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Electronic Imaging & Signal Processing

Compact near-infrared camera supports adaptive-optics demonstrator

A high-resolution diffraction-limited camera helps evaluate whether multiconjugate adaptive optics can improve terrestrial astronomical imaging.
2 October 2006, SPIE Newsroom. DOI: 10.1117/2.1200609.0372

The CAmera for Multi Conjugate Adaptive Optics (CAMCAO) was built as part of the European Southern Observatory's (ESO's) Multi-conjugate Adaptive Optics Demonstrator,1 where it is presently operating. After adaptive optics compensate for atmospheric turbulence, the CAMCAO samples the corrected point-spread function (PSF) in the IR. The adaptive-optics techniques used in the system have the potential to significantly increase the resolution possible from ground-based telescopes.

Meeting project goals meant the camera design and implementation had to balance the needs of several systems, which was challenging in some cases. Attaining low weight while ensuring long-term image stability was a major difficulty, and the baffling and cryogenic designs required particular attention. Also, the optical system performance had to permit PSF evaluation, so we chose a 2048×2048-pixel sensor to provide high resolution.

The CAMCAO is an Offner-relay-like system. This design re-images a field while forming a real pupil that can be used to baffle the system. Our camera's pupil is small, limiting the ratio of the focal length to the pupil diameter to f/15, and the image is magnified by 10%.2 We designed the CAMCAO optics system to be diffraction-limited at 2.2μm, but with an eight-position filter wheel (see Figure 1), it is capable of covering wavelengths from 1.0μm and 2.5μm. The imager is the HAWAII2 HgCdTe array from Rockwell. In addition to the high resolution, the camera has a relatively large field of view: it covers a 59×59arcsec section of the sky.

Figure 1. The optics for the Camera for Multi Conjugate Adaptive Optics include: (a) the entrance window; (b) the filter-wheel; and (i) the optical box. The cryogenics system includes (c) refilling and ventilation pipes; (d) the cold-support structure; and (g) the liquid nitrogen tank/cold plate. Other components include (e) the vacuum electro-valve; (f) data and control connectors; (h) the upper-cover.

The CAMCAO's re-imaging optics consist of five metallic mirrors mounted in a compact structure that is also massive, thus minimizing possible deformations. We made the light path from the filters to the detector light-tight. We paid special attention to parts—such as the cables and the filter-wheel shaft—that would be in contact with the external vessel, or would cross the radiation shields where proper baffling, light-tight feed-through, and heat-sinking were implemented. The filter wheel and its casing were designed to prevent stray light from reaching the re-imaging optics.

The system must be cryogenically cooled in a liquid nitrogen (lN2) bath cryostat. The entrance and exhaust pipes coil around the optical box, which allows it to operate while the camera turns over an horizontal axis. A second cooling option uses an optimized pulse-tube cryocooler.

We expended considerable effort on the cold support structure design.3 We needed a long cryogenic hold-time without refilling, but had to keep the total weight down to less than 90kg. The very strict image stability requirement called for less than 1/4 of a pixel (4.5μm) image displacement during the longest exposure, corresponding to a rotation of up to 30°. This was achieved using an optimized stainless-steel hexapod structure with very low thermal conductivity.

The CAMCAO assembly started with the mechanical portions, followed by the vacuum and cryogenics integration. During this procedure, the complex lN2 exhaust tube path and its interaction with the radiation shields had to be optimized to ease the assembly procedures and minimize the radiation entering the outer radiation shields. Re-testing during and after each lN2 cooling cycle has shown that the indium-sealed joints between the aluminium lN2 tank and the stainless-steel ventilation tubes could easily deteriorate due to high temperature gradients and the different contraction behaviors of the two materials. The problem was solved by using spring-loaded fasteners and an optimized indium seal.

The camera fulfils all cryogenic and vacuum stability requirements.3 In particular, the detector temperature is maintained at 80K by an accurate control loop, ensuring milliKelvin stability after cooling down at a rate slower than 0.5K/min.

Optical quality tests were performed for the individual components and on the integrated system. All metallic mirrors were characterized by the manufacturer, LT-Ultra Precision, and verified using our interferometer. The IR optical validation measurements were performed by re-imaging a point source in the camera focal plane and measuring the PSF with the detector. The computed Strehl ratio reached 95% in the central region of the field of view, with values larger than 90% in an area covering 88% of the focal plane. (The Strehl ratio compares the actual performance of a system to the best theoretical performance for imaging a point source of light.)

To measure the background—the detector intrinsic dark current plus the instrument background from residual leaks—the filter wheel was positioned in one of the blocked positions and a tungsten lamp was pointed at the entrance window. The reset clock was disabled while the readout and image-processing system acquired the frames. At 80K, the background measured 0.8 electrons per pixel per second, which is well below the specified maximum value of 10e-/pixel/s for the instrument background.

In conclusion, the CAMCAO was designed, manufactured, integrated and tested by a consortium of Portuguese institutions in collaboration with the ESO. The particular requirements for the CAMCAO's operation were met by implementing dedicated solutions to several critical issues in mechanical stability, baffling, and cryogenics.

After the CAMCAO has served its purpose in the Multi-conjugate Adaptive Optics Demonstrator, it will be attached to the ESO's ground-based Very Large Telescope.

This work was supported by the (FCT) and Programa Operacional Cincia, Tecnologia, Inovação (POCTI) (EC fund FEDER) under grant number POCTI/FNU/43843/2001.

Antonio Amorim, Jorge Lima, Filipe Duarte Santos 
Laboratory of Systems, Instrumentation and Modeling for Environment and Space Sciences, Instituto Dom Luiz, University of Lisbon
Lisbon, Portugal
João Alves, Enrico Marchetti, Gert Finger, Jean-Louis Lizon  
European Southern Observatory
Munich, Germany
José Rebordã
Aerospace Laboratory, Instituto Nacional de Engenharia,Tecnologia e Inovação (INETI)
Lisbon, Portugal
José Pinhão, Rui Marques 
Laboratório de Instrumentação e Física Experimental, Partículas and Department of Physics, University of Coimbra,
Coimbra, Portugal

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