The resulting shape is a round disk with flat sides. In order to improve spectral variability across the IFOV, the field stop was narrowed to 0.6° wide in the scan direction. The field stop has a 3-mm outer diameter chosen to make the IFOV be 1.1°. The pupil imaging design was selected to achieve good coregistration, which means that each wavelength is observing exactly the same column of atmosphere. The second telescope images the pupil of the first telescope onto the spectrometer entrance slits. The first telescope images the ground onto a field stop. The AIRS fore optics, consists of two identical back-to-back telescopes with an aperture stop in between to provide pupil imaging. All reflective surfaces (except the front scan mirror which is protected silver) are gold coated for broad band high IR reflectivity. ![]() This approach provides an extremely broad spectrally corrected instrument that avoids the difficulties of broad band multiple element anti-reflection coatings. The AIRS Spectrometer is a nearly all-reflective instrument with only filters, field flattener, dewar window and detector substrates as refractive components. Cryocoolers keep the focal plane at a cold operating temperature.įocal Plane Assembly: A second stage of filtering over each FPA array further defines the selected color band, rejects overlapping orders, and serves to reduce background photon levels. Schmidt Camera: The dispersed light is re-imaged onto the focal plane by a Schmidt camera.ĭewar: Light from the camera passes through a window into a dewar containing the focal plane. The light from one of the slits is passed through a mechanical chopper.ĭiffraction Grating: The energy is then relayed onto the grating surface where high spectral resolution separation occurs. Ultimately, these eleven slits are imaged onto the focal plane, where each slit image contains the energy from one selected grating order.Īfocal Relay, Chopper, and Collimator: The light from the slits is re-collimated appropriately to be dispersed by the diffraction grating. An order-sorting bandpass filter covers each slit, which forms the first stage of spectral separation. ![]() The common field stop ensures high spatial registration for all spectral samples.Īperture Filters and Slits: The collimated energy exiting the telescope is incident on the spectrometer entrance slit plane containing eleven individual apertures arranged in two staggered columns. Afocal Telescope: Light enters the system via the cross-track scan mirror and proceeds to a 4-mirror off-axis telescope assembly with a common 1.1°-diameter field stop.
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