The Neil Gehrels Swift Observatory

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Incorrect Exposure Times in some Aspect-Corrected Images

(Fixed in current pipeline as of December 1, 2005)

The exposure time keywords were incorrect for about 2% of the UVOT image data taken in 2005 March through 2005 September. Some earlier exposures are also likely to have problems. In some (but not all) of the cases the problem is caused by an error in the shift-and-add software in the instrument. This text file provides a list of these exposures and the correction factor needed for the exposure length. This problem is corrected in the current version (December 2005) of the Swift pipeline processing software. Exposure time keywords for the affected images will be fixed once the data is reprocessed with the current pipeline software.

For data processed with earlier versions of the pipeline, the reported exposures lengths may be incorrect. The problem can be corrected by following the recipe below.

  1. check for problems by comparing the start times (in mission elapsed seconds) in your data file with the start times in the list of affected images.
  2. make note of the ratio of measured/expected counts for any of your exposures that are in the list
  3. correct the exposure time keyword in only the extensions in question to reflect the equivalent livetime of the UVOT detector for the exposure in question. This can be done with the tools fv or fmodhead.

Example:

Let's fix the fourth extension in 

sw00131560001uvv_sk.img
whose measured to expected count ratio is shown in the list of affected images as 0.0732: 
#  MET         File Name             Exposure   Expected    Actual  Ratio
#                                                 Counts    Counts

139512177 sw00131560001uvv_rw.img vv139512177I  36006149   2635110 0.0732


The original EXPOSURE keyword in the image is 1500 seconds: 
EXPOSURE=     1500.44392000555 / Total exposure, with all known correction

Create an ASCII file called newtime with the original exposure time corrected for the ratio of true counts in the image to expected counts: 

EXPOSURE=     109.832 / Total exposure, corrected for aspect error
Run the tool fmodhead to insert the new value into the file in the appropriate extension:
fmodhead sw00131560001uvv_sk.img+4 newtime

Once the exposure time has been corrected in this way, standard tools that calculate magnitude based on measured counts over the elapsed exposure time (such as uvotmag) will return the correct answer.



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