HiCIAO: The Subaru Telescope's New High-Contrast Coronographic Imager for Adaptive Optics

Klaus W. Hodappa, Ryuji Suzukib,c, Motohide Tamurab, Lyu Abeb, Hiroshi Sutob, Ryo Kandorib, Junichi Morinob, Tetsuo Nishimurac, Hideki Takamic, Olivier Guyonc, Shane Jacobsona, Vern Stahlbergera, Hubert Yamadaa, Richard Sheltona, Jun Hashimotod, Alexander Tavrovb, Jun Nishikawab, Nobuharu Ukitab, Hideyuki Izumiurab, Masahiko Hayashic, Tadashi Nakajimab, Toru Yamadab, and Tomonori Usudac

aInstitute for Astronomy, 640 North A'ohoku Place, Hilo, HI 96720, USA bNational Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo 181-8588, Japan cSubaru Telescope, 650 North A'ohoku Place, Hilo, HI 96720, USA dGUAS, Osawa, Mitaka, Tokyo 181-8588, Japan

ABSTRACT

The High-Contrast Coronographic Imager for Adaptive Optics (HiCIAO), is a coronographic simultaneous differential imager for the new 188-actuator AO system at the Subaru Telescope Nasmyth focus. It is designed primarily to search for faint companions, brown dwarves and young giant planets around nearby stars, but will also allow observations of disks around young stars and of emission line regions near other bright central sources. HiCIAO will work in conjunction with the new Subaru Telescope 188-actuator adaptive optics system. It is designed as a flexible, experimental instrument that will grow from the initial, simple coronographic system into more complex, innovative optics as these technologies become available. The main component of HiCIAO is an infrared camera optimized for spectral simultaneous differential imaging that uses a Teledyne 2.5 μm HAWAII-2RG detector array operated by a Sidecar ASIC. This paper reports on the assembly, testing, and “first light” observations at the Subaru Telescope.

Keywords: Optical design, infrared camera, coronograph

1. INTRODUCTION

The High-Contrast Coronographic Imager for Adaptive Optics (HiCIAO) is a new instrument 1 designed for the Subaru Telescope of the National Astronomical Observatory of Japan (NAOJ). The new HiCIAO will supercede the capabilities of the existing CIAO2 (Coronographic Imager for Adaptive Optics) on Subaru. HiCIAO will be used behind a new, high order, curvature sensing and bimorph mirror adaptive optics system 3 that is currently being built for the Subaru Telescope. This new adaptive optics system with its 188 actuators will achieve better Strehl ratios than the current generation of adaptive optics systems and will therefore allow higher contrast coronographic observations. HiCIAO is the coronographic system and simultaneous differential imaging infrared camera to be used with this new adaptive optics system. The HiCIAO project and its scientific goals have described in earlier papers1,4. HiCIAO is designed as an experimental system, as opposed to a facility instrument, and will be modified over the coming years to incorporate more advanced coronographic techniques and upgrades to its adaptive optics capabilities. The main goal of HiCIAO is the imaging of young exoplanets by using their methane absorption to distinguish them from residual speckles of the much brighter central star, using spectral simultaneous differential imaging (SSDI)5 techniques.

2. CORONOGRAPH AND CAMERA OPTICS DESIGN

The first-light configuration of HiCIAO consists of a classical Lyot coronograph and an infrared camera designed for a Teledyne 2040×2040 pixels HAWAII-2RG 2.5 μm cutoff HgCdTe detector array. As shown in Fig. 1, the coronographic foreoptics consist of a field lens with the occulting spot deposited on its flat front surface, a doublet collimator, and a doublet camera lens. The optics are deliberately kept as simple as possible to reduce multiple reflections and the resulting reduction in image contrast, while still achieving adequate chromatic correction.

The coronographic foreoptics require several motion stages to select one of several field masks, occulting spots, and beam-splitting elements. Since the instrument is designed for the Nasmyth focus of the Subaru Telescope, the pupil mask needs to rotate against the field. Finally, all coronographic components can be fine adjusted on these high precision motorized stages to allow a precise centering of the coronograph relative to the adaptive optics system. Due to this complexity of the required motions, but also due to the difficulties of fabricating high quality cryogenic Wollaston prisms and finally, because of the relatively lower priority of the K band for the scientific goals of HiCIAO, it was decided to design the coronographic foreoptics for ambient temperature operation. A more detailed discussion of thermal background reduction in HiCIAO will be given in Section 2.5 below.

