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This paper presents a first-order performance analysis of the telescope - optical system for the Cryogenic Limb Array Etalon Spectrometer (CLAES), one of several experiments intended for flight on the Upper Atmospheric Research Satellite (UARS). The experiment is based around a solid Fabry-Perot spectrometer which provides spectral resolution of 0.25 cm-1 for atmospheric emission spectroscopy over the 3.5- to 12-μm infrared wavelength range. A solid hydrogen cryostat sized for a two-year in-orbit lifetime provides cooling for the detector array, spectrometer, and telescope optics.
The experiment involves the passive measurement of earth-limb radiance over a 10- to 60-km tangent altitude range. The instrument is required to provide near diffraction-limited performance over this 50 km or 1-deg field-of-view, and over the full wavelength range. The optical system must also provide a high degree of off-axis rejection and stray-light control, primarily to suppress intense emission from the earth surface which resides at -0.2 deg off-axis for the 10-km lowest altitude of observation.
The optical system consists of a Gregorian telescope and refractive re-imager arranged in the common "z" configuration with folded baffles and no obscuration. The 6-in. telescope primary mirror produces an intermediate focus where the astigmatic image spread is oriented along the horizontal limb direction. This allows for precise location of the field stop and very sharp cutoff of the field below -0.175 deg. The astigmatism and other geometric aberrations are corrected by the secondary mirror which produces an excellent image of the primary, allowing for location of a diffraction control or Lyot stop.
The re-imaging lens corrects for field curvature associated with the telescope and produces a flat field at the detector focal plane. Image quality is at the diffraction limit from 3.5 to 10 μm, with a point spread less than 10 percent of the airy disc diameter for example at 5 μm. The airy disc itself is less than 10 percent of a detector vertical dimension at this wavelength.
Some chromatic correction remains to be incorporated between 10 and 12μm, where the point spread exceeds the diffraction limit by about 20 percent. The off-axis scattering performance of the telescope is discussed in terms of the mirror scatter coefficient and point source rejection ratio. A mirror bidirectional reflectance distribution function (BRDF) of 1 x 10-4 at 1 deg with a 1/02 roll-off between 1 and 0.2 deg is realizable based on recent measurements. This results in an off-axis radiance term generally small in comparison with the system-limiting noise equivalent radiance (NER = 10-12 W/cm2/sr at 10 μm.) As the observational altitude increases the off-axis term becomes less and less significant.
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