack-Hartmann wavefront sensor. The FOV will be on the order of 120 to 180 arcsec, allowing us to measure the wavefront at various positions in the FOV. This information will be used to identify the source of aberrations. For example, since we are dealing with an off-axis system the source of astigmatism could be de-center of the secondary as well as deformation of the primary. Figure 3.4. Results of an active optics experiment at the Dunn Solar Telescope. A correlating Shack-Hartmann The extended FOV correlating Shack- wavefront sensor locked on granulation and a deformable mirror were used to correct optical aberrations in the Hartmann wavefront sensor will be almost telescope. The contrast of the recorded granulation images identical to what has been implemented for the improved substantially with the active optics servo loop MCAO experiments we performed at the DST. turned on. A large format (2k×2k) camera will be used to

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image the array of 2-3 arcmin subfields. The sensor operates at a moderate frame rate of a few fps and is considered an off-the-shelf item. The resolution will be of order 0.5 arcsec/pixel. The WFS will allow interactive selection of several (up to 6) 30×30 arcsec subfields, which are then processed using cross- correlation as in the case of the adaptive optics WFS. Several hundred wavefront measurements will be averaged to derive the fixed aberrations. This can be easily achieved with a high-performance off-the- shelf workstation or PC since the time-scale involved here is minutes. We have already successfully tested such an active optics wavefront sensor system at the DST (see Figure 3.4 above).

4. INSTRUMENTATION

The description of ATST instrumentation is divided into two parts: the instrument lab facility and the focal-plane instrumentation.

4.1 INSTRUMENT LAB FACILITY The Instrument Lab Facility is a set of common components that directly support science instruments and observers. The dominant requirements that affect the design of the instrument lab facility are derived from the telescope requirements associated with resolution, polarization sensitivity and accuracy, flexibility, adaptability, and availability. These were discussed in Section 1.1.2.

4.1.1 Polarimetry Analysis and Calibration The Polarimetry Analysis and Calibration system is used both to modulate the beam for determining the polarization state of solar features, and to calibrate out polarization introduced by the telescope. Because polarimetry is nearly ubiquitous in observational solar physics, ATST provides polarimetry analysis and calibration at the facility level, rather than making it part of the requirement for each instrument. The science requirement for sensitivity (10-5 relative to intensity) and accuracy (5×10-4 relative to intensity) place strong constraints on this system, and ultimately dictate the methods and strategies that will be used to do polarimetry.

Polarimetry at the Gregorian Station: Ideally the polarization introduced by the telescope and focal- plane instruments should be kept under 1%. This goal is only met in the Gregorian focal plane and, strictly speaking, only in the coordinate frame of the alt-az telescope. This will be the observing station for the coronal module of the Near-IR Spectro-polarimeter (see description below in Section 4.2.3), and other instruments for which the spatial and spectral resolution requirements are relatively low.

Time-multiplexed polarization modulation and analysis will be used at all observing stations because it is versatile, and the issues are well understood. The initial ATST facility-level modulators will include piezo-elastic modulators (PEMs), ferroelectric liquid crystal (FeLC) modulators, and rotating retarders. These modulators, as well as calibration polarizers, will be mounted in the turret assembly, which consists of three large rotating wheels with up to five positions each. The turret assembly is located above the Gregorian station as shown in Figure 4.1. One of the wheels is in the Gregorian focal plane; the other two precede it.

Telescope polarization at the Gregorian station will be calculated to the required accuracy using a detailed Mueller-matrix model of the telescope that includes the properties of aluminum coatings and oxide overcoatings. This model will be tested and improved by actual measurements. Polarization introduced by the focal-plane instrument will be measured using calibration polarizers in the turret assembly.

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Seeing and tracking errors introduce small changes to the sun’s image that, when sampled too slowly, can be misinterpreted as polarization of the solar source. Several strategies will be implemented to reduce this effect. The first is dual beam, or spatial modulation. This will be implemented by using a polarizing beam splitter as a polarization analyzer, spatially separating the s and p polarization states. Both beams can be analyzed independently but simultaneously to derive the four polarization states in a way that is less sensitive to seeing effects. It has the added advantage of utilizing all of the light introduced onto the polarization analyzer.

Spatial modulation alone still has shortcomings.

The two beams will generally not pass through the Figure 4.1. Turret assembly. instrument along exactly the same optical path. Differential aberrations may then become important. Furthermore, spatial modulation requires that different detectors or detector areas sample the two beams, which makes the measurements susceptible to differential-gain effects. Whenever possible the ATST polarization strategy will also include rapid polarization modulation (>1 kHz) and charge- caching camera systems. These specialized cameras, dubbed C3Po for Charge Caching CMOS for Polarimetry, will be optimized for highly sensitive and precise differential imaging. Chopping between the four linearly independent polarization states can be performed at speeds in the kHz domain to provide virtually simultaneous images without the need to read out the array at kHz frame rates (Figure 4.2). All independent image planes are observed with the same physical pixel on the detector, which renders normalized differences between image planes insensitive to the gain of individual pixels. The C3Po detectors will have a 100% geometrical fill factor and a quantum efficiency approaching unity. The technology can be applied to silicon to cover the 200 to 1100 nm wavelength range, and to infrared-sensitive materials such as HgCdTe for the 1000 to over 10,000 nm wavelength range. Rockwell has expressed an interest in providing these hybrid detectors to ATST. Another possible alternative is AIM, an independent entity of AEG Infrarot-Module, GmbH. They are part of a proposal to the EU to develop a C3Po detector for landmine detection. These detectors are considered to be a modest and achievable extension of the detector Figure 4.2. Block diagram of a C3Po. technology built into the most recent upgrade to the successful Zurich imaging polarimeters (ZIMPOL II).

Polarimetry at the Coudé Station: To meet the science requirements for polarimetry of solar features on the sun’s disk ATST must provide large instruments, slow beams, diffraction-limited imaging, and adaptive optics to correct the wavefront. Instruments that support these observations will reside at the coudé stations. The oblique angles of the transfer optics below the Gregorian focus will introduce considerably more telescope polarization than is present at the Gregorian station. This dictates that modulation and analysis must be performed near the Gregorian station prior to these strongly polarizing telescope mirrors. This allows the same turret-assembly polarimetry components to be used for both Gregorian and coudé observations.

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It will not be practical to pass two orthogonally polarized beams through the adaptive optics system. Thus, the coudé instruments can perform spatial modulation only after transitioning through the telescope’s feed optics. The C3Po strategy described above will also be available at coudé. Based on experience with existing strongly polarizing telescopes, we expect that ATST will meet polarization science requirements at coudé as well.

The individual components provided as part of the ATST polarimetry analysis and calibration system are summarized in Table 4.1.

Table 4.1. Polarimetry Analysis and Calibration System Components. Component Location Achromatic Linear Polarizer Calibration Wheel, Turret Assembly Achromatic Retarder Calibration Wheel, Turret Assembly UV Linear Polarizer Calibration Wheel, Turret Assembly UV Retarder Calibration Wheel, Turret Assembly UV/Vis PEM Modulator Wheel, Turret Assembly Rotating Achromatic Retarder Modulator Wheel, Turret Assembly UV Rotating Retarder Modulator Wheel, Turret Assembly Visible FeLC Modulator Wheel, Turret Assembly IR FeLC Modulator Wheel, Turret Assembly Vis/IR LCVR Modulator Wheel, Turret Assembly Achromatic analyzer Gregorian Focus Wheel, Turret Assembly 8 Polarizing Beam Splitters Gregorian or coudé stations 8 Linear Polarizers Gregorian or coudé stations 5 FeLCs Gregorian or coudé stations

4.1.2 Gregorian Station The Gregorian Station includes the Gregorian optical bench, transfer and re-imaging optics, mounting fixtures, and connections to utilities provided via System Interconnects. This observing station is mounted to the optical support structure, and provides a nominal plate scale (before re-imaging) of 3.95 arcsec/mm at f/13. Hence, the required 3-arcmin field of view has a diameter of 46 mm, and the goal five-arcmin field has a diameter of 76 mm. This station is suitable for instrumentation that is relatively compact, has relaxed spatial and spectral resolution requirements, and can tolerate a changing gravity vector. The Gregorian station has the advantage of providing an image with the minimum number of reflections, all of which are near normal. Thus, the Gregorian station provides the best throughput and the lowest levels of scattered light and telescope polarization. It has the disadvantage (compared with the coudé stations) of no high-order wavefront correction, images that are not diffraction limited over the full field of view, a changing gravity vector while tracking the sun, and stricter instrument size and weight limitations.

4.1.3 Coudé Stations The Coudé Stations include the optical tables, imaging optics, atmospheric dispersion compensators, standardized mounting fixtures, camera systems, and a connection to utilities provided via System Interconnects at both the upper and lower coudé levels. The majority of facility instrumentation and visitor instruments will be operated at these two stations. They have the advantage of full wavefront correction provided by the upstream high-order AO system, diffraction-limited images, and two large horizontal platforms that can accommodate multiple, large instruments in a constant gravity location.

Five facility instruments will eventually be permanently installed at the coudé stations. These are the Visible Spectro-polarimeter, the coudé module of the Near IR Spectro-polarimeter, the Visible Tunable

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Filter, the Near IR Tunable Filter and the Thermal-IR Polarimeter and Spectrometer. These are discussed in Section 4.2.

To accommodate the science requirement for a flexible, adaptable facility that can be quickly configured for a variety of diverse experiments, and to accomplish this in a cost-effective manner, ATST has adopted a strategy that calls for a high level of standardization in our approach to instrumentation wherever possible. This will be particularly true for observations performed at the two coudé stations. For example, all facility scientific instruments will conform to a prescribed optical height above the horizontal optical tables. This will allow common mounts with focus and decenter motions for cameras, filters, and imaging optics that will be provided as part of the laboratory instrument facility. Visitor instruments will be strongly encouraged to adopt the same standards, thus giving them access to facility components whenever possible.

Solar observations generally utilize multiple cameras in the course of a single observation. This need is derived from the wavelength diversity requirements placed on the instrumentation, and the need to make efficient use of the available light. The necessary cameras will be available “off the shelf” as part of the ATST instrument lab facility. This will allow scientist to take maximum advantage of observational targets of opportunity. This strategy of using uniform camera and controller systems will also minimize the cost of developing multiple camera systems and the software that runs them.

The camera systems that will be provided as part of the initial instrument lab facility are listed in Table 4.2. These same systems are available for use at the Gregorian focus as well.

Table 4.2. Initial Camera Systems. Camera Type Format Readout Count For use with Fast CCD 1k×1k 100 Hz 5 Vis Spectro-polarimeter, Vis Tunable Filter Large CCD 4k×4k 5 Hz 7 Vis Spectro-polarimeter, Vis Tunable Filter, Broadband Imager, Slit-jaw viewer Large Hybrid IR 2k×2k 5 Hz 5 Vis Spectro-polarimeter, NIR Spectro- polarimeter, IR Tunable Filter Visible Light C3Po 2k×2k 25 Hz 5 Vis Spectro-polarimeter, Vis Tunable Filter Near IR C3Po 2k×2k 25 Hz 4 Vis Spectro-polarimeter, NIR Spectro- polarimeter, IR Tunable Filter

4.1.4 Instrument Lab Thermal Control The Instrument Lab Facility Thermal Control system maintains air and component temperatures within the bounds necessary to meet self-induced seeing error budget. The coudé lab will be held at a constant, uniform temperature while the telescope assembly above it will track the ambient outside air temperature. Over much of the year there will be a large volume of warm air beneath the much cooler air within the telescope enclosure, producing a thermal instability. Many of our science use cases require simultaneous observations spanning wavelength ranges from the visible to beyond the glass cutoff wavelength in the near infrared. This precludes using a window to separate the two environments. Instead, the coudé stations will be isolated via a laminar “air window.”

