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Home > Data Analysis > UVOT > Summed Images

Summing UVOT images

You may want to sum images to detect faint sources. The tool uvotimsum will co-add the individual extensions present. More than one image can be given as input (comma-delimited, or the name of a text file contining a list of file names, one per line, preceded by @ symbol), and all extensions in the correct format will be summed.

Check that all your images are correctly aligned to the World Co-ordinate System: look in the image header for the keyword ASPCORR. If ASPCORR = 'DIRECT' then your images are already correctly aligned. If the default aspect correction has failed for the images in the archive (ASPCORR = 'NONE') then they cannot be co-added. In this case, performing a manual aspect correction using the tool uvotunicorr may allow you to correct for aspect shifts)

>uvotimsum
Name of input image file [ ] sw00374210000uuu_sk.img
Output file name [ ] seg000_usum.img

or

>uvotimsum
Name of input image file [ ] sw00374210000uuu_sk.img, sw00374210000uuu_sk.img 
Output file name [ ] seg000-001_usum.img

The input images can be used while gzipped.

The script uvotdetect can be run on this combined image (seg000_usum.img of seg000-001_usum.img) to determine whether the source has been detected, and uvotsource can be used to obtain the photometry measurements.

Please be aware that running uvotsource etc on an image formed via uvotimsum will not flag up data in low-throughput areas of the CCD. It is therefore advisable to check the separate extensions before combining data. Specific image extensions can be excluded from the addition; see fhelp uvotimsum.