AIRS Optics and Focal Planes
Infrared 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.
Focal Plane Assembly
The AIRS IR Focal Plane Assembly (FPA) represents a major advance in infrared technology, incorporating a number of state of the art features in a space qualified, high reliability configuration. Key to its operation is an advanced hybrid PV/PC HgCdTe focal plane consisting of 10 PV modules and 2 PC modules each individually optimized for a particular grating order/wavelength range (see diagram).
The PV modules consist of 1, 2 or 4 bi-linear arrays of back-side-illuminated HgCdTe detectors, each bump mounted to a low power, 1.2 m CMOS read-out integrated circuit (ROIC) that provides the first stage of signal integration and multiplexing. The 10 PV modules contain a total of 4208 detectors multiplexed down to 26 outputs, with each PV module having a unique set of requirements and constraints in terms of wavelength coverage, signal and background flux, and sensitivity (D*). Technologically, the most stressing are the long wavelength modules, particularly M10 which covers the band from 12.7 m to 13.7 m and required a detector material cutoff wavelength of 15.6 m. Considerable development in the areas of LWIR detector material growth, radiation tolerant ROIC design and fabrication, and detector/ROIC interconnect was required to satisfy system requirements. To mitigate risk in the longest wavelength region, 13.7 m to 15.4 m, PC HgCdTe detector technology was used. This spectral region is covered in two modules with individual leads for each of the 274 low impedance PC detectors, a distinct disadvantage attendant with this technology.
The set of 12 modules is mounted to a common ceramic motherboard. The motherboard contains a complex arrangement of power, command and control, and signal interconnects to all PV modules as well as individual lead-outs for the PC detectors. The AIRS FPA is unique in its hybrid PV/PC approach and required special care in the routing, shielding and grounding of very low noise (nV) PC signals in the presence of high level (V) PV signals. A total of 526 leads interconnect to the motherboard assembly using a series of 10 high-density, thin-film flex cables specifically designed for cryogenic operation. Modules are individually assembled, tested and positioned onto the motherboard using computer controlled stages to maintain the requisite optical alignment tolerances of +/-15 m. Overlaying the focal plane module is an IR bandpass filter assembly containing 17 individual filters used for grating order selection as well as background suppression. The filter set is precisely registered to the focal plane using a dark mirror coated frame for stray light control. The assembly also includes a cold shield and a field flattener lens, which is part of the Schmidt exit optics.