4.1.5 Instrument Control System The strategy outlined for modular instrumentation components places a large burden on the instrument control software. This is solved using the concept of a virtual instrument which is “assembled” both in a hardware and software sense as part of the observing setup procedure. The details of the virtual instrument and other important aspects of the instrument control system are discussed in the high-level controls chapter of this document (see Section 5.2.5).

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4.2 SCIENCE INSTRUMENTS The science instruments envisioned for ATST and their relative priorities (from the SRD) are listed in Table 4.3. The first four instruments will be built as part of the construction phase. The priority five and six instruments – and perhaps others – will be built early in the operations phase.

Table 4.3. Science Instruments. Priority Instrument Fore-Optics Dispersing System Detector System Visible-Light Broadband 1 Phase Diversity Interference Filters Visible Imager Visible Polarization Medium Dispersion 2 Visible Spectro-polarimeter Visible or special Analyzer Spectrograph Near-IR Spectro-polarimeter Near-IR Polarization Medium Dispersion 3 Near-IR (Disk and Corona) Analyzer Spectrograph 4 Visible Tunable Filter Polarization Analyzer Visible Tunable Filter Visible Near-IR Tunable 5 Near-IR Tunable Filter Polarization Analyzer Near-IR Filter Thermal-IR Polarimeter & Medium resolution, 6 Polarization Analyzer Thermal-IR Spectrometer cold grating Visible/near-IR high- Visible/Near-IR High- Visible and near- 7 dispersion Dispersion Spectrograph IR spectrograph Swiss Polarization Visible spectrograph ZIMPOL Detector UV-Polarimeter Contribution modulation system or narrow-band filter System

4.2.1 Visible Light Broadband Imager Design Requirements

The primary science requirement of the Visible Light Broadband Filter instrument (VLBI) is to obtain the highest possible spatial and temporal resolution image sequences from the ATST. The study of small- scale magnetoconvective processes both inside and outside of sunspots requires spatial resolutions on the order of 0.01 arcsec and temporal cadence values of 5 seconds or less. The spatial resolution requirement is near the projected ATST diffraction limit, thus the VLBI cannot significantly degrade the image quality delivered by the telescope. The VLBI must also be capable of making broadband filter images in a range of scientifically important visible spectral bands on a rapid cadence. This drives the design to use simple, high optical-fidelity, thin-film interference filters for spectral selection. Table 4.4 lists the baseline science requirements of the VLBI derived primarily from the SRD and secondarily from optomechanical design considerations.

Table 4.4. VLBI Instrument Requirements. A. Optical Requirements Value Goal Priority Source Notes 380-800 Spectral Range nm 330-1100 nm 1 SRD CN bandhead 388 nm Unvignetted circular Field-of-View 3 arcmin 5 arcmin 1 SRD diameter 0.02 Spatial Resolution arcsec 0.01 arcsec 1 SRD 330 nm Spectral Resolution 0.01 nm 0.01 nm 1 SRD Short exposure imaging Beam Speed F/20-F/45 Variable 2 SRD Multiple plate scales -2 -3 Scattered Light 10 I0 10 I0 2 SRD Sunspot umbral imaging -2 -2 Instrumental 10 I0 10 I0 3 SRD No polarimetric capabilities

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Polarization required Mounting Horizontal Multi-config 2 Coudé or Gregorian focus B. Interference Filter Requirements Value Goal Priority Source Notes λ/4 Φ25 Optical Quality mm λ/4 Φ100mm 1 A.3 0.01-0.1 Bandpass nm 1 A.1 Varies with spectral region Transmission 40% 60-70% 1 SRD Temporal resolution Out-band Blocking 10-4 UV/IR 10-5 UV/IR 1 No active thermal control Operating Temp. 17±3 ûC 17±10 ûC 1 Telescope ambient Thermal Stability ∆λ<0.01nm 1 No active thermal control Mounting 3—5 Parallelism arcmin < 2 arcmin 2 A.3 Fringe avoidance C. Mechanism Requirements Value Goal Priority Source Notes Filter wheel Speed 2 sec 1 sec 1 SRD Including settle time Focus Lens Speed 2 sec 1 sec 1 SRD Temporal cadence < 5 sec Camera Exposure <100 msec 10 msec 1 A.3 Freeze atmospheric seeing

Engineering requirements dictate that a fully functional subsystem of the VLBI be available at the ATST first light. Therefore a “First Light Imager” (FLI) subsystem of the VLBI is currently envisioned. This requirement for the FLI subsystem drives the overall VLBI design to a modular concept described in the following section.

Instrument Description

The baseline VLBI consists of an optical relay unit (collimating and camera lens systems), one or more rotating filter wheel mechanisms containing the broadband interference filters, and a focal plane camera mounting system. Each of these systems will be designed to be removable, replaceable, and/or reconfigurable within a horizontally mounted enclosure.

The optical relay unit consists of a field lens and stop at the nominal telescope focal plane followed by a collimating lens unit (CLU) which collimates the light prior to the interference filter stages. Following the filters, the camera lens unit focuses the image plane onto the camera system. For a given camera pixel size, no single focal ratio will provide a Nyquist sampled detector plane for all wavelengths in the VLBI spectral range. Therefore the VLBI optical relay system will provide a range of focal ratios from F/20 to F/45 in order to optimize a given detector’s sampling. The current concept uses a varifocal zoom lens to provide a continuously variable focal ratio that will accommodate any camera system and pixel size.

The VLBI filter wheel unit will consist of up to three rotating mechanisms containing four broadband interference filters each. The filter wheel mechanisms will be brushless DC motor driven with optical encoders for positional readout. Each filter will be a three- cavity thin-film interference filter with a nominal diameter of 10 cm. The large diameter is required to ensure a 3-arcmin field of view at the nominal ATST focal length. A list of VLBI spectral ranges, specified in the SRD, is shown in Table 4.5.

The availability of large-format (8k×8k), high quantum efficiency (QE ~90%), fast-readout (~1 sec) detectors is currently extremely limited. Thus it is not possible to design the VLBI for a particular camera that will meet the ATST spatial resolution, temporal cadence, and field-of-view science requirements. We circumvent this by incorporating a flexible detector mounting using a three-axis linear-motion stage with micron accuracy. Any given detector can thus be placed anywhere in the 8 to 10 cm diameter focal plane

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to cover the full field. In addition, the VLBI camera system incorporates two separate detector stages in a split beam configuration. This allows two cameras to be independently focused and defocused for “Phase Diversity” (PD) imaging, thus enabling instrumental aberration measurement as well as image restoration to the diffraction limit.

Table 4.5. Nominal VLBI Interference Filter Specifications Central Bandpass Filter Wavelength nm nm CN molecular band 388.3 0.1 Ca II H & K lines 393.3 & 396.8 0.01 CH molecular G-band 430.5 0.1 Blue continuum 450.4 0.05 Green continuum 555.0 0.05 H-alpha 656.3 0.01 Red continuum 668.4 0.05 TiO sunspot bands 705.7 0.1 Ca II chromospheric magnetic 854.2 0.01

For the FLI subsystem, we envision the VLBI camera stages using one or two standard, readily available 4k×4k CCD cameras, and a single filter wheel (or in the simplest cases, a single filter mounted directly to the camera system input beam). As the commissioning phase of the telescope progresses, the FLI system can be built into the larger VLBI system.

4.2.2 Visible Spectro-polarimeter Design Requirements

The Visible Spectro-Polarimeter (ViSP) is the instrument responsible for the spectral analysis of the visible solar light and its polarization state, recording the wavelength dependence of the full Stokes vector (I, Q, U, V) at each spatial point in the field of view.

In order to meet the science requirements, the ViSP must be able to

• Observe the small-scale magnetic elements (flux-tubes) in the solar photosphere with an angular resolution of at least 0.05 arcsec, or about 40 km. • Cover a large field of view of at least 3 arcmin. • Routinely attain a polarimetric precision of 10-4 times the continuum intensity. In addition to this requirement, it would be highly desirable to reach the 10-5 level at least in particular configurations. • Minimize seeing-induced cross talk. It should be small compared to the polarimetric precision quoted above. • Fully resolve spectral features, including those arising from hyperfine structure or magneto-optical effects. The spectral resolution should be at least 3.5 pm at a wavelength of 600 nm. • Observe at least three different spectral ranges in the visible simultaneously (wavelength diversity), in the range from 380 nm to 900 nm.

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• The ViSP should be able to operate simultaneously with the infrared spectro-polarimeter (NIRSP). A compensator for atmospheric differential refraction is needed in order to ensure that both instruments are observing the same field. Instrument Description

The basic optical layout of the ViSP appears in Figure 4.3. The instrument concept is based on modern spectro-polarimeters, with a slit that scans the field of view and a spectrograph that images the slit spectrum on a 2D detector at each scanning step. However, it incorporates significant technological advances coupled with an innovative design that are necessary to fulfill its stringent requirements, far beyond those of any other present instrument of its kind.

The light beam first passes through a retarder with a time-dependent retardance (modulator) and later through a polarization analyzer, which can be a linear polarizer or a polarizing beam-splitter depending on the operation mode (see below). Reflections of the light beam along its path in the telescope introduce significant instrumental polarization. In order to attain the polarimetric precision set forth by the science requirements, the modulator and the calibration optics must be placed at the Gregorian focus. At this location, the light beam has only undergone two reflections and the instrumental polarization can be controlled at the level required.

Figure 4.3. ViSP basic optical layout.

Unfortunately, it is not possible to fulfill the wavelength diversity requirement and the 10-5 precision goal simultaneously. In order to avoid the seeing-induced cross-talk, it is necessary to either run the modulator at nearly kHz frequencies or to split the beam just before the detector into two beams with opposite polarizations. In the first case (single-beam scheme), both the modulation and the analysis are done very rapidly at the Gregorian focus. Subsequent reflections in the optical path to the detector will not affect the measurements, since the beam has been analyzed already. This scheme, which permits highly accurate polarization measurements, involves the use of FeLC. These devices are not achromatic and need to be tuned to a specific wavelength. Therefore, it is not possible to meet the wavelength diversity requirement with this setup. A dual-beam scheme, on the other hand, does not need such a fast modulation. The relatively slow (tens of Hz) time modulation of two simultaneous images with opposite polarization is used to correct the undesirable effects of seeing, at least to first order. The modulator and analyzer (a polarizing beam-splitter) can be made achromatic over a broad range of wavelengths, which permits the simultaneous observation of several spectral domains. The dual-beam setup has the analyzer at the end of the optical path, right before the detector in the coudé focus. Multiple inclined reflections exist between the modulator and the analyzer, introducing spurious polarization. Imperfections in the calibration to

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correct such instrumental polarization and small residual cross talk from the seeing may compromise the measurements at a level of 10-4.

The ViSP will have three different operation modes: The single and dual beam modes described above (for polarimetry at 10-5 and wavelength diversity, respectively), and a “hybrid” mode that combines advantages from both schemes.

