4.4 Fabry-Perot Characteristics

Fabry-Perot Optimum Offsets

The Fabry-Perots have one fixed and one moving etalon. The movement of the moving etalon is controlled by three coils each driven by a servo-loop. In order to operate correctly the moving etalon has to be parallel to the fixed one to a very high degree of accuracy - this is achieved by applying a fixed offset to the control signals for two of the coils. These fixed offsets have been optimised during Performance Verification. The offsets are constant over the scan range for FPS, however, for FPL (owing to a fault in the wiring harness) they are constantly altered over the scan range to keep the etalons parallel.

Fabry-Perot Spectral Element Size

This is taken as the full width at half maximum of the Airy profile it varies with wavelength over the band of each Fabry-Perot but the resolving power, defined as , is more or less constant.

Fabry-Perot Miniscans

For extended range wavelength scans on the Fabry-Perots (AOT L03), a small range is scanned at each grating setting, these are termed miniscans. The range over which the Fabry-Perot is scanned for each miniscan is limited to keep the spectrum at the peak of the grating resolution element response.

Fabry-Perot Scan Range

The range over which the Fabry-Perot is scanned for L04 is expressed in numbers of resolution elements - the maximum range is 15 resolution elements.