6.1.2 Camera Performance

The FLITECAM detector is a 1024x1024 pixel indium antimonide (InSb) Raytheon ALADDIN III array cryogenically cooled to 30 K. The detector has 27 μ pixels which translate to a plate scale of 0.475" per pixel, resulting in a nearly 8' square FOV. The detector is optimized for use between 1 - 5.5 microns, and has a read noise of approximately 40 e- and dark current ≤ 1 e- sec-1.

The on-axis image quality (IQ) of the camera is excellent and images obtained with FLITECAM are expected to be diffraction limited from 3 - 5.5 μm. From 1 - 3 μm, the IQ will be limited by contributions from the telescope optics, telescope jitter, shear layer seeing, and the diffraction limit. Ground based tests under the best conditions at the Lick Observatory indicated that the IQ is stable within 1.5' of the optical axis, but that beyond this distance, coma begins to appear. The extent of the coma has not been parameterized.

The detector well depth is relatively shallow (~ 80,000 e-) which, when combined with the detector quantum efficiency (QE), may necessitate relatively short exposure times, particularly on bright sources or in regions of high sky background. The shortest exposure time available for a full 1024x1024 detector readout is 0.2 seconds. However, faster readout times can be achieved using sub-arrays -- 0.08 seconds for a 512x512 sub-frame and 0.015 sec (~67 Hz) for a 64x64 pixel sub-frame. These subarrays are required to be symmetric and relative to the center of the detector. An additional ``movie'' mode is avaiable to sample a single 512x512 quadrant at ~0.08 seconds without any deadtime between images, or down to the 32x32 sub-frame (closest to the detector center) at < 0.015 seconds. Users interested in these modes should consult the FLITECAM Instrument Scientist via the SOFIA Help desk (sofia_help@sofia.usra.edu).

Care must be taken not to over-expose the detector since charge persistence can be a significant problem for InSb arrays. If the detector is over-exposed, it is necessary to take several long exposures (~5 minutes each) of blanks mounted in the filter wheel to allow the detector to recover. To help mitigate the problem of accidental saturation of the detector, filter changes are carefully orchestrated to ensure that the sky is never viewed through the two open filter wheel positions.