1. High Precision Polarimeter (HPP): The modulator is a FeLC and the analyzer is a linear polarizer, both at the Gregorian focus. This mode allows for 10-5 polarimetry. The ViSP needs to be tuned to a specific wavelength and cannot operate in combination with the NIRSP. HPP requires a charge- caching device as a detector. 2. Fast Achromatic Polarimeter (FAP): This is the hybrid mode. It uses a fast (~1 kHz) rotating wave plate as an achromatic modulator at the Gregorian focus. The analyzer is made of a FeLC combined with a linear polarizer, and it is located at the coudé focus immediately before the detector (one analyzer is required for each detector). Seeing-induced cross-talk is prevented by the fast modulation, but multiple reflections between the modulator and analyzer complicate the calibration. The FAP meets the wavelength diversity and 10-4 polarimetric precision requirements. FAP requires charge- caching detectors, and it works with the NIRSP. 3. Slow Achromatic Polarimeter (SAP): The modulator is an achromatic rotating wave plate at a frequency of ~10 Hz, located at the Gregorian focus. The analyzer is a polarizing beam-splitter before the detector. Residual seeing-induced cross talk and calibration errors limit the polarimetric precision to a few times 10-4. This mode is capable of wavelength diversity. Conventional CCDs can be used for detectors, and it works with the NIRSP. The ViSP design is contained in one plane, allowing easy access to the components for upgrades and adjustments. The slit width is adjustable to allow for various trade-offs between resolution and photon flux. A turntable contains several gratings that can be selected to meet the needs of the observing program.

4.2.3 Near-IR Spectro-polarimeter Design Requirements

The Near-IR Spectro-Polarimeter (NIRSP) is the instrument responsible for the spectral analysis of the near infrared solar light and its polarization state. As with the ViSP, this instrument will record the wavelength dependence of the full Stokes vector (I, Q, U, V) at each spatial point in the field of view. Infrared spectroscopy requirements are diverse because of the broad flux and spectral resolution conditions inherent to photospheric and coronal physical conditions. In order to meet these requirements we describe a modular NIRSP system that satisfies both the low flux, and lower angular and spectral resolution requirements of the corona, and the higher resolution (spectral and spatial) needs for observing the photosphere.

Our philosophy in designing the NIRSP has been to achieve ATST spectroscopic infrared science observing requirements with multi-use optical components (and designs) wherever possible. For example, coronal spectroscopy will be obtained almost exclusively from the Gregorian focus, ahead of the many reflections that bring light into the coudé instrument room. In many cases the spectral resolution for coronal observing is dictated by the few-million degree temperature coronal line profiles. In practice the necessary resolution is somewhat higher than what coronal line-widths dictate because we often need sufficient spectral resolution to separate K and F coronal and scattered-light photospheric spectral features (for example for calibration). This is achieved with our resolution 4×104 coronal spectrometer. Spatial resolution of even an arc second will provide revolutionary new information about the faint corona's

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magnetic field. Disk observations must be obtained after the image is corrected by ATST adaptive optics at the coudé focus. Thus we require resolution of at least 3×105 with diffraction limited spatial resolution here. As we illustrate below the full requirements can be achieved with common optical components, but with distinct F/6.6 Gregorian and F/40 coudé systems.

Instrument Description

Infrared detector technology is a significant driver for the NIRSP. While the final instrument design will be decided in a year or more, we feel that the most prudent NIRSP concept now should be based on the stable Rockwell detector HgCdTe technology. This proposal assumes Hawaii-2-style 2k×2k format detectors. Coronal science requirements also dictate the need to observe into the thermal IR in order, for example, to reach the important 3.9 µm SiIX emission line for magnetic diagnostics. With common camera elements we describe below a 1-5 µm range coronal and photospheric NIRSP system. This also allows photospheric thermal IR spectroscopy without significant budget or technical performance impact.

The NIRSP-C and NIRSP-G (coudé and Gregorian) optical configurations are scaled versions of a common reflecting Littrow design. These optics are described below in Table 4.6 and Figure 4.4. NIRSP- G is illustrated here with the ATST-supplied focal relay that yields an F/6.6 input beam. The largest optic is the collimator and camera 35 cm parabolic mirror. The grating is a standard R2 87 line/mm echelle, the same grating used for NIRSP-C. Table 4.6 lists the optical specifications for both configurations. Pixel binning is used to properly sample the larger NIRSP-G slit.

With slit choices described in Table 4.6 the diffractive NIRSP grating illumination is comparable to the geometric illumination. NIRSP-G pixels will be binned 6×6 to yield 0.85-arcsec spatial and 242 mÅ spectral resolution for coronal observing. This Littrow configuration minimizes off-axis angles from the parabolic mirror so that the geometrical performance is close to diffraction limited.

Table 4.6. NIRSP Coudé and Gregorian Optical Specifications NIRSP-G NIRSP-C Grating R2: 87 line/mm R2: 87 line/mm Grating size 400x200mm 400x100mm Collimator focal length 1.2m 3m Focal Ratio 6.6 40 Plate scale 0.127 mm/arcsec 0.776 mm/arcsec Slit Width 108µm 36µm Pixel scale (at 1 µm) 242 mÅ 32 mÅ Pixel scale (arcsec) 0.142" 0.046" Design/Diffraction limited FOV 290" 290" Hawaii-2 FOV 290" 47" Cold blocking filter BW 1% 1% System QE 5% 5% Grating emissivity 50% 50%

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Figure 4.4. Optical layout for the NIRSP-G configuration. NIRSP-C is identical but scaled in length by a factor of approximately 2.5.

While the optical configurations are similar, the mechanical structures are quite different. This results from the vastly different background conditions for coronal and disk observing, i.e. NIRSP-C is not intended for coronal observing conditions. The dashed lines in Figure 4.5 show the expected solar signal and background versus wavelength assuming an optical table-mounted warm spectrograph, but with a 1% bandwidth cold order-sorting and blocking filter in the IR camera Dewar module. The figure shows the mean photospheric flux versus the purely thermal background contribution. The background is much lower than the solar signal blueward of about 5.5 µm at which point the background dominates the photospheric Figure 4.5. Signal and background flux calculations for NIRSP-C flux. This shows that a warm NIRSP-C (solid lines) and NIRSP-G (dashed lines). Assuming the optical configuration of Table 1 the expected signal and background per will satisfy most requirements for high NIRSP resolution element is plotted versus wavelength. A warm resolution photospheric observations. spectrograph is adequate for disk observations at wavelengths shortward of about 5.5 µm (dashed lines) while a cold NIRSP-G Coronal observations with a warm NIRSP- configuration allows coronal observations shortward of about 4.5 µm. A warm NIRSP-G spectrograph does not satisfy coronal G configuration would be dominated by observing requirements. thermal background photons at all

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wavelengths longward of about 1.9 µm. By cooling the grating and spectrograph optics to approximately 85K the NIRSP-G will allow coronal spectroscopy up to a wavelength of at least 4.5 µm. This is illustrated in Figure 4.5, which shows the coronal signal and background for a warm and cooled NIRSP-G configuration as described in Table 4.6.

The lower spatial and spectral resolution required for coronal observations also permits a compact cooled spectrograph design. Our working concept is illustrated in Figure 4.6. The IfA group recently designed, built, and commissioned an even larger cooled IR echelle spectrograph for the AEOS telescope which is the basis for some of the schedule and cost estimates for the ATST version. This NIRSP-G design uses (not shown) six stepper-based mechanisms utilizing vacuum cryogenic feed-throughs. Additional manual external mechanical adjustments for M1 collimation and grating β are also provided. The design includes provisions for mounting a polarizing beam splitter behind the cold slit. The slit viewer and integral slit- viewer filter wheel and the final science detector can be removed from the NIRSP-G without warming the Dewar or breaking vacuum. The entire cylindrical volume of the NIRSP-G is supported at both ends by ATST-supplied circular bearings, which provide for instrument rotation as needed for the alt-az telescope configuration.

Figure 4.6. Mechanical concept illustrating NIRSP-G. This "transparent aluminum" schematic diagram illustrates the critical components of NIRSP-G. The overall Dewar length is 1.2 m and its mass is approximately 900 kg. Light enters from a slit wheel assembly on the right. Removable HgCdTe cameras are indicated on the top surface. Radiation shielding, cold straps and most mechanisms are not indicated.

The NIRSP-C configuration will be constructed on a conventional optical table in the ATST coudé space. The optical layout is identical to Figure 4.6 (except for scale). Infrared slit-viewer and final science cameras for NIRSP-C are identical to the NIRSP-G cameras. Of course all mechanism controls and their hardware and software interfaces will be common for all NIRSP components. NIRSP-C relies on the ATST coudé room rotation and does not use separate instrument rotation control.

4.2.4 Visible Tunable Filter Design Requirements

The Visible Tunable Filter will be used to obtain narrow spectral bandwidth observations over an extended area of the sun. This capability will provide us with rapid 3D-imaging spectrometry, Stokes spectro-polarimetry, and accurate surface photometry. It will also deliver spectroheliograms to measure

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Doppler velocity, transverse flows, provide a feature tracking capability, and generally permit the study of evolutionary changes of solar activity. Investigators will use it in conjunction with the AO system for high spatio-spectral imaging. The filter will operate at four spectral bandwidths. These four modes and their associated requirements are given in Table 4.7.

Table 4.7. Visible Tunable Filter Modes and Requirements. Filter Mode Observations Passband Field of View Typical Spectral Desired Peak FWHM (pm) (arcmin) Lines (nm) Transmission A. Narrow Spectro- 2.0 1 FeI: 524.70, 525.02, >50% Passband polarimetry using 525.06, 630.15, (Dual/Triple I,Q,U,V Stokes 630.28 Etalon fractional 629.87, 868.8 Configuration) parameters CaII: 863.5

3D Spectrometry FeI: 569.1,557.6, & 3D Tomography 684.27 & Flow Geometry CI:538.03 B. Medium Filter Vector 12.0 3 FeI: 525.02, 525.06, >60% Passband Magnetograms 630.15, 630.28 (Single or Dual CaII: 863.5 Etalon Configuration) Filtergrams CaII: 863.5 MgI : 517.2 C. Intermediate Dopplergrams 20-30 3 HI: 656.3 >70% Passband FeI: 543.45, 557.6, (Single Etalon 630.15 Configuration) High-Speed Imagery & Flares HI: 656.3 D. Broad Advective Flows- 100-1000 3 CN: 430.5 >80% Passband Transverse Flows (Interference CaI: 399.3 Blocking Filters Movies & Active CN: 430.5 only) Region Evolution Continuum 450.8

It will meet the minimum system requirements shown in Table 4.8.

Table 4.8. Minimum System Requirements Minimum Aperture 200 mm Spectral Range 450 to 750 nm Spectral Resolution 200,000 Minimum Peak Transmission 50% with blocking filters Maximum Ghost Transmission 10-4 Maximum Stray Light 10-3 Drift Stability < 0.1 pm per hr

Design Description

Our design for the Visible Tunable Filter is a triple etalon system based, in part, on the successful German TESOS system, shown in Figure 4.7. We have selected a multiple Fabry-Perot (FP) spectral filter for the following reasons:

1. It can provide the required spectral resolution for high-resolution spectral imaging, Stokes profile analysis and filter magnetograms (spectral resolution ~250,000 at 500 nm);

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2. It has the high etendue (light throughput) to obtain a sufficient number of spectral samples within appropriate solar oscillation periods, and the required magnetic sensitivity on the timescale that solar features change; 3. It is mechanically and optically simpler in design than a Lyot filter; 4. It provides the rapid tuning between wavelengths that is required for finding the line center and adjusting the wavelength setting for Doppler-induced shifts; Figure 4.7. The TESOS instrument at the German Vacuum 5. It is a single system capable of simple Tower Telescope (Tenerife, Spain). The design of the ATST spectroscopy, Stokes line profiles, and Visible Tunable Filter will use the experience gained from the filter magnetograms. The etalons can development of this successful instrument. accommodate large aperture (~200 mm) filter systems allowing extended field-of-views without spectral degradation.

