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Questions about Swift? Try our guide to Swift or the guide to data processing and analysis. If these don't solve your problem, please feel free to e-mail us at swifthelp@le.ac.uk.

List of acronyms and abbreviations

We are located in the Department of Physics & Astronomy , at the University of Leicester (directions).

Position Determination

The source detection routine uvotdetect detects all sources in a UVOT sky image, centroids and can optionally perform photometry. It works with individual or summed images but the FITS extension must be given; the image file can be used while zipped. An example run for a u-filter sky image might be: 

>uvotdetect plotsrc=yes 
Input image file[]:sw00374210000uuu_sk.img+1 
Output file name[]:detectU.fits 
Exposure map file name or NONE[]:sw00374210000uuu_ex.img
Detection threshold (0.5:100) []:3

You should, of course, check that the images were successfully aspect-corrected (ASPCORR = 'DIRECT' in the image header) before analysing the data, otherwise any positions you determine will likely be incorrect. While it is possible to run the aspect correction by hand (see Summing UVOT images), if the automatic aspect correction failed, it is unlikely a simple re-run will solve the issue.

The output file detectU.fits contains a table of detected source positions, count rates and 1-sigma errors for each parameter. The option plotsrc=yes also plots the field in DS9 and marks detected sources, producing something like this:

If you do not give an exposure map, then the exposure time is read from the EXPOSURE keyword in the FITS header of the image. If an exposure map is given, then you should add the option expopt=BETA for individual images or expopt=ALPHA for summed images.

To perform photometry on the detected sources, set the option calibrate=yes - this includes coincidence loss correction. Generally, the task uvotsource is preferred for photometry, however this method should agree within the errors with uvotsource results for point sources.

For extended sources, uvotsource must be used; this uses circular apertures for source extraction as was used in photometric zeropoint calibration, rather than ellipses as used by uvotdetect.

Please note that, if there are clearly sources in the image which are not being detected, try running uvotdetect including zerobkg=-1 on the command line. This forces the script to calculate the background outside SExtractor, and can help in some situations.

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