The camera lens is a BaF2 / fused silica doublet operating at ambient temperature. This is a less than ideal combination of optical materials, but it was chosen so that the mechanically robust fused silica lens can also be used as the cryostat window. The most important limitations to the performance of a spectral simultaneous differential imager (SSDI) are optical aberration differentials between the beams that limit the ability to match and subtract the speckle cloud. To minimize these differential-path aberrations, the infrared camera lenses (the window) are placed as close as is mechanically possible to the beamsplitting Wollaston prism. The first cryogenic optical element is the common-path filter, which is used for direct imaging and dual-beam polarimetric imaging. For use in quadruple-beam imaging, which is the mode of operation for spectral simultaneous differential imaging, the four individual filters (one for each of the four beams generated by the double Wollaston prism) are placed as close as possible to the detector, to minimize the impact of any surface imperfections on the wavefront in each of the channels.

The coronographic foreoptics deliver a collimated beam to the infrared camera. Therefore, the mechanical coupling between the foreoptics and the camera is not very critical. The warm fore-optics are designed as a separate unit that will be mounted onto an interface bracket that is directly bolted to the AO optical bench. The HiCIAO infrared camera will sit on its own support system on the Nasmyth platform. The foreoptics interface is also designed to allow the use of the Subaru IRCS6 spectrograph at the AO focus, taking the place of the HiCIAO camera. For IRCS, a system of mirrors directs the AO system output past the coronographic foreoptics to a different focus location. The IRCS cryostat uses the same attachment points to the Nasmyth platform as HiCAIO. The changeover between these two instruments will take several hours and is a daytime task.

2.1 Direct Imaging Mode

The HiCIAO camera is designed as a flexible system that can be configured into different modes of operation in an optical bench environment at the Subaru Telescope Nasmyth focus. The most basic mode of operation is direct imaging at the AO focus, with or without a coronographic occulting spot.

Figure 1: Raytrace of the HiCIAO infrared camera in simple imaging mode. The optics consist of a field lens, the doublet collimator, and the doublet camera lens, which also serves as the cryostat window.

Figure 2: A view of the open HiCIAO cryostat, showing the 3 filter wheels and the optical baffle tube. The detector on the right side of the optical path is not installed in this picture.

The HiCIAO instrument contains three almost identical rotary mechanisms: The Common Path Filter Wheel, the Pupil Wheel and the Differential Path Filter Wheel. The basic design is that of a modified Geneva intermittent motion mechanism that includes a detent mechanism, very similar to the mechanisms designed for NIRI7 and IRCS6. Our design has a Geneva wheel with 48 slots. The two-pin driver is located at the periphery of the Geneva wheel and advances the Geneva wheel by 2 slots for every 360° driver turn. The driver is also coupled to a cam, the function of which is to lift/lower the detent in/out of the detent groove in the Geneva wheel as it is being turned. The nominal filter positions are defined by the position of the detent arm and are accurate to approximately 50 arc-seconds. The mechanism is driven with a Phytron cryogenic stepper motor coupled to the driver with a flexible coupling.

2.2 Differential Imaging Modes

The HiCIAO infrared camera offers two simultaneous differential imaging modes. For polarimetric simultaneous differential imaging (PSDI) mode, a single Wollaston prism (Figure 3) splits up the image into two orthogonally polarized images. In conjunction with a rotating waveplate in front of the AO system and of HiCIAO, this can be used to obtain imaging polarimetry. In particular, the polarization of an object near a bright star, for example a scattering dust cloud or a dust condensation in a protostellar disk, can be used to distinguish the object from the less polarized residual speckles of the bright star.

For spectral simultaneous differential imaging 5 , we will use a set of two Wollaston prisms (material YLF, angles 27° and 38°), with their optical axes at 45° relative to each other, to produce the four images of the object. Each of these images is recorded through individual filters, designed to match a spectral feature in the object, for example, the Methane absorption band in brown dwarves and young gas giant planets. Figure 4 shows a picture of the four-filter unit during installation in the differential filter wheel of HiCIAO. Figure 5 (right) shows an example of imaging the planetary disk and ring area of Saturn, where the planetary disk shows strong methane absorption while the rings do not. We expect that the use of four sub-images and filters will improve out ability to distinguish the methane absorption signature of a young planet from the methane-free stellar speckles compared to instruments that only use two wavelengths.

By far the best birefringent material for use in the H band is LiYF4, commonly referred to as YLF, due to its very low chromatic effects8. The chromatic effects in YLF are about a factor of 36 (in the H band) lower than in the more commonly used Calcite. In the J band, Calcite is about a factor of 5 worse than YLF, and Calcite is not usable at all in the K band due to its absorption. YLF also has the advantages of reasonable cost and low anisotropy in its thermal expansion, leading to a relative insensitivity of the optical components to temperature changes. The Wollaston prisms for HiCIAO were fabricated by Bernhard Halle in Germany.