A triple etalon system offers several additional advantages, including superior spectral purity and out-of- band rejection, excellent throughput, and wider bandpass blocking filters that will remain stable over timescales of years. It is also a conservative choice in terms of the finesse requirements on the individual etalons.

Table 4.9 shows the relevant parameters of the triple etalon system. FSR = Free spectral range, FWHM = full width at half maximum, F = finesse, R = reflectance, D = gap distance, and M = order. This configuration has the same gap ratios as the TESOS system, however the FWHM of our system is 2 pm compared to TESOS 3 pm FWHM. TESOS has finesses of 30-40, while ours are just above 50.

Table 4.9. Triple etalon system. Etalon System FSR (nm) FWHM (pm) F R D (µm) M 1 0.106 2.02 52.6 .94 1300 4952 2 0.172 3.27 52.6 .94 802 3055 3 0.242 4.59 52.6 .94 571 2175

Papers by Gary, Balasubramaniam, and Sigwarth (2003), and Gary and Balasubramaniam (2003) summarize the triple-etalon Fabry-Perot filter of choice for the ATST visible narrow-band filter and consider the overall instrument requirements. The ATST filter optical design employing a triple Fabry- Perot etalon system requires that each etalon have a flatness of ~λ/200 before coating, with the optical finesse 10-50.

The heritage for the use of etalons and multiple etalon systems in solar physics comes from observatories in both the United States and Europe, including the NSO/Sacramento Peak, NSO/Kitt Peak, the German Vacuum Tower Telescope, Big Bear Solar Observatory, and the High Altitude Observatory. The design of the ATST multiple etalon system relies on the existing experience and expertise from this and other experience.

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4.2.5 Additional Operations Phase Instrumentation Infrared Tunable Filter

The top-level design requirements for the Infrared Tunable Filter call for a wide passband range (at least including HeI10830Å, FeI15648.5Å, FeI15652.0 Å, etc), large field of view, and narrow passband for measurements of solar magnetic field at deeper layers in solar atmosphere. This filter system can be operated as a spectro-polarimeter, a filter-imaging magnetograph, or a high-resolution imager. The Design Requirements for the IR Tunable Filter are shown in Table 4.10. Table 4.10. IR Tunable Filter Design Requirements. Properties Requirements Comments Spectral Coverage 1.0~1.7µm (scanning) Resolving Power > 150,000 FOV 1~3 arcmin Bandpass 0.1Å@15650 Å Spatial Resolution < 0.1 arcsec AO – 0.09 arcsec at 1.6µm Operation Mode Narrow/Medium/Broad band Consideration of flexibility to serve different observation purposes. This shall be realized by taking in/out individual filter from the system. Aperture > 36 mm (Lyot), > 150 mm Compared to the currently designed similar filter (FPI) system at BBSO, ATST shall have better performance. Throughput > 40% (Lyot), > 80% (FPI) Instrumental Principles regarding filters. Polarizer will be a major drag in this design. Scattered Light < 10-3 Stability ~0.05 Å/hour (FPI & Lyot) Due to the properties of components of Lyot and FPI.

The preliminary design combines an interference pre-filter, a tunable Lyot filter and a single Fabry-Perot etalon.

Thermal Infrared Polarimeter and Spectrometer

The Thermal Infrared Polarimeter and Spectrometer (TIPS) will perform vector polarimetry and infrared spectroscopy of the sun’s atmosphere. It will cover the entire thermal-infrared from 5µm out to the long- wave telluric cut-off at 28 µm. The TIPS will study the magnetic structure, dynamics, chemistry, and physical state of sunspots, plages, flares, prominences, and quiet regions. In the polarimetry mode, vector magnetic fields will be measured using the Mg I emission lines at 12.3 µm. With these lines, the most magnetically sensitive in the solar spectrum, TIPS will measure the strength and three-dimensional configuration of fields in the upper photosphere. In the spectroscopy mode TIPS will record spectra at high spectral and spatial resolution.

The primary candidate instrument for the TIPS is a cryogenic grating spectrometer. The instrument will be placed at one of the coudé stations. A large echelle grating will provide the required high spectral resolution. One spatial dimension will be imaged along an input slit and the second spatial dimension will be mapped by stepping the slit across the field. The spectrometer will use a large format (1024×1024) As:Si detector array with operation optimized for rapid cadence.

5. HIGH LEVEL CONTROLS & SOFTWARE

The ATST software provides the means to control and coordinate observations performed with the telescope and instruments. Numerous types of software will be in use on ATST, ranging from the lowest

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level servo or logic controller to the highest-level queue and scheduling processes. Each of these software components fulfills some part of the science requirements for the ATST mission.

The ATST software system is designed to operate the ATST through all stages of observational detail, from science program submission into observation scheduling, on to instrument configuration and data collection, through data reduction and archiving, and finally to data retrieval. This chapter describes how the software performs each of these tasks and how the requirements for each task drive the design in a particular direction.

5.1 SOFTWARE DESIGN REQUIREMENTS The software requirements have been derived from several sources. First, the SRD defines a number of functional and performance requirements that may be traced through the whole ATST software design. Chief among these are the requirements for flexibility, adaptability, and availability. Second, additional requirements have been discovered at ATST workshops or design reviews. And third, the technical engineering requirements further constrain the software design in behavior.

The software design is based upon the scientific and operational requirements. These requirements may be categorized into four principal areas.

Instruments: The software system must operate all instruments through their complete range of functionality. Instruments must coordinate observations with other instruments and with other components of the ATST. The instrument configuration must be flexible and dynamic to support a variety of experiment setups. Future instruments should not be constrained in their design by ATST software.

Telescope: The telescope must be capable of software control for acquiring, tracking, guiding, and offsetting on and around the sun. The required accuracies for each function should be met by both the mechanical and software system. The telescope software must provide the science image to the requested location with the required image quality.

Observations: Observers must be able to operate the telescope and instruments in a variety of ways. Observations may be taken at the telescope or remotely, they may be performed in real-time or scheduled, and they may be synchronized with other local or remote observations.

Data Handling: The instruments may generate a large volume of data. The data must be transferred from the instruments to a permanent storage facility. The data must contain pertinent state information about the observation.

5.2 SOFTWARE DESIGN DESCRIPTION The four categories above are the major systems responsible for each aspect of operation of the software. The Observatory Control System (OCS) coordinates programs, experiments, and observations. The Data Handling System (DHS) manages the flow, storage, and processing of image data. The Telescope Control System (TCS) operates the mechanical and optical structures. The Instrument Control System (ICS) coordinates the instruments and their associated calibration systems. Together the principal systems provide a lifecycle structure within which an observing program may reside.

Communications: The principal systems need to communicate with each other to coordinate telescope, instrument, data, and observer activities. The communication channels are well defined and simple, since the majority of activities occur within each system and not between them. There are two types of communications activities, commands and events. Commands are synchronized activities between a

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client, who requests an operation, and a server, who performs the operation. An example of a command is a request to configure an instrument's mechanical assemblies. The requesting client, usually the OCS's instrument user interface, issues a configuration command to the instrument's server process. The command is parsed for accuracy and completeness, the readiness of the instrument is checked, and the result of the configuration operation is returned to the client. Note that the returned result indicates only that the configuration was accepted and the required movements or reconfigurations were begun. Command completion is returned though the event channel. By performing actions in this two-step process—called command-action—the server remains available to execute other commands while the last command is underway. This is extremely useful if the next command is an abort.

Observations: An observing program is the basis for all that goes on in the control system. The origin of the observing program is in the science proposal, where an observer selects one of the many possible operational scenarios. For instance, the observer may choose to use the ATST in its diffraction-limited mode. The observer further selects the instruments and targets. Finally, the operational strategy, or sequence of operations, is determined. All of this information is incorporated in an observing program. Sometimes an observing program may be a richly complex series of instructions and system interactions, possibly describing a synoptic, or regularly scheduled, observation. Sometimes it may be a simple set of operations to release telescope and instrument control to the observer for setup or serendipitous observations. Regardless of the complexity or lack thereof, the observing program encapsulates everything necessary to execute the planned observations.

5.2.1 Common Services The Common Services provide libraries, tools, and hardware required by all systems. The software elements include the communications library, container and component interfaces, persistent stores, and the configuration library. Common Services also include hardware for the time base, network equipment, and terminal servers. These terms and concepts are defined in the sections that follow.

5.2.2 Observatory Control System The OCS has the following responsibilities: • Management of system resources • Management of experiments, science programs, observations, and configurations • Coordination of the TCS, ICS, and the DHS • Management of ATST systems during coordinated observing with other observatories • Essential services for software operations • User interfaces for observatory operations

In general the OCS assumes managerial responsibilities for the ATST system and directs the activities of the remaining principal systems. Services that are central to the operation of ATST software are provided by the OCS. The OCS acts as the interface between users and the ATST systems during normal operation, allowing users to construct science programs and virtual instruments for use in an experiment, monitor and control the experiment, and obtain science data from the experiment.

The OCS also provides basic services to support system maintenance and general system engineering operations. This includes tools to examine system diagnostic information, handle alarm conditions, monitor safety systems, and perform routine engineering tasks.

Functional Organization of the OCS: The OCS can also be viewed as organized hierarchically into broad functional categories: application support, experiment support, and resource management. The top levels of these categories are shown in Figure 5.1.

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The application services provided by the OCS include the event, alarm, log, and persistent store services. The application framework includes APIs and libraries as well as a general framework for building and deploying ATST applications. The TCS, ICS and DHS (as well as the OCS itself) are resources that are managed by the OCS. The OCS provides for direct operator control of these resources as needed. However, the normal operational model is to allow experiments as much resource control as practical over the resources that are allocated to that experiment. Figure 5.1. Functional Categories of the OCS

Performing Experiments with the OCS: Experiments are the heart of ATST operations, and the control system is designed with this in mind. A laboratory-style environment provides flexible support to carry out experiments that are likely not understood or defined at the time the laboratory itself is designed. An experiment undertaken at the ATST requires a Virtual Instrument and a Science Program of Observations. The OCS interacts with the ICS to create and manage virtual instruments. Science program management is the sole responsibility of the OCS.

OCS Services: The OCS is responsible for the fundamental services provided to all ATST software. These services are best described by examining the information flows supported by these services.

Command/action/response directives: Direct control of one system component by another component is accomplished using the Command/Action/Response model pioneered by Gemini. In ATST, commands are implemented as state-change directives. To effect a change in behavior of a target component the controlling component describes the conditions necessary to accomplish the state-change by providing the target component with a set of attributes (name, value pairs) that characterize the difference between the existing state and the desired state. This set of attributes is a configuration. A configuration may be simple, consisting of only a few attributes, or it may be quite large with hundreds of attributes.

