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The infrared detectors used by SWS exhibit memory effects. That
means, their responsivity is not constant and their response to a sudden
flux change is not `instantaneous' at the time resolution of the SWS
readouts, but exhibits multiple time constants. Both effects
depend on the present illumination, the illumination and ionizing
radiation history during the last hours, and the detector operating
parameters (bias, temperature). The most important impacts on SWS data
are long-term drifts in the flux calibration scale and distortions of
spectral features by short-term memory effects.
Various measures have been taken to minimize the impact of memory effects:
- Detector operating parameters have been selected to be in regions
of more acceptable memory effects.
- Internal flux calibrations and frequency switching are used to
monitor long term responsivity drifts.
- The AOTs have been designed for better elimination of short term
memory effects. All line spectra are scanned at least twice for
redundancy, which in case of the grating means: the grating going up and
down to identify line asymmetries caused by memory effects. All grating
AOTs have been designed such that differential memory effects between
the 12 detectors of the arrays do not directly transform to false
spectral features, but only to small errors in flux
calibration .
Despite these precautions, memory effects will still influence the flux
calibration and the quality of line profiles to a higher extent than in
wavelength regions where more well behaved detectors are in use. Methods
are being developed to reduce these further in the course of the off-line
data reduction.
SWS Consortium
Wed Aug 7 17:20:29 MET DST 1996