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.
(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.
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 50,000 hours.
Although ASTER operates with a low duty cycle (8% average
data collection time) the cryocooler operates 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 varies 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 +4% or better. The
combined data rate for all six SWIR bands, including
supplementary telemetry and engineering telemetry, is 23 Mbps.