Optical transmission filters for space spectrometers

Spectrometers are widely used for observation of Earth and planets from Space. Traditional instruments are based of push-broom methods while compact instruments, without moving parts, are presently under study. Special optical filters with non uniform performance, attached to a CCD detector, allow the construction of a compact spectrometer that offers great advantages because of its reduced weight and dimensions. These filters show a narrow transmission band which peak wavelength is displaced along the surface of the filter itself. In this way the image of the observed object will be completely detected while the CCD is moved with respect to the object surface. The variable filters are able to work from ultraviolet to infrared and are obtained by depositing a graded coating either on a quartz substrate or directly on the CCD surface. Depending on the spectrum of interest, different materials are employed. Silicon oxide, tantalum oxide and silver have been chosen for the range 400-2500nm. The filter dimensions are of the order of few millimeters. The design and technique of fabrication of such filters will be described and results on their optical performance will be reported.