Connections: System components operating in a distributed environment must be able to locate those other components that they need to communicate with. In ATST, a connection service tracks the locations and status of all system components and provides a name service that is consulted when a connection needs to be established. If necessary, the connection service is capable of directing that a non- running component be started in order to satisfy a connection request.

Event Notifications: Status information is communicated throughout ATST using the event service. Events are messages that are broadcast from some source. System components that are interested in particular events must subscribe to the appropriate event channel. Events are published by name and contains sets of attributes as values. Subscription to events is also by name but wildcards may be used by a subscriber to receive classes of events through a single channel. The ATST event service is reliable and high-performance. Events from a given publisher are also delivered to subscribers in the same order in which they are published. All events are time stamped and identify their source – both the generating component and the configuration that was active in that component when the event was generated.

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Alarms: System alarms have the same structure and distribution properties as events but are functionally distinct. Alarms denote abnormal conditions that require operator intervention. Alarms are not considered an integral part of the ATST safety system, however. Ensuring safety is solely the responsibility of the Global Interlock System. Alarms are useful for monitoring safety status as well as other abnormal conditions and software systems may be implemented to refuse many commands when unchecked alarms exist. Alarms are tagged in the same manner as events.

Log Messages: Log messages are simple string messages that record system activity. As with events and alarms, log messages are transmitted using a publish/subscribe mechanism and are time stamped and source tagged. All log messages are categorized as one of debug, note, warning, and alarm (the alarm log message category is not isomorphic to system alarms – all system alarms are logged using an alarm log message but the handling of system alarms does not depend upon this logging). In addition, log messages in the debug category have an associated level, and debug messages are only published if their level is less than or equal to the current debug level of the originating component.

Persistent Stores Access: A great deal of information in ATST needs to be recorded for arbitrary periods that are independent of the lifetimes of specific system components. In addition, system components need access to initialization parameters on startup and reinitializations. Finally, information specific to an experiment (virtual instrument details, science programs, configurations, and science header data) is preserved. ATST uses various persistent stores for these types of information. System components have access to these stores either directly or through database proxy services.

Alarms and log messages are always recorded in persistent stores. Events are not normally recorded but a high-performance engineering archive is available for recording events upon demand. These persistent stores are searchable using a general query mechanism under program control.

Application Framework: The OCS also provides the application framework supporting consistent operation in a distributed environment. The ATST application framework is based on the Container/Component Model made popular by EJB (Enterprise Java Beans), Microsoft's .NET initiative, and CORBA's CCM and patterned after the model developed as part of the ALMA project (Figure 5.2). The OCS provides containers that wrap system components in a common environment providing uniform access to services. Component developers focus on

implementing the functionality required of each component and rely on access to a Figure 5.2. OCS-2: The classic container/Component Model container for basic services.

5.2.3 Data Handling System The DHS is responsible for: • Bulk data transport • Quick look channels • Data storage, retrieval, and distribution • Data reduction pipelines support

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The DHS manages the flow of scientific data collected by ATST instruments. The data reduction pipelines support is a potential upgrade that is supported by the initial DHS design.

Because of the performance requirements placed on the DHS, parts of its functionality are distributed across other system components. For example, instrument camera systems perform any data processing required to reduce data output to meet bandwidth restrictions imposed by the implementation of the bulk data transport. Similarly, instrument component developers are responsible for providing the processing steps required to convert raw quick-look data into meaningful quality-control information.

Bulk Data Transport: The role of the bulk data transport is to reliably transfer science data from scientific cameras sources to data store targets (Figure 5.3). Physically, the bulk data transport uses multiple data channels on a high-performance switched network. The use of a switched network allows for increased flexibility – data channels can be established between any data source and target.

Figure 5.3. DHS-1: DHS 'pipeline' showing distributed functionality The ATST scientific cameras are individually capable of generating large amounts of data quite rapidly. This is compounded by the fact that multiple experiments may run simultaneously, each using multiple cameras. The bulk data transport is to be implemented using the latest stable technology for high speed data transfers and operates using data channels that are physically distinct from other system communications. This ensures that system control and monitoring activities may continue unaffected by bulk data transport loads.

Quick Look Channels: ATST cameras can generate quick-look images and post them onto quick look data channel streams. The quick look facility is a publish/subscribe mechanism allowing applications to accept, process, and display quick look data from any source (Figure 5.4). This allows, for example, an operator’s GUI to display quick look data while a separate process performs automatic analysis of quick look data with feedback into the ATST image quality control system. At the same time, a third process may be sub sampling the same quick look data channel and recording selected images. The publish/ subscribe mechanism also simplifies display of quick look data at multiple stations in the observatory.

Data Storage, Retrieval and Distribution: ATST provides temporary storage for all scientific data products and permanent storage for calibration data products. The temporary storage acts as cache between the high- speed bulk data transport and Figure 5.4. DHS-2: Routing of Quick look data using publish/subscribe

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slower distribution media (DVD, tapes, removable hard drives, etc.). All science data, whether located in temporary or permanent storage, maintains associations via a relational database with the configurations, observations, and experiments that were involved in the creation of the data. Header information is also archived and associated with data products. Any header attribute may be used as a key to retrieve one or more data products from the store.

When an experiment completes, the data products and all related ancillary products associated with that experiment are retrieved from the store and made available on distribution media for that experiment’s investigators.

Support for Data Reduction Pipelines: Support for on-line data reduction processing is a future upgrade to the ATST system. The use of a switched network for bulk data transport and database access to all data products simplifies the process of integrating this upgrade into ATST.

5.2.4 Telescope Control System The TCS is the central coordination facility for the delivery of the solar image to the instrument. It is responsible for the precise pointing and tracking calculations necessary to observe the sun. The TCS does not itself operate any mechanical components; rather it delegates this responsibility to the various ATST telescope subsystems and manages them according to the observation requests. The TCS does interact with the other principal systems, most notably the OCS and ICS. Observation configurations generated by the OCS are sent to the TCS for proper telescope positioning and configuration. Coordinated events are returned by the TCS so the OCS (and associated observer) is informed about the telescope's status. If an instrument uses the telescope in any coordinated fashion, such as scanning or calibration, the ICS and TCS synchronize these activities through the TCS interface (Figure 5.5).

The TCS also manages the wavefront image reconstruction process. The high-speed adaptive optics corrections take place in the Adaptive Optics Control System (AOCS), a TCS subsystem. The TCS manages the state of the AO system, including the offload of accumulated errors to other subsystems.

The TCS delegates the high-speed control loops of the telescope components to its subsystems. Figure 5.5 shows the major TCS subsystems and their relationships. The scope of a subsystem's functionality is limited both by construction and control. Generally, if both the error detection and error correction of a simple control loop is handled locally, that loop is part of a subsystem. For instance, the M1 mirror assembly has a number of axial force actuators that detect applied forces and apply corrective forces to position the OCS actuator at the correct position. The force map required to figure the primary mirror is downloaded from the TCS to the cfgs

M1 Control System (M1CS), but the M1CS is responsible ICS cfgs TCS events for positioning and maintaining the actuators in their proper positions. The TCS coordinates the control loop by Interlock Trajectory downloading “set-points”; the M1CS provides the actual cfgs control loop function. Similar control methods are used for other subsystems like the AOCS and the Mount Control FOCS ECS MCS System (MCS). M1CS AOCS M2CS

The description of the TCS subsystems can be found in their Image quality data appropriate mechanical or optical system description. These systems are developed and delivered by the subsystem Global Interlocks vendor and follow an interface to the TCS. A brief Figure 5.5. Telescope Control System. description of how they interact with the TCS is given here.

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Enclosure Control System: The ECS operates the enclosure carousel and shutter drives to properly position the entrance aperture. The enclosure needs to be moved to an accuracy of better than one-half degree, and must avoid collisions with the telescope mount assembly due to the shape and dimensions of the enclosure volume. The TCS provides a stream of altitude and azimuth trajectory data to the ECS.

Mount Control System: The MCS operates the telescope altitude and azimuth drives to properly position the telescope mount assembly. It also controls position of the Gregorian and coudé rotators. The TCS provides trajectory information at 20 Hz to the MCS.

M1 Control System: The M1CS controls the axial support actuators used to shape the figure of the primary mirror. The TCS provides the default shape based upon the current position on the sky. Additional shape information from the active optics may be delivered by the TCS.

M2 Control System: The M2CS operates the tip-tilt-focus actuators on the secondary mirror. The TCS provides the default positions based upon the current position on the sky. Additional tip-tilt-focus information from the adaptive optics may be delivered by the TCS.

Feed Optics Control System: The FOCS controls the smaller mirrors delivering the image to the coudé or Gregorian instruments, and it controls the calibration equipment located at the Gregorian platform. The TCS provides the required commands needed to position the optical elements.

Adaptive Optics Control System: The AOCS controls the wavefront correction hardware, including the adaptive optics real-time controller and the active optics wavefront sensors. The TCS receives accumulated image correction data and distributes it to the appropriate subsystems.

Acquisition and Guiding Control System: The AGCS operates the external acquisition and guide telescope. Non-recurring errors detected by the AGCS are used by the TCS to both update the pointing model and feed back drive errors to the mount.

Pointing and Tracking: The TCS has the responsibility for target acquisition and tracking. This requires a number of simple yet important steps. First, the principal target for the ATST is the sun. This requires a calculation of the solar position and rate. The second step is the conversion to an appropriate coordinate system. Solar observations are usually carried out in either heliocentric or heliographic coordinate systems.

Closing the loop on the TCS pointing is achieved by guide error signals from the acquisition and guiding camera and from the adaptive optics system. Figure 5.6 shows the TCS servo loops required to track features on the sun. Although the AO system has a very good resolution of the positional error, the features tracked by the AO system may be moving with relation to the solar center. The AO signal is useful the TCS when the tracking of the telescope is to be unlocked from the solar disk, thus allowing the telescope to follow this moving feature.

Figure 5.6. The TCS servo loops required for tracking and guiding.

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Active and Adaptive Optics: The TCS is responsible for controlling the image quality parameters of the telescope optics. The adaptive and active optics control systems provide residual wavefront error data in the form of Zernike coefficients that is propagated to other telescope subsystems. Figure 5.7 shows the schematic flow of Zernike coefficients from the adaptive optics system to the other TCS subsystems. Tip- tilt-focus offloads at about 100 Hz are sent to the M2 mirror controller for removal of moderate bandwidth errors (on the order of 10 Hz) such as wind shake. Any accumulated M2 bias needs to be removed to prevent pupil wander; this error is further offloaded to the mount controller at about 1 Hz.

M2 Tip-tilt bias offload ~0.01 Hz TTF bias offload ~10 Hz

Low-order figure offload ~0.1 Hz aO/AOCS Adaptive Optics/ Active Optics Active Optics 10 Hz Control System ~2 KHz

WFS Deformable M5 Mirror Mount M1 ~2 KHz M6 Tip-Tilt Mirror ~2 KHz Adaptive Optics ~2 KHz

WFS

Figure 5.7. Adaptive optics control loops controlled by the TCS.

5.2.5 Instrument Control System The ICS is responsible for managing virtual instruments. The ICS provides the mechanisms for associating components into virtual instruments, determining the availability of components, and holding the representations of the virtual instruments. The ICS also maintains a set of active instruments: those virtual instruments that are physically realized and actively sharing the light beam. Thus the ICS enables multiple experiments to take place simultaneously.

The ICS assumes no active role during the carrying out of an experiment. The OCS directs control of a virtual instrument during an experiment.

