26 July 2004
Optical filters, part of the High Resolution Dynamics Limb Sounder (HIRDLS) instrument, manufactured at the University of Reading have successfully been launched on the Earth Observing System (EOS) Aura mission from Vandenberg AFB, California.
The HIRDLS instrument is a Limb Viewing Radiometer which is scheduled for launch on the NASA Earth Observing System (EOS) Aura mission. Measurements of radiance from the thermal emission of trace chemical elements in the atmosphere are performed through a cooled infrared optical system. It has 21 spectral channels precisely defined by our filters contained within a focal plane detector assembly array. We are also responsible for the 21 "Ghost Image" suppressing filters positioned at the cold (60K) detector plane. These are very small in size - 1.180 mm x 0.440 mm.
The infrared filters manufactured at the University are vital elements used in the instrument to select the gases whose emissions are to be measured. They do this by only allowing a very tightly defined band of wavelengths of infrared light through them - very precisely placed in regard to the gas emission spectral line. The filters reject all other wavelengths. The instrument is designed to look at 21 different gases/height combinations thus needing 21 different wavelengths to be selected by the filters. The light is gathered from the limb and focussed down through a system of lenses and mirrors passing through an array of 21 band defining filters on the way, and eventually enters the detector can. Here it finally passes through a further 21 filter array (whose function is to reduce the amount of optical cross talk) before it falls onto the detectors. The detectors are of very small size to maximise the ratio of signal to noise - this in turn means that the filters immediately in front of them are also very small, 1.39 mm by 0.63 mm (about the size of a grain of sugar). These filters operate at the detector temperature of 65K.