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Diffraction Limited Imaging at the KPNO 4-m (1Dec94) (from NOAO HIGHLIGHTS!, NOAO Newsletter No. 40, 1 December 1994) Diffraction limited imaging at infrared wavelengths is a key goal of Gemini and other large telescope projects. Ian Gatley, Ron Probst, Steve Ridgway, and Mike Merrill of the KPNO IR group, working in collaboration with Frank Low (Steward) recently got a sneak preview of this technique at the 4-m by reconfiguring the Cryogenic Optical Bench into a high resolution camera called the Diffraction Limited IR Imager, or DLIRIM (pronounced "delirium"). This was an experiment to explore the technical and scientific aspects of operating in this mode, and to define requirements for future instruments. DLIRIM turns the necessity for taking extremely quick pictures to operate in the thermal infrared into a virtue, by sampling the speckle pattern at rapid time scales. Thermal emission of the atmosphere in the L' band (3.5-4.1 um) requires high speed operation--50 millisecond integrations--to avoid saturation, a capability provided by the KPNO Wildfire electronics. This is also fast enough to "freeze" the atmospheric seeing, which together with the long wavelength can produce excellent image quality. In speckle terms, a point source image often has a single dominant speckle. To get the highest resolution available on Kitt Peak, Gatley et al. used the 4-m telescope (3.8 meters effective aperture), known from wavefront curvature analysis to have high quality surface figure. The DLIRIM configuration replaces COB's internal reimaging camera module with a camera providing 0.1' per pixel on a 256 X 256 InSb array, with the data system modified to store long strings of contiguous 50 msec frames. To obtain dynamic range while retaining image quality, DLIRIM uses shift-and-add frame addition with a bright point source in the field as a reference. This is a postprocessing analog to tip-tilt correction, with the distinction that by centering on a dominant speckle some high order correction is obtained as well. DLIRIM on the 4-m clearly produced diffraction limited cores on point sources in the L' band. Segments of the Airy rings could be seen in the raw single frames on bright stars, and a uniform point source core FWHM was achieved across the full 25" field with the reference source in one corner. Limited data in the K band showed more of the core-halo image structure predicted for adaptive optics systems at shorter wavelengths. These results were obtained in very unstable atmospheric conditions with rapidly varying low cloud plus high cirrus. The 4-m optical seeing monitor, operating at 30 Hz, reported typically 1.25" FWHM seeing on bright stars near our IR targets. [Figure not included] Figure 1. The field of the Becklin-Neugebauer source in Orion at 3.6 um. This 2 X 2 mosaic was obtained by using BN as the reference and placing it sequentially in each corner of the array field. It represents 90% of the field of the Gemini telescopes, using a 1024 X 1024 array at 0.05" per pixel. The inset shows two point sources separated by 0.4". Dynamic range between these sources and BN itself is 7.5 magnitudes. DLIRIM demonstrates that the KPNO site and 4-m telescope can deliver outstanding performance in the infrared. DLIRIM also gives a technical preview of what this type of imaging science entails, and the requirements it imposes on a facility instrument. While DLIRIM was an ad hoc experiment taking best advantage of existing capabilities, it gives a clear picture of things to come. Indeed, the DLIRIM camera is designed to utilize the 1024 X 1024 arrays being jointly developed by NOAO and USNO as they become available. Ron Probst for the Infrared Group
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