Requirements: The ATST is required to provide the flexibility inherent in a laboratory environment. This is a key science requirement, and has a significant impact on the system design both in mechanical systems and in software. The DST at Sunspot, NM, is specifically mentioned as a model that well illustrates the desired flexibility. On the DST a series of optical benches on a protected rotating platform provide the principal support for observing. Scientists can construct instruments specific to their experimental needs from existing components. While a few instruments are “facility” and consist of a fixed set of components, even these instruments may be combined with other components using dichroics, beam splitters and slit-jaw reflections. The ATST mechanical systems provide a similar flexibility through optical benches on a two level rotating coudé platform in the telescope pier.

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The Experiment: Observers at ATST are interested in performing Experiments (Figure 5.8). A central tenant of the ATST control system model is that the system should be adapted to the requirements of the experiment. A laboratory environment provides flexible support to carry out experiments that are likely not understood or defined at the time the laboratory is designed. Consequently, experiments are a formal concept within the model.

Figure 5.8. Information flow in an Experiment

In ATST, an experiment includes a science program of Observations and a Virtual Instrument capable of performing those observations. (The experiment also ultimately includes Results but that aspect is not relevant to this discussion.) Observations contain sequences of operational steps describing the behavior of the instrument. Each operational step consists of a set of configuration parameters and a simple command describing a state change within the instrument, collectively referred to as a Configuration (Figure 5.9). This use of science programs is typical of modern observatory operations and matches similar functionality provided at SOLIS, Gemini, VLT, ALMA, and other observatories.

What differentiates the ATST approach from these other observatories is that, instead of adapting experiments to fit within the bounds imposed by instruments consisting of fixed components, the ATST observer can construct a virtual instrument from available components to meet the needs of the particular experiment. This provides a great deal of flexibility in the nature of experiments that can be performed at ATST.

The Virtual Instrument: Instruments consist

of one or more Components. Some Figure 5.9. A possible instrument configuration components may be purely mechanical with no associated software (e.g., a dichroic filter). Others may be purely software (a sequencer). Most, however, include both mechanical and software aspects (cameras, scanners, etc.). These last components are called Devices.

In a conventional instrument the set of components that comprise the instrument are fixed and permanently associated with each other. Nevertheless, there is some software that understands these associations. Thus the primary difference between a virtual instrument and a conventional instrument is merely that the associations within a virtual instrument are not fixed but rather managed by software. A subtle difference that is implemented in the ATST virtual instrument model is that telescope components

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can also be associated as part of a virtual instrument. For example, an experiment that needs to perform drift scanning across the solar disk can include the telescope mount as a component, while an experiment performing coronal observations is likely to include the occulter as a component.

Scientists assemble the requisite components and combine them into the virtual instrument. Virtual instruments are then named and saved for potential future use. Some virtual instruments are used so often that the physical component associations are also maintained. This would be the case for ATST facility instruments.

From a control perspective, once the component associations have been made the control of a virtual instrument is identical to that of a conventional instrument. This simplifies the integration of instrument operation within the otherwise conventional ATST control system.

Components are hierarchical and may be composed from other components. In particular, instruments themselves are components. Composition of instruments is a common feature of operation of the DST and is expected to be a key operational characteristic of ATST as well. With a few additional components several facility instruments may be associated into a new virtual instrument.

Some experiments require cooperation between ATST and off-site observatories (including off-planet). A virtual instrument can include special proxy components for coordination with off site facilities.

The Life Cycle of a Virtual Instrument: The first step when performing an experiment with ATST is to construct a virtual instrument (Figure 5.10). This can be done either by browsing and selecting an existing virtual instrument or constructing a new virtual instrument from a catalog of system components. The components are then configured as needed for this experiment. While many components can be configured by setting a few parameters, others – such as sequencing components – may take more effort to configure, depending on the requirements of the experiment. Once the virtual instrument is defined, it is registered with the ICS, which records the instrument.

A science program that uses that instrument can now be constructed and added to the experiment. Observations within the program may be controlled through sequences of configurations or interactively. In either case the observations are scheduled with the OCS for execution.

As the time for observing approaches, the scientist lays out the physical systems associated with the virtual instruments components and prepares for observing. The OCS also notifies the ICS that a particular virtual instrument is needed for an upcoming observation and the ICS confirms that it is available and enabled. When enabled, the virtual instrument assumes control over its components, and observations proceed according to a prescribed sequence.

Once the observations in the science program have been completed, the OCS notifies the ICS that the virtual instrument is no longer needed. The ICS then deactivates the instrument. Once deactivated, the virtual instrument’s physical layout may be preserved for future use in other experiments or the instrument's physical systems may be made available for use in other virtual instruments. Figure 5.10. The lifecycle of a virtual instrument

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The Role of the OCS: The OCS acts as the interface between the scientist and the ATST control system as a whole. The OCS maintains the science programs and sequences the observations of experiments by providing configurations to the associated virtual instruments. The actual sequencing may be accomplished using scripts or through graphical user interfaces for interactive observing.

Implementation of the Virtual Instrument Model: Virtual instrument component software is constructed using a Container/Component Model as the basic framework. Containers provide access to services required by the components and are responsible for managing component life cycles. This allows component developers to concentrate on the functionality required in the component and provides a common implementation for standard features. This approach also enhances the distributability of components. Software only components can be instanced on arbitrary computer systems easily as long as a container is available to hold the component.

6. ENCLOSURE

The Enclosure is the large structure that surrounds and provides protection for the Telescope Assembly (Figure 6.1). It includes a variety of mechanical subassemblies, bogies, controllers, drives, and equipment that are used to point, track, and slew it in synch with the Telescope during science observations. The Enclosure is comprised of five major components: (1) the Enclosure Structure and Drive Systems; (2) the Utility Transfer System; (3) the Thermal Systems; (4) the Ancillary Mechanical Systems; and (5) the Enclosure Control System. These five items are described in detail, below.

Figure 6.1. The Enclosure.

6.1 ENCLOSURE DESIGN REQUIREMENTS The Enclosure serves a number of important roles and functions during science observations. Of these, three are considered to be the top-level, or most important to the performance of ATST. These top-level functional requirements are as follows:

Telescope Protection: The Enclosure provides protection for the telescope and optics under all weather conditions expected at the site, including both survival and operational conditions. For example, the Enclosure protects the telescope from wind-induced vibration and mirror buffeting during science observations, while still allowing adequate passive wind flushing throughout the dome.

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Pointing, Tracking, and Slewing: The Enclosure accurately and repeatably points, tracks and slews along with the telescope over its full required range of travel. The specifications for pointing, tracking, and slewing are based on the requirement to remain in synch with the telescope during observations.

Throughput, Thermal, Stray Light: The Enclosure provides for an unobstructed optical path from the sun to the telescope. It does this without imparting excess thermal energy (i.e., degrading seeing) or adding deleterious stray light into the science light paths.

In addition to these top-level requirements, there are a number of second-level functions that the Enclosure provides. For example, the Enclosure has a variety of safety systems and features included to protect personnel and the telescope from damage (e.g., failsafe brakes; GIS interface, etc.). The complete specifications and design for the Enclosure, including all the top-level and second-level requirements, are outlined in the Enclosure Design Requirements Document (ATST Document #SPEC-0010).

6.2 ENCLOSURE DESIGN DESCRIPTION The Enclosure is a hybrid design, incorporating the best features of traditional corotating and non- corotating enclosures. To minimize the exterior surface area exposed to direct sunlight, the Enclosure features steeply sloped sides on either side of the entrance aperture (Figure 6.2). This results in a close, form-fitting arrangement around the telescope, which requires the Enclosure to track along with the telescope during normal observations (i.e., c rotating). At certain discreet positions, however, such as zenith pointing, the Enclosure can remain stationary while the telescope is independently moved in azimuth (i.e., non-corotating) for maintenance and engineering operations.

Figure 6.2. Hybrid enclosure design. Carousel: The carousel is the large structure that forms the basic building envelope of the Enclosure. It rotates about an azimuth axis that is coincident with the telescope azimuth axis. The carousel design is based on modern dome construction methods: i.e., a large steel ring beam and dual arch girders, along with intermediate framing and supports.

The carousel structural framework is clad with a dual-skin type system that is comprised of steel outer panels, composite inner panels, and spray-on urethane insulation that is applied on the interior. Chilled air is circulated in the space between the outer and inner panels (see Thermal Systems, below). The exterior

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of the carousel is painted with a white TiO2-based paint for low insolation absorption and high emission properties.

Shutter: The shutter is the large assembly that rotates about a horizontal axis coincident with the telescope altitude axis. The shutter is an up-and-over one-piece design. It is constructed of a structural framework clad in a dual-skin system that is similar to the carousel.

Unlike the steel carousel, however, the shutter exterior skin is fabricated from aluminum. This reduces weight and increases thermal conductivity, which is important because the shutter receives the highest insolation density during normal science observations. The interior side of the aluminum exterior skin is also fitted with an array of welded fins that aid the thermal transfer to the chilled air system

Attached to the shutter is a three-meter long entrance aperture tube. This effectively raises the entrance aperture above the skin boundary layer. An eight-sided cover closes off the end of the tube when the telescope is not observing the sun (see Ancillary Mechanical Systems, below). The entire shutter assembly is driven in altitude by a traditional system of guide rollers and geared drives.

Azimuth Track and Bogies: The azimuth track is the large steel ring that is mounted to the top of the stationary enclosure base. The Enclosure carousel is supported on top of the azimuth track via an array of support bogies, or rollers. This type of system is employed by most modern observatory enclosures.

There are 24 bogies arrayed on the underside of the carousel, four of which are drive units. The remainder provides vertical support only. A series of lateral guide rollers provide lateral definition. The bogies are designed to minimize vibration transfer from the enclosure into the foundation. Seismic restraints are also incorporated at the azimuth track interface.

Enclosure Drive System: The enclosure drive system is the assembly of bogie drives, encoders, and controllers that allow the carousel and shutter to move during telescope operations. The design of the enclosure drive system is dictated primarily by the requirement to stay in synch with the telescope during observations:

• Blind Pointing. Blind pointing within 20 arcmin. This figure is considerably less stringent than the telescope-pointing requirement. This is due to the slight over-size of the entrance aperture relative to primary mirror size, i.e., the enclosure does not have to track as precisely as the telescope; it has to track only good enough to keep sunlight off the telescope mount assembly; • Tracking Rates. Tracking rate is nominally solar rate; • Slew Speed. Slew speed is up to 3 degrees/sec; • Range of Motion. ±270 degrees in azimuth; 10-90-degrees in altitude. The carousel is driven in azimuth by four chain-driven bogie drive units. The shutter is driven in altitude by a dual chain drive system. The drive motors are commanded by industry-standard digital servo- controller/amplifiers. Position data are sent via the ECS (see below). Position feedback on both axes is via rotary encoders. Fail-safe brakes are mounted on both axes. Shock-absorbing over-travel stops on both axes limit the range of motion in altitude and azimuth.

Utility Transfer Systems: There are a number of electrical power cables, data and signal lines, and coolant supply and return hoses that must cross onto the two major rotating axes of the Enclosure. This functionality is supplied by the Utility Transfer System, or UTS. The UTS is comprised of two major subassemblies: (1) the azimuth cable wrap; and (2) and altitude cable drape. It also includes the cable

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ways, junction boxes, and other equipment used to support and distribute the utilities onto and throughout the Enclosure.

