|Backward Looking Telescope (RED) - The VNIR subsystem backward looking
telescope is of the same design as the nadir telescope and contains only a
single silicon charge coupled detector line array and no calibration lamps
as it is only used to acquire a stereo pair image.
Nadir Looking Telescope
(Dark Blue) - The VNIR subsystem nadir looking telescope is a
reflecting-refracting improved Schmidt design. The focal plane of this
telescope contains three 5000 silicon charge coupled detector line arrays.
The nadir and backward looking telescope pair are used for same orbit stereo
imaging and can be rotated as a unit +/- 24 degrees to provide extensive
cross-track pointing capability. Light from either of two halogen lamps are
used periodically for subsystem calibration.
The VNIR subsystem consists of two independent telescope assemblies to minimize image
distortion in the backward and nadir looking telescopes. The detectors for each of the
bands listed in Table II consist of 5000 element silicon charge coupled detectors
Only 4000 of these detectors are used at any one time. A time lag occurs between the
acquisition of the backward image and the nadir image. During this time earth rotation
displaces the image center. The VNIR subsystem automatically extracts the correct 4000
pixels based on orbit position information supplied by the EOS platform.
The VNIR optical system is a reflecting-refracting improved Schmidt design. The backward
looking telescope focal plane contains only a single detector array (Band 3 of Table II)
and uses an interference filter for wavelength discrimination. The focal plane of the
nadir telescope contains 3 line arrays (Bands 1-3 of Table II) and uses a dichroic prism
and interference filters for spectral separation allowing all three bands to view the same
area simultaneously. The telescope and detectors are maintained at 296 + 3K using thermal
control and cooling from a platform provided cold plate. On-board calibration of the two
VNIR telescopes is accomplished with either of two independent calibration devices for
each telescope. The radiation source is a halogen lamp. A diverging beam from the
lamp filament is input to the first optical element (Schmidt corrector) of the telescope
subsystem filling part of the aperture. The detector elements are uniformly irradiated by
this beam. In each calibration device, two silicon photo-diodes are used to monitor the
radiance of the lamp. One photo-diode monitors the filament directly and the second
monitors the calibration beam just in front of the first optical element of the telescope.
The temperature of the lamp base and the photo-diodes is also monitored. Provision for
electrical calibration of the electronic components is also provided.
The system signal-to-noise is controlled by specifying the NE delta rho to be < 0.5%
referenced to a diffuse target with a 70% albedo at the equator during equinox. The
absolute radiometric accuracy is to be + 4% or better.
The VNIR subsystem produces by far the highest data rate of the three ASTER imaging
subsystems. With all four bands operating (3 nadir and 1 backward) the data rate including
image data, supplemental information and subsystem engineering data is 62 Mbps.