VNIR

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.

VNIR Design.
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 (CCD's). 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.