Figure 3: The single YLF Wollaston prism during installation in its holder.

Figure 4: The four-filter unit used for spectral differential imaging during installation.

Figure 5 (left panel) shows the two split images in the single-Wollaston polarimetric imaging mode, the right panel shows the four-fold split in the double Wollaston spectral differential imaging mode. The summary in Fig. 6 shows the methane absorption spectrum in detail and illustrates how the different filters are affected by methane absorption. The image in the upper center (1.60 μm), upper right (1.62 μm), and lower left (1.64 μm) are taken through filters that are progressively deeper in the methane absorption band, so that the disk of the planet is progressively darker. The rings, being composed of small ice and dust particles and not containing gaseous methane, are not subject to methane absorption and therefore have equal brightness in all four filters.

Figure 5 shows the location of the two orthogonally polarized images in single Wollaston (polarimetric simultaneous imaging) mode (left), and the four images produced by the double Wollaston prism for spectral simultaneous differential imaging (right). The images were obtained during the first light engineering run without adaptive optics.

Figure 6: Summary of the imaging results obtained with HiCIAO, explaining the use of different filters to distinguish methane absorption objects (Saturn’s atmosphere) from objects with a continuum spectrum (Saturn’s rings). Also shown is a comparison of the point spread function of the HiCIAO optics compared to the theoretical design predictions. 2.3 Pupil Viewer

A pupil viewing system is very useful for the alignment of the instrument Lyot stop with the telescope so that the best image quality and coronographic suppression can be achieved. It is also useful for the optimal reduction of thermal background. The HiCIAO pupil viewer is an achromatic doublet lens system that gets inserted approximately in the middle between the pupil plane and the focal plane, so that it re-images the pupil plane onto the focal plane. The achromatic doublet consists of BaF2 and Schott glass SF6, a good achromatic combination. The pupil viewer lenses are mounted on a filter wheel mechanism largely identical to those used for the filter wheels.

Figure. 7: Pupil viewer image of the Subaru Telescope mirrors and secondary support structure (black) and the Lyot stop mask (grey) during, but before completion, of the alignment process. The background structure is the inside of the Subaru enclosure building, individual rivets are clearly visible.

2.4 Filters

The common path filter wheel, used for direct imaging and single-Wollaston polarimetric observations, has space for 11 filters and currently carries the Y, J, H, and K broad-band filters. The differential path filter wheel has space for 5 sets of filters with 4 filters in each set, plus an open position. At this time, we have installed only two of these differential imaging filter sets, the set designed to detect Methane absorption and one to detect H2 emission. One differential filter wheel position will remain open, and another is used as a blank-off position. Due to the close proximity of the differential path filters to the focal plane, wavefront errors in these filters are not very critical. Filter substrates better than one wavelength (633 nm) are acceptable and this substrate quality is easily achievable.

2.5 Thermal Background

A major decision in the design of HiCIAO was the operating temperature for the optical components. Since the core science of HiCIAO will be done in the H band, we decided not to operate the foreoptics at cryogenic temperatures. The HiCIAO instrument was designed as a very flexible experimental system and would have required a very large number of cryogenic mechanisms at prohibitive cost if it were built as a fully cryogenic system. The following design features reduce the impact of thermal emission for observations in the K band:

1. The distance between the optical pupil (with the warm Lyot stop) and the cold stop inside the cryostat was kept to a minimum. The cold stop is located just in front of the first (common path) filter wheel. Aside from the thermal considerations, placing this filter wheel as close as possible to the pupil is useful to minimize non-common path wavefront problems in the optics and filters.

2. A "background reduction mirror" (Fig. 8) on the back surface of the warm Lyot stop reflects light from the inside of the cryostat back into the optical path. This has the effect of effectively replacing an ε = 1 surface at ambient temperature with a surface at the temperature of the inside of the cryostat, plus some scattered light.

3. For the two beamsplitting modes (single and double Wollaston), part of the field of view must be masked off, to avoid overlapping stellar images from different field points. These masks are flat mirrors facing the inside of the cryostat.

Figure 8: Background reduction mirror behind the pupil mask rotator

Overall, HiCIAO will be background-limited in all filters for integration times longer than a few seconds. This is true for direct imaging and polarimetric imaging of faint sources. For coronographic observations near bright stars, the limiting noise source will be speckle noise, setting an even higher effective noise than photon shot noise from the backgrounds. The detector performance required for HiCIAO is not challenging for HAWAII-2RG detector arrays. For HiCIAO in its basic mode of operation, there is no benefit to be gained from developing new extremely low noise readout algorithms for the detector array.

