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7.2 Overall Calibration

The overall strategy is discussed in the SWS calibration papers. In summary internal calibration sources are used to tie together extensive ground based calibrations with in-flight observations of astronomical calibration sources. The three types of calibrations that are carried out are:

Determination of focal plane geometry, array geometry and alignment

The relative positions of the various entrance apertures and detectors need to be known to ensure that one grating wavelength calibration can be applied to all combinations of entrance slit and detector. The respective offsets have been calibrated in instrument level tests on the ground. Beam profile measurements and wavelength calibrations on astronomical sources are used for in-orbit checks.

Determination of wavelength scales

In ground-based instrument level tests, the relation between grating position readout and physical grating angle was determined by measurement of wavelength references in the form of vapour absorption lines (H tex2html_wrap_inline7437 O, NH tex2html_wrap_inline7439 , HCl). The spectral features provided by the internal grating wavelength calibrator have been tied to that scale. For the Fabry-Pérot , the position-gap relation has been determined from the spectrum of the internal F-P wavelength calibrator, which is known to high accuracy from fourier transform spectroscopy. H tex2html_wrap_inline7437 O and NH tex2html_wrap_inline7439 vapour absorption lines have been used for additional checks and for determination of the variation of effective F-P gap with wavelength.

In orbit, the grating position-angle relation and the F-P position-gap relation was re-established during PV , first using the internal calibrators and then astronomical sources. It is checked regularly during the mission.

Determination of photometric sensitivity

The photometric sensitivity has been determined on ground by scanning the spectrum of a calibrated blackbody source within the test cryostat. These tests resulted both in detailed spectral response functions for the various AOT bands   and in a first calibration of the signal created by the internal stimulators. The signal from the stimulators will be used to monitor variations in the broad band sensitivity of the detectors, e.g. due to memory effects.

In orbit, an extensive program was executed to determine the photometric sensitivity on astronomical sources. It was started during PV and carries on at a lower level during the mission. The dark current  measurements needed both on ground and in orbit are done with SWS shutter closed.

next up previous contents index
Next: 7.3 Flux calibration Up: 7 SWS Calibration Previous: 7.1 Introduction

K. Leech with contributions from
the SWS Instrument Dedicated Team (SIDT)
and the SWS Instrument Support Team (SIST)