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Small Scale Sensitivity Check

In the UVOT we have discovered some small areas where the throughput is lower than for the rest of the detector. The UV filters show the effect more strongly than the optical. In the UV the bad areas cover just 0.6% of the whole field of view, however this is 7% of the central 5’x5’ square. For the optical filters, 0.7% of the central area is affected.

HEASoft 6.30 and later

From HEASoft version 6.30, and CALDB version 20211108 (released 2022-02-25), onwards, the small scale sensitivity check is performed by default by the software.

The UVOT FTools will now by default use a mask to screen out sources detected on areas of low sensitivity on the detector. The magnitude of the source will be given as 99, and the corrected count rates for that source will appear negative. Additionally, the Small Scale Sensitivity factor - SSS_FACTOR in the FITS file - will be -99.9. These data should generally be disgarded.

A set of three masks are defined for each filter. The default level (Low) removes the smallest number of sources and is designed to cover all the most strongly affected locations. Users can choose instead to use NONE (i.e., disable the use of a mask), or a MID or HIGH level of screening.

The tools (specifically uvotsource, uvotevtlc, uvotlc, uvotmaghist, uvotproduct, uvotpipeline, uvotexpmap) now have the parameters sssfile (default CALDB) and ssstype (default LOW).

That is, running uvotsource is the equivalent of running

>uvotsource sssfile=CALDB ssstype=low

For more information on which level of screening is appropriate for your science requirements please see the release note.

HEASoft 6.29c or earlier

If you think your data may be affected by the areas of lower throughput, you should run uvotsource (or other relevant UVOT tools) with the additional command lssfile=sssfile5.fits (sssfile5.fits can be downloaded).

If the source of interest is located in a bad area, its magnitude will come out as 99 (and the Large Scale Sensitivity factor - LSS_FACTOR in the FITS file - as -99.9) and these data should be disgarded.
IMPORTANT: This check disables the usual LSS correction, so remember to reset this by including lssfile=CALDB for further analysis of sources not affected by the problem.

See here for more information.