The Canarias Camera Experiment (CIRCE): Progress of the Opto- and Cryo- mechanical Design and Manufacture

M. L. Edwardsa,S.S.Eikenberrya,M.Charcos-Llorensa,A.Marin-Franchb N. Lassoa, S. N. Rainesa, J. Juliana, K. Hannaa,C.Packhama,M.Rodgersc, R. M. Bandyopadhyaya aUniversity of Florida, Department of , 211 Bryant Space Science Center, Gainesville, FL, 32611 bInstituto de Astrofsica de Canarias, Via Lactea s/n, E-38200 La Laguna, cOptical Research Associates, 3280 East Foothill Boulevard, Suite 300 Pasadena, California 91107-3103

ABSTRACT We present the current status of the Canarias InfraRed Camera Experiment (CIRCE) an all-reflective near-IR, imager, spectrograph, and polarimeter for the 10.4-meter Gran Telescopio Canarias (GTC). In particular, we review the progress of the opto- and cryo- mechanical design and manufacture, focusing on the custom filter, lyot, and grism wheels, lightweight optics, and mirror brackets. We also outline our progress with the optical bench. Finally, we discuss a number of CIRCE’s features that both complement and augment the planned suite of GTC facility instruments. Keywords: instrumentation:miscellaneous, infrared:general

1. INTRODUCTION The Canarias Infrared Camera Experiment (CIRCE) is a near-infrared (1 - 2.5 micron) visitor instrument for the 10.4-meter Gran Telescopio Canarias (GTC). Located on the island of in the , the telescope is a joint venture between Spain, , and the (UF). While the final suite of GTC instruments will sample all crucial optical and infrared regimes, the first- generation of facility instruments do not include a NIR camera. CIRCE fills the crucial gap between the tele- scope’s inception and the commissioning of the second-generation NIR facility instrument EMIR. Envisioned as a “workhorse” camera, capable of filling the NIR needs of the GTC community, CIRCE will have three basic modes, imaging, low- to moderate- resolution , and imaging and spectro- polarimetry. Additional features like high-speed imaging , further complement and augment the GTC’s capabilities. In this paper, we review an analysis of the CIRCE optical layout. We then discuss our progress on the opto- and cryo- mechanical design.

2. OPTICAL DESIGN While CIRCE’s basic collimator/camera layout is similar to many modern NIR cameras, the all-reflective aspheric design is more novel. Significant advances in the manufacture of diamond-turned optics have made them a viable alternative to lenses, especially since they often offer better throughput and image quality than all-refractive systems. Figure 1 shows the CIRCE optical layout, as designed by M. Rodgers at Optical Research Associates (ORA). Incoming light will pass through the cryostat entrance window (1) to the telescope focal plane (2) where a slit wheel will house an imaging field stop, a long slit for grism spectroscopy, and a partial-field imaging stop for polarimetry. Two fold mirrors aft of the telescope focal plane will direct the light to a two mirror collimator (Send correspondence to M.L.E – E-mail: [email protected]fl.edu, Telephone: 1-352-392-2052 ext. 253)

Ground-based and Airborne Instrumentation for Astronomy II, edited by Ian S. McLean, Mark M. Casali, Proc. of SPIE Vol. 7014, 70142L, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.788424

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Figure 1. The all-reflective aspheric optical design of CIRCE.

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Figure 2. The J-band encircled energy. Note that even at the corners the EE within two pixels is at ∼ 75% which will produce an image of the pupil at the Lyot stop (3). Filter and grism wheels located fore and aft (4) of the stop will allow for narrow- and broad- band imaging, grism spectroscopy, and Wollaston polarimetry and spectropolarimetry Finally, a four mirror imager will focus the light onto the detector focal plane, where we will place a HAWAII-2 2048×2048-pixel infrared detector (5). CIRCE’s 0.10 arcsec plate scale provides seeing limited images even in the most excellent atmospheric condi- tions at the GTC site. Additionally, the planned 3.4 arcmin × 3.4 arcmin field is also competitive with similar instruments on 8 - 10 meter class telescopes. CIRCE spectroscopy includes two grisms, both of which share the grating ruling masters custom-built for FLAMINGOS-2. The first grism will cover the 1.25 − 2.4µm bandpass instantaneously at a resolution of R=410 at 1.25µm and R=725 at 2.20µm. The second grism will cover a single band instantaneously at a resolution of R ∼ 1500 in its 3rd order (K-band), 4th order (H-band), or 5th order (J-band). These values assume a 3-pixel slit. The CIRCE optics will maintain seeing-limited image quality with the grism in the optical path over the entire bandpass. CIRCE’s optical design includes an imaging- and spectro- polarimetric mode A detailed explanation of CIRCE’s polarimetric capabilities are the subject of another paper in this proceeding.2 In addition to these three main modes, CIRCE will also have a sub-framing readout mode that will enable high-speed imaging pho- tometry in any filter (broad- or narrow- band). The MCE-3 electronics can support continuous frame rates faster than 1 Hz over fields-of-view exceeding 1 × 1-arcmin.

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Downloaded from SPIE Digital Library on 28 Apr 2010 to 128.227.184.43. Terms of Use: http://spiedl.org/terms Figure 3. Solid model of the CIRCE optical layout including brackets and mirrors

We present the J-band encircled energy diagram for CIRCE in Figure 2. We see that at the center of the detector the encircled energy is > 90% in two pixels. This value remains high over the entire field; the EE within two pixels in the corner is ∼ 75%.

