AIRS

IR Spectrometer Assembly

The multi-aperture, array grating spectrometer is an advanced pupil-imaging design providing high spectral resolution, wide spectral coverage and full spectral multiplex operation essential to meeting the AIRS science objectives. The design approach uses a coarse echelle grating in combination with high definition bandpass filters to create a two-dimensional color map compatible with state of the art IR FPA technology.

Upwelling radiance enters the system (see figure at right) via the cross-track scan mirror, where it is directed into a 4-mirror off-axis telescope assembly with a common 1.1 field stop to ensure high spatial registration for all spectral samples. The collimated energy exiting the telescope is incident on the spectrometer entrance slit plane containing eleven individual apertures arranged in two staggered columns. For precise radiometry, these slits are conjugate with the system entrance pupil and each is covered with an order-sorting bandpass filter, which forms the first stage of spectral separation. Ultimately, these eleven slits are imaged onto the focal plane, where each slit image contains the energy from one selected grating order. The entire wavelength range is mapped onto the FPA using orders 3-11. 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.

The relationship between the entrance slits, grating orders, and FPA layout is illustrated in the lower right portion of the optics diagram. Energy passing through the entrance slits is re-imaged within the system where a tuning fork chopper (357 Hz) is incorporated for reduction of 1/f noise in the PC channels. The energy is then relayed onto a coarse (13 l/mm) grating surface (photo at right) where high spectral resolution separation occurs. Energy from each entrance slit is dispersed by the grating and re-imaged onto the focal plane by a wide field, off-axis F1.7/F2.0 Schmidt camera which provides a nominal 100 m x 200 m spectral resolution element format. Schmidt aberrations are corrected by an aspheric surface on the grating and a field flattener incorporated within the FPA assembly. A provision for commandable, micron level adjustment of alignment and focus is built into the Schmidt mirror assembly via three precision actuators and can be used in flight if necessary. So far in the mission, no alignment adjustments have been made.