The azimuth cable wrap is a large, non-powered serpentine style wrap using COTS-type cable chain components. It is located underneath the observing floor, and is accessed via level 4. The wrap is over- sized by 50% to allow future upgrades and expansions. The allowable range of travel of the wrap is 270 degrees. The altitude cable drape is a standard, sliding type drape that extends and retracts as the shutter moves in altitude. Guides and hangers are used to keep hose stresses to a minimum. The range of travel of the drape is 10 to 90 degrees. The sunshade is also serviced via a smaller, identical-type drape (see Ancillary Mechanical Systems, below).

Thermal Systems: The ATST enclosure has a large surface area exposed to solar radiation. Left uncooled, the skin surface would rise to a temperature some tens of degrees above the ambient air temperature, producing rising bubbles of warm air and intolerably large thermal image degradation (seeing). Analyses show that the dome skin temperature must be kept slightly below the ambient air temperature to minimize seeing. Two features of the enclosure skin accomplish this.

The skin is painted with white oxide paint, which has both a small solar absorptivity (reducing the absorbed heat flux from the sun) and a large infrared emissivity (assisting radiative transfer to the cold sky). It is also chilled by recirculating cold air in a duct system immediately interior of the skin. Heat exchangers located in the lower portion of the carousel transfer skin heat to liquid coolant. Fans circulate air over the heat exchanger elements, through the enclosure skin, and back to the heat exchangers. The skin is divided into zones, each having its own heat exchanger system. The zones allow fine control over the spatial distribution of temperature in the dome skin.

A second source of enclosure seeing is a temperature difference between the dome interior air and the exterior air. Large amounts of flushing reduce this seeing source to acceptable levels. Passive flushing is provided by a number of independently controllable vent gates. Outside wind blows through the vent gates and over the telescope structure, removing thermal turbulence and assisting the thermal control of the mount and optics. When ambient conditions are excessively windy, the vent gates can be partially closed to throttle the interior wind speed. The enclosure is also fitted with an active ventilation system, used as a backup when exterior conditions prevent the operation of the passive system of gates. Figures 6.3 and 6.4 show the flows expected, based on computational fluid dynamics (CFD).

Ancillary Mechanical Systems

Sunshade: As the shutter assembly rotates up in altitude, a gap between the carousel arch girders is exposed. A deployable sunshade is used to close off this gap and keep sunlight from entering the enclosure, where it would warm the floor and result in localized seeing.

The sunshade is passively lifted by the shutter as it rises. The total range of travel for this sunshade is 0 to 45 degrees. The sunshade is constructed of a double-wall system, similar to that of the shutter. A chilled air system is used to actively cool the assembly.

Entrance Aperture Tube Cover: To seal off the enclosure from the outside elements during non- operational periods, a motorized cover is employed at the end of the entrance aperture tube. This cover provides complete protection in all survival weather conditions expected at the observatory site. The cover design is based on a traditional eight-sided petal arrangement, similar to that used on large telescope mirror covers (e.g., Mayall 4m telescope). The assembly can be opened or closed in any orientation of the shutter, however under normal operation, the unit is opened only at the lowest shutter angle (i.e., 10 degrees).

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Figure 6.3. Expected flows through enclosure. Figure 6.4. Expected flows through enclosure. Jib Cranes: Four deployable jib cranes are used inside the carousel for maintenance, operations, and engineering purposes, such as installing Gregorian instruments, servicing the telescope, installing baffles, etc. These cranes are mounted to the four inside “corners” of the carousel. During normal operations, the cranes are locked in their stowed positions, flush with the interior walls of the carousel. During use the cranes can be rotated out and used for lifting objects off the observing floor.

Sensor Arrays: The Enclosure is outfitted with an array of functional, safety, and diagnostic sensors that are distributed on and around the system. These sensors are used to continuously monitor the performance and health of the enclosure, and to provide feedback to the TCS, OCS, and GIS systems. The sensor arrays include (1) Thermal Sensors; (2) Air Flow Rate Sensors; and (3) Limit Switches.

Enclosure Control System: The ECS provides control for the enclosure and ancillary components. It operates the azimuth drives, shutter, sunshade, dome cranes, ventilation gates, and thermal management. It is controlled by the TCS for all operations except low-level engineering activities and safety interlock situations. The ECS is connected directly with the GIS to perform safety operations.

7. SITE INFRASTRUCTURE AND SUPPORT FACILITIES

Site Infrastructure is the compilation of technical requirements, specific site characteristics and a conceptual layout of the ATST facilities at each of the candidate sites. Support Facilities include the Lower Enclosure under the dome (Enclosure); the attached Support Building and certain equipment within these buildings including integral Facility Equipment; a mirror Cleaning and Coating Facility; and Handling Equipment. Each of these items is described in more detail below.

7.1 SITE SELECTION TECHNICAL DESCRIPTION AND IMPACT During the D&D phase the ATST tested six candidate sites. We determined that it would be feasible to build ATST at all six sites, but significant logistical and cost differences exist between them. These were brought to the attention of project management and the Site Selection Working Group, and became part of the information used to down-select to three sites. These are Big Bear Lake, CA; Haleakala, HI; and La Palma in the Spanish Canary Islands.

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7.1.1 Technical Site Requirements Accessibility - A range of vehicles, from standard passenger cars to large construction cranes and flatbed trucks must be able to reach the facility, both during construction and in long-term operation. This applies to the roads leading to the site as well as to the local access in the immediate vicinity of the telescope.

Dimensions – To accommodate the observatory structures a minimum horizontal area of approximately 200 ft. by 200 ft. is necessary, and topography that will allow the creation of a suitable platform. A site larger than the minimum would allow a more flexible site layout, and would also facilitate construction staging.

Structural Characteristics – The soil/rock of the site must have sufficient bearing capacity to support the loads imposed by the telescope pier and the building foundations while also allowing adequate isolation between the two. Stiffer natural substrates that increase the lowest resonant frequency of the telescope support system are considered very advantageous. The lateral force factors (seismic and wind) inherent to the site must be of a magnitude that can be safely designed for without prohibitively expensive structural measures.

Manageable permitting process – The environmental issues inherent to the site must be such that the construction of ATST would not likely be precluded based on the applicable environmental protection statutes. Any necessary construction permits issued by the regional authorities must also be obtainable.

Utility infrastructure – Sufficient electrical power, data/telephone connection, and domestic water/sewer service must be achievable at the site. Existing infrastructure that can be extended to ATST and a low cost connection to local utility company lines are considered very advantageous.

7.1.2 Conceptual Design of the Potential Sites Existing site characteristics, special logistical considerations, and a tentative layout for the ATST facility have been developed for each of the candidate sites. The position of the telescope and building as shown in the illustrations is speculative and will be refined for optimum seeing conditions at the final selected site.

Big Bear Lake:

This site is at Big Bear Solar Observatory in the mountains near San Bernardino, California. The existing observatory is on a peninsular causeway extending into Big Bear Lake. There is also an existing on-shore support compound with space for additional development. The New Jersey Institute of Technology operates BBSO, which is on property owned by CalTech and the Municipal Water District. The conceptual plan is to create a new telescope site by widening a section of the existing causeway, or by extending a new causeway off the shoreline (Figure 7.1). An on-shore component of an ATST facility at Big Bear would be likely as well, which could incorporate the Remote Operations Building and possibly some of the spaces programmed for the attached Support Building, both of which are described in later sections.

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Big Bear Solar Observatory 0 100' 200 feet 0 20 m 60 meters NORTH and Potential ATST Sites scale

se line existing lea approximate

space for potential new on-shore facilities approximate existing lease line existing observatory existing causeway

ATST Primary option widen exist. t e s causeway n u s

e ic t ls o s Alternate options r e for new causeway m m u and site s

Figure 7.1. Potential Big Bear site. Construction of ATST would require an Environmental Impact Report and an extensive array of permits from the lake–related authorities Big Bear Lake is a very popular recreational resource for the L.A. area and is surrounded by tourist related businesses and expensive vacation cabins. While the existing solar observatory is a popular landmark on the lake, a new one more than twice the size may meet with some resistance from homeowners and environmentalists in the area

Any of the options for creating a site out in the lake would require surrounding the worksite with a cofferdam, dewatering pumps to keep the site dry; and extensive dredging and excavation. These special requirements and some barge-mounted work would add about four months to the overall construction schedule. The seismic risk in this area is very high, which, combined with the complications of building on a lakebed, would result in higher than normal structural costs. There are existing utility services in the area that could be extended to serve ATST, although the electric, telephone/data and domestic water systems would require upgrading. Regional wage rates are somewhat higher in this area than the national average. There is a healthy competitive environment for construction contracting in the vicinity and extensive resources for materials and equipment within convenient driving distance.

Haleakala:

This site is at Haleakala Observatory on the island of Maui, within two hours of coastal cites and less than a one-hour drive from the observatory’s base lab facility. The entire compound, including a large Air Force telescope complex, is owned by the University of Hawaii and managed by the Institute for Astronomy (IfA). The potential site area identified for conceptual planning of ATST includes the existing Mees Telescope, although there appears to be sufficient space for ATST even if that facility remains (Figure 7.2). We may also collaborate with the Air Force in a planned new mirror-coating facility.

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road access main HO

test tower Mees Teles. ATST

Potential Site for ATST at Haleakala Observatory 0 100' 200 feet NORTH

0 20m 60 meters

scale

Figure 7.2. Potential Haleakala site. No local building permits are required for construction at HO, however, environmental permitting is a significant cost, schedule, and public relations factor. An Environmental Impact Statement and a Conservation District Use Permit would be required, which together would take 12 to 18 months and cost about $200K. These estimates rely on the applicability of recent biological surveys and other studies that were done to support the HO Long Range Development Plan. The nesting season of an endangered bird species will restrict the schedule for heavy excavation. The presence and visibility of a new large telescope on a spiritually significant mountain would likely be an issue during public review of the EIS.

Relatively little excavation would be required to create a suitable level platform for the ATST structure. The volcanic gravel and cinder on the site has inherently low bearing capacity, so the pier and building foundations would have to be wider than normal or extend down to more solid rock layers well below the surface. This factor along with the probable requirement to mix all concrete on site would increase the structural costs of the project. The utility infrastructure at HO, especially electrical power and data connection, is well-developed and reportedly has sufficient reserve capacity to serve ATST. The most significant cost consideration is the very high regional wage and price factor, which would result in construction project costs that are 40 to 50% higher than the national average. Long-term operational costs would also be affected by these higher wages and material prices.

La Palma:

This site is at the Roque de los Muchachos Observatory (ORM) on the Canary Island of La Palma. The observatory is on the northern rim of a caldera in the center of the island, about an hour away from coastal cities. The ORM property is owned by Spain and is home to the observatories of various European institutions and their common support facilities. It is operated by the Astrophysical Institute of the Canaries with headquarters on the island of Tenerife. The legal and operational procedures for providing tenant sites at ORM are well-established by a cooperative agreement instituted by the government of Spain. The primary potential site for ATST has ample dimensions, but somewhat inconvenient

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topography. There is much open space on the observatory compound and several alternative sites have been identified (Figure 7.3). The adjacent William Hershel Telescope has a 4 m mirror coating facility, which, if deemed suitable for ATST, offers an opportunity for a major cost savings to the project. ATST also could participate in a currently ongoing project to develop a sea-level observatory support facility on La Palma.

