The topics below were discussed at the 29 January LWS Calibration Meeting
in Vilspa Spain.
Effect of Cosmic Ray Glitches
This topic was covered in the
7 Feb 1996 letter sent by the Vilspa ESA helpdesk. The cosmic
ray hits are ocurring at the rate which is at the upper end of the rate
expected before launch, and have had a greater effect on the LWS detectors
than expected. As the letter stated, cosmic ray hits have required that
the nominal responsivity of the detectors be reduced (by commanding a lowering
the bias voltages in the integrating circuits). This reduction in sensitivity
is characterized by the table provided in the letter. These cosmic ray
hits have both increased the noise and have forced a loss in responsivity.
When a detector is struck by a cosmic rays, there are effects in the responsivity
of a detector immediatly following the hit. The responsivity fluxuates
or drifts, adding low frequency noise to a spectral scan.
To help the user identify the data that has been affected by glitch-induced
detector instabilities, a second SPD (Standard Product Data) file may be
included in the user-delivered data products giving the un-deglitched data.
Users may then be able to see where the worst hits ocurred and assess the
quality of the subsequent data for themselves.
A second problem (discussed below) is that of "fringes" in the data.
To assist the users in correcting fringes, the final data product
files may also include a column of flux data that has not been
divided by the nominal bandpass filter functions for each detector. In
this way, the user may more easily rescale the data using one of several
LWS-supplied bandpass filters that can correct for fringes.
Also, during a single revolution (one sidereal day in which there are 16
hours of data collection), there is a slow change in the responsivity of
the (irradiated) detectors over time. This drift is removed by the
data reduction pipeline.
One consequence of the cosmic ray glitches is to make "fast mode" for
the all AOTS (grating and Fabry-Perot scans) the preferred mode.
Cosmic ray glitches are thought to produce correlated noise in the detectors,
making the most conventional of integration schemes potentially non-optimal.
The search for the best averaging scheme or median extraction scheme for
data reduction is actively being sought. Note, the observer receives all
scans individually that make up an observation. The pipeline processing
does not average scans across the same spectral range.
More information on glitches is available.
Other Sensitivity Issues
The noise in the detectors in ideal conditions should be independent
of the signal strength. This however is not found to be the case for LWS.
With higher signal strength the noise level rises in all detectors (except
SW1), and these rises are now being taken in to account by the PGA, PH,
and time estimator logic. Fortunately, the noise rises severely only with
the strongest of signals. The effect will therefore not be a problem for
those measuring the flux of strong continuum or strong line sources, but
will be a factor for those looking at weak lines on strong continuum.
Grating Range Scans
As mentioned in the
7 Feb 1996 letter from ESA, those doing full range grating scans, LWS01,
are given the following tip: "if you can do without the range 187um - 198um,
and you can live with a reduction in sensitivity (about a factor of 2)
in the range 170um - 187um, then specify a range of 43um to 170um."
You will still get data up to 187um but the observation will take 20% less
time. This may be useful for those whose observing times are driven by
the short wavelength sensitivities and are already beyond what they need
for sensitivity at the long wavelengths.
More information
on the scan range is available.
Fabry-Perot Work at Long Wavelengths
The FP that operates at long wavelengths, FPL, contrary to expectations,
requires variable alignment that "moves" with wavelength. The LWS AOT03
has not yet been commisioned at long wavelengths for this reason. FPL will
become available when a patch to onboard software is installed. The short
wavelength FPS is covering wavelengths up to about 95um and is working
well.