5.3 Fabry-Pérot Flux Calibration

The flux calibration in Fabry-Pérot mode is more complex than for the grating. Ideally the relationship between photocurrent and flux for the Fabry-Pérots would have been directly established using observations of sources with known spectral characteristics. However, the transmission of the Fabry-Pérots is such that only the very brightest objects (Jupiter and Saturn) would have made suitable candidates for such observations. These have relatively poorly known far-infrared spectra and, even with sources as bright as these, the observations would have been prohibitively long. Therefore a boot strap method is used whereby the photocurrent is first converted to flux using the grating mode relationship and the signals from the illuminator operations; this also removes the signature of the instrument RSRF in grating mode.
From OLP Version 8 onwards, a `throughput correction' is applied, thereby giving the FP flux in units of W cm m. The throughput correction is the FP transmission multiplied by the FP resolution element, the two factors being undissociable in continuum observations; this has been derived using Mars as the calibrator.
The Fabry-Pérot photometric calibration is derived from observations of Mars made with the FPs set at a fixed gap and the grating scanned over its full range. In this observation mode the various order and wavelength combinations of the FP are selected as the wavelength falling onto the detectors changes due to the grating movement. An example of the output data are shown in Figure 5.3. The peaks of the orders represent the convolution of the instrument relative spectral response (RSRF), the spectrum of Mars and the product of the transmission efficiency and effective spectral element width of the FP used.

Figure 5.3: Derivation of the FPS throughput with Mars `mixed-mode' observations. Red line: Third order polynomial fit to the peaks. Blue lines: .

Figure 5.4: Derivation of the FPL throughput with Mars `mixed-mode' observations. Note the break around m between LW1 and LW2. Red line: Second order polynomial fit to the peaks. Blue lines: .

With knowledge of the instrument spectral response from grating measurements and a model of the Martian spectrum the instrument and input spectrum can be removed giving the FP function versus wavelength which can be deduced by fitting the peaks with a low order polynomial (see Figure 5.3). In the case of the FPS this is a straightforward fit and there is no dependence on detector or grating order. For the FPL the situation is more complex as there is an apparent break between the detectors up to and including LW1 and from LW2 through LW5 (see Figure 5.4). This break has no explanation at present, but it is clearly present for all FPL observations and the derived calibration coefficients do correct for it.

The derived coefficients of for FPS and the two sections of FPL (SW4-LW1 and LW2-LW5) are stored in the calibration file LCTP and used in Auto-Analysis to correct all FP data to W cm m.