In the Optical Assembly the incident radiance and irradiance are collected and split according to wavelength and focussed on the detector modules of the two optical channels. The Optical Bench consists of a single aluminium structure in which all elements are mounted. Attached to this aluminium structure are the two Detector Modules. The Optical Assembly Thermal Hardware consists of a thermal radiator and heaters. The thermal radiator is thermally connected to the detector modules via cold-fingers and flex-links. The heaters are attached to the Optical Bench structure and the CCD detectors.
OMI has the following on-board calibration/characterisation
A White Light Source (WLS) and Light Emitting Diodes (LEDs).
The white light source (C07 in the figure of the optical bench) is a Tungsten Halogen lamp with a UV-transparant quartz bulb. The main purpose of the WLS is calibration of the detector pixel-to-pixel gain variations.
Two green LED's, emitting at 570 nm, are positioned close
to the CCD detectors. The position of the green LED's in
the VIS channel is at the aperture 208. In the UV channels
two LED's are placed between lenses (110) and (111).
The primary use of the LED's is identification of bad detector pixels, to obtain the relative electronic gain values, and to calibrate the non-linearity of the electronics in flight.
The signals of the
read-out register after a draindump (which accompanies
every image) can be used to determine the readout noise
and the electronic offset values.
The WLS is used once per week. The LED's are used once per day for characterisation of bad and dead detector pixels, once per week to monitor the electronic gain values and once per month to monitor the non-linearity of the electronics.