You may want to sum images to detect faint sources. To sum extensions within an image use uvotimsum
. If you wish to sum images from different observations, the separate FITS files should be merged first, using fappend
.
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 not, run
the task uvotskycorr
TWICE (first to ID
the sources and then use that output correction file to WCS-correct the
image).
You will need a star catalogue file
(get usnob1.spec here), and the spacecraft
attitude file *sat.fits from the auxil directory of your observation.
Assuming the ASPCORR keyword is not set to DIRECT, here are the steps to try and perform the aspect correction (however note that, more often than not, if the data are not already aspect-corrected in the archive, these steps will fail. In this case, performing a manual aspect correction using the tool uvotunicorr
may allow you to correct for aspect shifts):
>uvotskycorr catspec=usnob1.spec What to do (ID|SKY) [ ] ID Input SKY image file name(s)[ ] sw00374210000uuu_sk.img.gz Input corrections file[ ] NONE Input attitude file[ ] sw00374210000sat.fits.gz Output file name[ ] Ucorr.fits >uvotskycorr catspec=usnob1.spec What to do (ID|SKY) [ ] SKY Input SKY image file name(s)[ ] sw00374210000uuu_sk.img.gz Input corrections file[ ] Ucorr.fits Input attitude file[ ] sw00374210000sat.fits.gz Output file name[ ]
Once the aspect correction has been performed on all the images and extensions, they can be summed as required:
>uvotimsum Name of input image file [ ] sw00374210000uuu_sk.img.gz Output file name [ ] Usum.fits
or, if multiple Obs IDs are required (note the image must now be uncompressed):
>cp Usum.fits Usum_all.fits >fappend 001sum.fits Usum_all.fits
where 001sum.fits
was created using uvotimsum
on the segment 001 data (sw00374210001uuu_sk.img.gz). As many images as required can be added together in this manner. Use uvotimsum exclude=NONE
when summing the extensions in Usum_all.fits, since the aspect correction keyword is not copied when uvotimsum is run. uvotdetect can be run on this combined image (Usum_all.fits
) to determine whether the source has been detected.