It is sometimes desirable to sum UVOT exposures to improve the signal-to-noise, or to detect faint sources. This tool simplifies summing a multiple extension FITS image file. It determines the range on the sky of the input images, checks that the input image filters match and calculates time related keywords.
The input exposures are assumed to be in SKY coordinates. No attempt is made to align individual exposures to a common frame of reference. Use uvotskycorr to do this. The output pixel size is set to "pixsize" degrees. If "pixsize" is less than or equal to zero then the output pixel size is equal to size of the largest input pixel. The user can exclude exposures using the "exclude" keyword.
Two rebinning methods are available. GRID assumes that all images are oriented in the same direction. This means that image rotations are ignored during image rebinning. XIMAGE makes no assumptions about image rotation. Both rotation and translation are performed during the rebinning.
This tool will sum exposures with different frame times within the constraints of the flexframetime and ignoreframetime parameters. If images with differing frame times are summed, the UVOT photometry tools will yield questionable photometry on the summed images. For this reason, by default only slight variation in the input image frame times is allowed. If images with differing frame times are summed, the frame time of the sum is estimated as the exposure time weighted mean of the input frame times.
The primary output HDU contains keywords taken from the primary input HDU. The first extension of the output contains the summed image.
The special value ASPCORR:NONE can be used to exclude extensions that do not have the ASPCORR keyword. The special value DUPEXPID can be used to exclude all but the largest of input HDUs sharing the same EXPID- this avoids including IMAGE&EVENT mode exposures twice in the sum. The special value SETTLING can be used to exclude extensions with OBS_MODE=SETTLING. The special values OVERLAP:(WEAK|STRONG) will exclude any images which do not overlap the transitive closure of images overlapping the primary image (which is determined by finding the image which has the largest overlapping exposure or number of overlapping images). The special value DEFAULT is treated as exclude=ASPCORR:NONE,DUPEXPID,SETTLING.
The mask file can be created by running uvotexpmap with the "maskfile" parameter. The mask uses "1"s to indicate a pixel to be included in the summed image and "0"s for pixels to be excluded.
hdu:weight
where the primary HDU is number 0 and the first extension is 1. Entries are optional- HDUs with no corresponding entry are assumed to have weight unity (1.0).
When a weight file is provided, outfile is written with EXPOSURE = 0 to indicate that photometry is not appropriate.
Use ignoreframetime=true to sum input images regardless of the frame times.
1. Run uvotimsum specifying input file and output pixel size, overwriting the output file (which will be prompted for) if necessary
uvotimsum infile=swuv.img pixsize=0.0001 clobber=yes
2. Run uvotimsum excluding HDU 3, the HDU named RALPH, and HDUs 6-14
from the output sum
uvotimsum exclude=3,RALPH,6-14
3. Run uvotimsum to produce an exposure map corresponding to the
output of example 2
uvotimsum infile=swuv_ex.img expmap=yes exclude=3,RALPH,6-14
4. Run uvotimsum with a weightfile
Say we have an image file with two extensions and want the sum of 2.5 times the first extension and 0.5 times the second extension. Create the file weights.txt with contents
1: 2.5
2: 0.5
and issue the command
uvotimsum infile=swuv_ex.img weightfile=weights.txt
uvotexpmap, creates UVOT exposure maps and mask files
ximage, has the ability to interactively sum images