5.3 Astronomical Calibration procedure

5.3.1 Wavelength calibration

5.3.1.1 Grating calibration

Grating mode calibration

This is done using first a full grating scan on a source with known lines to identify the lines suitable for wavelength calibration, those that are strong enough to give good signal-to-noise and that are narrow enough to determine an accurate line centre. Individual scans of these lines are then made to provide accurate line positions. These line positions are fitted to the known wavelengths with a third order polynomial to give a grating calibration that is adequate for the low resolution mode of the instrument.

Spectral element

The size of the grating spectral element is determined using the mixed mode observations. Detailed FP scans are made with the grating at a fixed position, giving a very detailed profile of the grating response function: from this, the size of the spectral element (Full Width Half Maximum) is determined. The observation is done for several detectors and grating positions to get the wavelength dependence of the profile and spectral element.

5.3.1.2 Fabry-Perot calibration

Wavelength Calibration

The wavelength calibration of the Fabry-Perots is carried out by observing strong, narrow lines with known wavelengths. The centres of these lines are determined as a function of the commanded position of the Fabry-Perot. The corresponding order of interference and etalon gap are calculated abd the gap is fitted as a third order polynomial in commanded position.

Spectral element

The size of the Fabry-Perot spectral element is determined using the wavelength calibration observations. The Fabry-Perots are scanned over lines known to be narrow, on sources with a reasonably well known velocity dispersion. The Fabry-Perot profile is determined by deconvolving the observed lines shape with the expected line shape given the source structure and velocity dispersion. From this the size of the spectral element (Full Width Half Maximum) is determined. The observation is done for several lines to get the wavelength dependence of the profile and spectral element. Note that at this moment this calibration has not been done, due to the lack of suitable sources for this calibration. It is expected that the calibration source G0.6-0.6 can be used for this purpose.

5.3.2 Relative Response and Flux calibration

5.3.2.1 Grating relative response

The absolute flux calibration is done by directly comparing the detector photo current of a standard source with the known (model) flux (corrected for the optical characteristics of the telescope and the LWS instrument).

The LWS has been calibrated by observing Uranus, a source for which a good model exists and which is point-like in the LWS beam. At each wavelength the expected flux density (power per unit area) from Uranus, within the spectral resolution element, is calculated from the model and the corresponding photocurrent is measured. This calibration therefore provides a direct relationship - for a point source - between the photocurrent and the flux density at the entrance pupil of the ISO telescope. This relationship is then used to assign a flux density to the observed photocurrent from any other source.

The observations for the flux calibration are repeated for several bias levels. The grating relative response needs to be known before the FP relative response calibration and the FP transmission calibration can be performed.

For the photometric calibration and the relative response calibration several sources were used during PV phase and during the routine calibration observations. Table 5.1 gives the sources for the different calibrations.

Table 5.1: Sources used for the photometric calibration and the relative response calibration of the LWS grating and Fabry Perot subsystems. The primary source for the grating flux calibration is Uranus, the other sources are observed regularly for checking purposes.

5.3.2.2 Fabry Perot relative response

For the Fabry Perot response the FP is scanned with a high oversampling with the grating at a fixed position (mixed mode observations). Again here the source spectrum is removed (by means of a model or a grating scan). Finally the profile is normalized to the absolute transmission at a given wavelength (the FP transmission is determined in a separate procedure, see below).

5.3.2.3 Fabry Perot transmission

The FP transmission is determined by using scans of strong lines with both the grating and the Fabry Perot. Comparing the output of the detectors for these scans of the same line for grating and Fabry Perot gives the the Fabry Perot transmission at the wavelengths of the lines.