SWIR

Cryocooler (Green) - The platinum Silicide-Silicon Schottky barrier linear detector array in each of the six SWIR channels are cooled to 80 K using a mechanical split Stirling cycle cooler of long life and low vibration design. Pointing Module (Blue) - The pointing mirror can point +/- 8.54 degrees from the nadir direction to allow coverage of any point on the earth over the spacecraft's 16 day mapping cycle. This mirror is also periodically used to direct light from either of two calibration lamps into the subsystem's telescope. Telescope (Red) - The SWIR subsystem uses a single fixed aspheric refracting telescope.

SWIR Design.
The SWIR subsystem uses a single aspheric refracting telescope. The detector in each of the six bands (Table II) is a Platinum Silicide-Silicon (PtSi-Si) Schottky barrier linear array cooled to 80K. Cooling is provided by a split Stirling cycle cryocooler with opposed compressors and an active balancer to compensate for the expander displacer. The on-orbit design life of this cooler is to be 50,000 hours. Although ASTER will operate with a low duty cycle (8% average data collection time) the cryocooler will operate continuously because the cool-down and stabilization time is long. No cyrocooler has yet demonstrated this length of performance and the development of this long-life cooler is one of several major technical challenges facing the ASTER team.

The cryocooler is a major source of heat. Because the cooler is attached to the SWIR telescope, which must be free to move to provide cross-track pointing, this heat cannot be removed using a platform provided cold plate. This heat is transferred to a local radiator attached to the cooler compressor and radiated to space.

Six optical bandpass filters are used to provide spectral separation. No prisms or dichroic elements are used for this purpose. A calibration device similar to that used for the VNIR subsystem is used for inflight calibration. The exception is that the SWIR subsystem has only one such device.

The NE delta rho will vary from 0.5 to 1.3% across the bands from short to long wavelength. These performance estimates may be optimistic for the bandpasses given in Table II. since bands 5-9 are narrower than those used in developing the conceptual design. The absolute radiometric accuracy is to be +4% or better. The combined data rate for all six SWIR bands, including supplementary telemetry and engineering telemetry, is 23 Mbps.