3. LABORATORY AND TELESCOPE TESTS

After the instrument was assembled in the laboratory, we performed some tests to verify the instrument’s fundamental functionality as an infrared camera. The tests included various items such as optical performance, functionality of the Wollaston prisms, alignment of differential filters, configuration of the split beam on the detector, and detector characterization. The optical performance was evaluated with a pinhole (5 μm diameter) placed closely to the field lens where the telescope Nasmyth focus is located when observing. Figure 6 compares a radial light profile obtained in the laboratory with the one simulated by the Zemax ray-tracing program. The two radial profiles are consistent in terms of the position of the ring and the proportion of the intensity between the core and the ring, indicating that the optics perform as designed.

Figure 9: HiCIAO installed at the Subaru Telescope Nasmyth focus.

The HiCIAO instrument was delivered to the Subaru Telescope in November, 2007. HiCIAO was then installed at the Nasmyth focus of the Subaru telescope. Since the 188-actuator adaptive optics system was not yet available, HiCAIO was installed directly at the (uncorrected) Nasmyth focus in a temporary configuration with the coronographic foreoptics mounted directly on the infrared camera cryostat (Fig. 9).

The purpose of the first light observation without AO was to check the physical fitting and alignment with the telescope, the functionality of the three observation modes using the objects on the sky, and the commands coded in the Subaru Observation Software System. Most of our original plans were achieved without serious problems. We will have a number of additional engineering nights with the 188-actuator AO system in the future to verify the performance of the three observation modes with and without coronagraph. 4. DETECTOR ARRAY

HiCIAO uses a Teledyne 2.5 μm cutoff-wavelength HgCdTe HAWAII-2RG detector array9. Two of the attractive features that the HAWAII-2RG devices offers are capacitive reference pixels and reversible shift registers. The reference pixels will allow signal corrections which eliminate the need for tight thermal control of the detector. Reversible shift registers give flexibility in reading out various portions of the detector array. This will give us the flexibility to read the two or four beams in a truly simultaneous manner. It will allow us to read portions of the array faster while integrating on the rest of the field, a capability essential to internal wavefront sensing for infrared-bright objects.

The HiCAIO instrument is currently operated by an external, ambient temperature Teledyne Sidecar ASIC10. This configuration had been chosen because, at the time when we finalized the instrument design, only the ambient temperature "ASIC development kit" was available. During the initial detector testing, several shortcomings of this approach were identified. In the 32 channel readout mode that we are using for HiCIAO, the detector heated up during operation and a pattern of hot pixels with excessive shot noise was clearly visible. This issue was corrected by improving the thermal connection to the heat sink. A second problem is a defective channel in the development kit ASIC, which will be corrected in the near future with an upgrade to a new cryo-ASIC system. Finally, the ambient temperature ASIC development kit, being connected to the HAWAII-2RG detector array by a cable of approx. 30 cm length, did not perform as well as has been reported for the cryo ASIC, with the noise being about twice what it should be. This issue should also be resolved with the use of the cryo ASIC.

5. SOFTWARE

The software system at the Subaru Telescope has three main components: the Telescope Control System (TCS), the Subaru Observation Software System (SOSS), and the Instrument Control System (ICS). TCS and ICS control the telescope and the instrument, respectively, while SOSS interconnects the TCS and ICS and schedules the commands to the telescope and the instrument. The ICS is the instrument-specific software that controls all functions of a specific instrument. ICS consists of a main Linux PC, Windows PC for the detector control, two opto-mechanical controllers, and some serial terminal devices. The main computer manages the instrument devices via the Ethernet LAN. It is responsible for non-time-critical observational tasks such as temperature control/monitor, vacuum monitor, power monitor, starting-up/shutdown of the devices. It also has interfaces to the users and SOSS and distributes the commands to individual instrument devices. The sending and receiving of commands and status between the main computer and the serial devices is mediated by a terminal server that has multiple RS232C ports. The interface to the SOSS is coded using the SUN RPC library provided by the observatory. Graphical interfaces developed using Tk program language are available for observers to control the instrument devices, to monitor the status, and to perform quick look and analysis during the observation. Observers can execute the observation sequences either in a script using abstract commands executed from the SOSS or individually using the graphical interfaces from the main computer.

Figure 9: Screenshot of the HiCIAO instrument control GUI

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