3. OPTO-MECHANICAL DESIGN The opto-mechanical phase, including the design and manufacture of lightweight mirrors, brackets, and the optical bench is rapidly progressing. Figure 3 shows CIRCE’s opto-mechanical layout, a solid model of CIRCE’s eight mirrors and their brackets. Five of the large mirrors have a honeycombed back surface, reducing the weight of these optics by 20 - 25% Test mirrors and mirror blanks based on these models are currently in fabrication at the University of Florida Physics and Astronomy machine shop. Machining of the brackets is complete. In Figure 4, we show a picture of two completed “practice” mirror blanks and their brackets. The brackets will be anodized black to prevent internal reflection within the dewar. To substantially reduce cost and lead time, the large (1 meter diameter × 1.5 meter long) optical bench was designed in multiple pieces so that it could be manufactured on machines available in-house at UF. Flexure modeling shows that this option is viable; the large cryogen tank beneath the bench offers adequate stiffness to support the optics and cryo-mechanisms. A solid model of this novel optical bench designed by M. Charcos is presented in Figure 5 Janos Technology will complete the mirrors and analyze the optics employing a newly developed technique that emphasizes ensemble testing of the mirrors, brackets, and bench. This novel solution to optical analysis allows for immediate correction of alignment problems in situ. Thus completed bench, with mirrors and brackets in place and aligned, will arrive at UF ready for integration.

4. CRYO-MECHANICAL DESIGN One of the key ideas for CIRCE was to design and manufacture the cryo-mechanisms, dewar, and electronics using drawings and expertise gained from previous UF instruments. The obvious advantage of this plan was that we could save time by modifying already existing designs or assemblies instead of starting from scratch. The CIRCE filter box benefited from this philosophy; the final design of the assembly is a hybrid of the Flamingos 1 and 21 pupil boxes. While the size of the wheels, number of teeth on the gears, and other crucial dimensions changed to accommodate the differences between the optical designs, the basic structure of the CIRCE box and internal mechanisms is very similar to its predecessors.

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Downloaded from SPIE Digital Library on 28 Apr 2010 to 128.227.184.43. Terms of Use: http://spiedl.org/terms Figure 4. Imager 2 and 3 blanks and their brackets.

Figure 5. Solid model of the CIRCE optical bench

The CIRCE filter box will hold five geared wheels. Of the five wheels, three will contain narrow- and broad- band filters. Each filter wheel will have five spaces, four for 70 mm-diameter filters and one left blank as a pass-through. The design of the grism wheel, positioned at the aft of the filter box, is similar to that of the filter wheels. The grism wheel will house two grisms and two filters with one “open” space for use when CIRCE is in imaging mode. The lyot wheel is identical to the filter wheels. It will be located in the center of the bank of five wheels and therefore inaccessible except by disassembling the box. We plan to place a pupil stop and masks for testing and integration in this wheel. The CIRCE focal plane mechanism at the telescope focal plane offered a new challenge. The small space between the entrance window and the focal plane, called for a compact design, while the envelope of the GTC prohibited one large wheels with full and half field masks and slits for spectroscopy. To overcome this constraints, M. Charcos designed a novel solution; two translation stages that insert focal plane and decker masks into the optical path and a rotation mechanism rotating the HWP. We present a solid model of these stages in Figure 6.

We currently have the filter, grism, and lyot gear blanks in-hand as well as the J, H, and Ks broadband filters and cryogenic motors. The designs for the focal plane mechanism, filter box, and all associated mechanisms are complete and machining will soon be complete. Upon completion, we will test each cryo-mechanism prior to integration using a specially designed test cryostat available at the University of Florida.

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Figure 6. Solid model of the CIRCE focal plane mechanism

5. CONCLUSION The Canarias InfraRed Camera Experiment (CIRCE), a near-infrared visitor instrument for the GTC, will provide a unique and powerful complement to the telescope’s facility instruments. The design phase has ended and currently the machining and manufacture of the optical, opto-mechanical and cyro-mechanical components is underway. Within a few months, we will begin integrating all mechanical components. We will also begin work on all necessary cabling; a review of the current electrical design and manufacture can be found in another paper in this proceeding.3 We anticipate that CIRCE will be ready for the telescope in 2009.

REFERENCES [1] S. S. Eikenberry et al. “FLAMINGOS-2: the facility near-infrared wide-field imager and multi-object spec- trograph for Gemini” in Ground-based and Airborne Instrumentation for Astronomy,I.S.McLeanandM. Iye, ed., Proc. SPIE 6269, pp 39, 2006. [2] M. Charcos-Llorens, M. L. Edwards, S.S. Eikenberry A. Marin-Franch, C. Packham, J. Julian, N. Raines. “IR polarimetry system with the Canarias InfraRed Camera Experiment (CIRCE)” These Proceedings. 2008. [3] N. Lasso Cabreraa, K. T. Hanna, S.S. Eikenberry, M. L. Edwards, M. Charcos-Llorens, A. Marin-Franch, J. Cenarrob “Readout electronics and fast photometry with the Canarias InfraRed Camera Experiment (CIRCE)” These Proceedings. 2008.

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