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Scanner Mechanisms and Baffles

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Scanner Mechanisms and Baffles

During nominal measurements light enters the instrument via the azimuth (ASM) or elevation (ESM) scan mechanisms. Both are located below the lower part of the level 1 of the OU. Mechanically, each scanner comprises a mirror block, bearings, a drive motor and encoders. Bearings use a special lubrication reducing life limitation risks in the scanner’s quasi-continuous in-orbit operations. The scanning mirrors have uncoated, polished aluminium surfaces with a size of 90 mm × 60 mm for the ESM and 125 mm × 110 mm for the ASM. Whilst the ASM captures radiation coming from regions ahead of the spacecraft, the ESM either views the ASM or the region directly underneath the spacecraft. In the first case (limb observation), the light collected from the ASM is reflected into the spectrometer, in the second case (nadir observation) the ASM is not involved in the optical path.

Scanning is required in order to steer the Line-of-Sight both for executing particular observation geometries and for collecting light not only from the limited size of the ground projection of the Instantaneous Field of View (IFoV, see below) but from a wider ground scene. Although both scan devices can be rotated by 360°, baffles limit the effective field of view. This results in the Total Clear Field of View (TCFoV) which depends on the observation mode as listed in (table 2-4) and sketched in (fig. 2-8)

 

Observing Geometry  Total Clear Field of View (TCFoV) 
Nadir  32 / 31 (ESM, across track left / right) 
Limb, Occultation  88 (ASM, azimuth 316 - 44 ) 8 (ESM, elevation 19.5 - 27.5 from X-Y plane downwards) 
Sub-solar  1.7 (azimuth) 14.8 (ESM, elevation 53.7 - 68.5 from X-Y plane upwards) 

Tab. 2-4: SCIAMACHY Total Clear Field of View

 

click to enlarge

fig. 2-8:

image
Sketch of SCIAMACHY's TCFoV and observation geometries. (Graphics: DLR-IMF)
 

For the limb and occultation LoS, the baffles provide a symmetric range on either side of the flight direction while vertically they restrict viewing from slightly below the horizon to an altitude of about 380 km, i.e. well above the top of the atmosphere at 100 km. The nadir LoS is limited to an area of about +31° (right) to -32° (left) across track. For a special type of measurement, the rectangularly shaped Nadir Calibration Window (NCW) can be opened temporarily allowing light incident from above to enter the instrument via the ESM mirror. Its elongated TCFoV of 1.7°´14.8° is designed to view the sun at high elevation when the spacecraft crosses the orbital sub-solar point.

 

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