Figure 7.3. Potential La Palma site.

A building permit from the local municipal authority would be required as well as environmental approval from the National Park Board and another governmental agency. The cost of the building permit would be 4% of the civil contract value (~$200K) and the cost of the required environmental impact study is expected to be less than $50K. At the primary site there is a possible visibility issue from a scenic viewpoint in the adjacent National Park, but otherwise there are no significant environmental issues anticipated.

The methodology for establishing an adequate foundation is somewhat uncertain as the local bearing conditions depend on the unique subsurface volcanic rock formations. The seismic risk is historically low, with only small magnitude motion induced by remote volcanic activity. The utility installation costs would be relatively low as the infrastructure on ORM is well developed. The regional cost data for Spain, and specifically the Canary Islands, indicate overall construction costs that are marginally higher (~6%) than the U.S. national average. Utility charges and common use fees that are assessed to tenants at ORM are roughly equivalent to tenant charges at observatories in the continental U.S.

7.2 LOWER ENCLOSURE AND SUPPORT BUILDING These two building elements (identified in Figure 7.4) are spatially and structurally contiguous and have similar functional requirements. They are treated as a single entity for this analysis.

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Lifts for: - mirror & instruments - personnel

Support Building Lower Enclosure

interior mechanical spaces exterior equipment yard

Figure 7.4. Building design and layout. Structural Requirements: The structure of the Lower Enclosure and Support Building carries the weight of the roof and exterior walls; the interior floor loads of all levels; lateral seismic and wind loads; and the dynamic loading of the rotating enclosure above. The building structure must be isolated from the telescope pier to prevent unacceptable levels of vibration from reaching any critical optical elements. The foundations must be sufficient to safely transmit all these loads to the bearing capacity of the soil.

Thermal Requirements: The design of the Lower Enclosure and Support Building must minimize any contribution to thermal turbulence in critical optical paths. This will impact: the height of the support building and its proximity to the Enclosure; the appropriate location for heat generating mechanical equipment; the selection of exterior materials and finishes; the potential need for active cooling of exterior building surfaces; and the appropriate thermal separation of interior spaces. The CFD analysis that is being used to model the air flow performance of the enclosure will be extended to thermally optimize the design of the entire facility, once the site dependent variables of wind speed and direction, topography, and available site space are more definitively established.

Functional Space Requirements: Table 7.1 lists the anticipated building spaces and their dimensions. The location of these spaces in either the Support Building or the Lower Enclosure, as indicated, may change as the design evolves. The identified space requirements and how best to accommodate them are also somewhat dependent on final site selection.

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Table 7.1. Functional Space Requirements Space Description Area (net) Height ft2 m2 ft m Control Room 400 37 9 2.7 Computer Room 200 19 9 2.7 Instrument Prep Lab 600 56 10 3.1 Shop 500 47 10 3.1 High-bay Receiving/ Mirror Prep 800 74 20 6.1 Mirror Coating Area 600 56 20 6.1 in conceptual Site Manager's Office 120 11 8 2.4 Visiting Observer's Office 120 11 8 2.4

Shared Office Space (~4 people) 400 37 8 2.4 design Kitchen/Break Area 150 14 8 2.4 Restrooms (2) 100 9 8 2.4

Mech. Equip Spaces (upper & lower) 1,200 112 9 2.7 Building in Support Machine and Service Rooms 250 23 9 2.7 Platform Lift and Elevator (shafts) 460 43 65 19.8 located Exterior Mechanical Equip. Space 700 65 N/A N/A Basement Utility Area 1,500 140 8 2.4

Total (in Support Building) 8,100 753 Stationary Instrument Space (future) 800 74 9 2.7 Instrument Storage 400 37 9 2.7 Visitor Gallery 500 47 9 2.7 in Lower

Telescope Maint. & Utility Floor (Level 4) 2500 233 10 3.1 Enclosure Telescope./Dome Fixed Floor (Level 5) 1900 177 up to dome Base of Pier* 2,000 186 8 2.4 Lower Coudé Platform* 1750 163 11 3.4 Upper Coudé Platform* 1430 133 11 3.4

Total (in Lower Enclosure) 11,280 1,049

19,380 ft2 1,802 m2 Total Required Building Space (Net)

*While the requirements and design of the Coudé Stations and Pier are covered elsewhere, their area is included here to provide a more comprehensive total of the proposed building size.

The gross building area in the conceptual design (including corridors, stairs, wall thicknesses, etc.) is ~24,250 ft2 (2255 m2), ~ 25% larger than the net totals in the table.

Building Design: The building design and layout is depicted in Figures 7.5 and 7.6. The basic construction of the Lower Enclosure and Support Building will be a steel-framed structure on concrete foundations with standard metal panel roofing and siding. This is a conventional building system that is economical, flexible, and adapts well to a variety of site conditions and lateral load cases. The exterior finish will be determined by the best thermal performance (probably high-titanium white) with some

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consideration given to environmental impact issues if necessary. The interior build-out will be similar to standard office and light commercial construction.

Enclosure & Support Building Section ~105 ft. (32 m) ft. (32 ~105

Level 5 dome Level 4 catwalk

Level 3 ~46 ft. (14 m) platform lift platform Level 2 Upper Coude

Level 1 Lower Coude ground level Level 0 - Basement (TBD) cable wrap Depth of pier (TBD) Figure 7.5. Building design and layout.

82 ft. (25 m)

elev. a 7' x 6' office re common a mach. b break a office l rm. ry space room a w.c. jan. n o ti office ta s re tu n u ru f LOWER lity uti COUDÉ PLATFORM chiller platform lift instrument shop (19' x 19') lab monorail crane compressor max instrument envelope

70 m) ft. (21 exterior mechanical

s pump vac. t a crane bridge o t r io V n cable/utility tray i a a s ry e it ar o la g recieving mechanical r b in coating generator G g a a ta & mirror equip l re s e) facility le a & ov r e prep area space tank oil y ag ab or o st n t oil pump pe (o

0510 30 ft. north LEVEL 1 (LOWER COUDE PLATFORM) 0 5 10 m Ground Floor Figure 7.6. Building design and layout.

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The conceptual facility design presented here has been developed to be prototypical for any of the candidate sites. In future development it will be tailored to fit the specific conditions of the ultimately selected site.

7.3 FACILITY EQUIPMENT Facility equipment includes the outfitting and furnishing of the control room, shops, labs, offices and other ancillary spaces. It also includes the special observatory-related mechanical and electrical equipment that will be permanently installed in the facility to serve the utility needs of the telescope and instruments, and to address the extensive cooling requirements of ATST. This does not include normal building utility items such as lighting, domestic plumbing and general air conditioning, which are incorporated into the budget and design of the building itself.

The following is a list of the necessary special utility equipment identified to date: • Electrical Generator – Capacity of ~200 KVA. Voltage, fuel and anticipated duty cycle are site dependent. • Uninterruptible Power Supplies – Two units serving a total load of ~50 KVA. • Chillers – Two units of ~30 ton capacity with appropriate temperature range to serve the cooling requirements of optical components, the heat stop, the telescope mount, the enclosure, and other special heat sources. • Glycol (or other liquid coolant) – Supply and distribution piping as required. • Special Fans – For active ventilation of the telescope enclosure and adjacent spaces. • HEPA Filtration – Air handlers and filters as required for particulate control in the coudé labs, mirror coating area and possibly in specified stationary lab areas. • Cryogens – Supply, distribution and processing for liquid nitrogen, compressed helium, or other coolants as required for special instrument related systems. • Air compressor(s), vacuum pumps, and other equipment required for general utility use and special telescope-related applications Multiple areas are incorporated into the conceptual design of the facility for housing the special utility equipment, which will have varying requirements for proximity to the telescope and for the non- detrimental exhausting of waste heat.

7.4 MIRROR COATING AND CLEANING FACILITIES An appropriate area and the necessary equipment are included in the ATST facility for the handling, cleaning and recoating of the primary mirror and smaller reflective optics. This equipment includes the M1 assembly handling cart, M1 lifter and coating plant. The availability of an appropriate existing coating facility or the potential shared use and co-development of this facility with neighboring observatories is site dependent.

Description: The coating plant utilizes a sputtering system to deposit a coating of pure reflective aluminum. The coating plant itself is a large clam-shell stainless steel vacuum chamber used to apply the coating to the mirror surface. The coating plant assembly includes vacuum pumping systems, chilled water delivery, the magnetron assembly, and the vacuum tank itself. Additional associated equipment includes a drainage system and holding tank for stripping fluids, compressed air delivery, and gas cylinder racks.

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Operation: The upper cover of the chamber is lifted clear of the lower half on jacking screws so that the lower half can be moved on floor rails to the mirror stripping/cleaning area. After stripping, the mirror is lifted by a crane and held suspended while the mirror cart is moved out of the way and the lower half of the coating chamber moved under the mirror. The mirror is lowered onto a turntable support frame in the coating chamber lower half and the chamber is moved back under its cover. During the coating process the mirror must be positioned central to the axis of rotation of the turntable and be retained in that position. The front surface of the mirror must remain in a normal plane to the axis of rotation.

Additional Parameters: • The chamber interface to the coolant lines is by manually operated valves; there will be a flow requirement and outflow temperature requirement. • For the installation of the plant the mobile section of the vessel will use air bearings and drives to position the vessel within the coating plant chamber. Compressed air couplings normally used to support the mirror handling cart will provide compressed air to the chamber air skates when the chamber is installed in each observatory. Handling trolleys will be used for the maintenance and installation of the magnetron systems. • A rack for 8 gas cylinders will be required for the Argon supply used in coating the mirror.

7.5 HANDLING EQUIPMENT The most challenging requirement for material handling is transporting the primary mirror from the telescope to the coating facility and back, which may occur as often as every six months. A platform lift is provided for that purpose. The mirror in its cell and cart is approximately 5 m in diameter by 2 m high and weighs close to 15 tons. The capacity and dimensions of the lift are based on the size and weight of the mirror. The design of the lift also accommodates the maximum defined volume and weight for instruments that will be moved to and from the coudé stations.

For personnel and smaller equipment a standard building elevator is also provided that serves all five levels of the Lower Enclosure and the two levels of the Support Facility. For disassembling the primary mirror from its cell and for loading and unloading large instruments and other equipment, a bridge type crane with ~20 ton capacity is provided in the high-bay receiving and mirror prep area. A smaller capacity monorail crane is provided in the instrument lab. Appropriate hatches and removable flooring are designed into the upper levels of the Lower Enclosure to allow the jib cranes on the Enclosure (described elsewhere) to be used for material handling in that area. Appropriate additional equipment (portable scissors lifts, fork lifts, special purpose hoists, etc.) will be provided as well.

All handling equipment, especially the lifts and cranes that are integral to the structural design of the building, will have the highest affordable capacity and be configured for maximum flexibility as future requirements are difficult to predict.

7.6 REMOTE OPERATIONS BUILDING To augment the attached Support Building, there is an identified need for a facility that would serve ATST functions that do not require direct proximity to the observatory. This would allow for a smaller structure and less heat generation adjacent to the telescope. The appropriate nature and location of the Remote Operations Building is dependent on final site selection. The functional requirements for this facility would be mostly for administrative offices with some auxiliary lab/shop space geared toward long-term maintenance/storage of instruments and equipment. A high-speed data connection, for effective teleconferencing, for real time communication with the observatory, and for transmission of data to home institutions will be provided.

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