Sometimes it is handy to visualize an entire visit or focal plane at once. We have a stack tool for this in pipe_drivers, called visualizeVisit.py
, but it uses camera geometry rather than WCS information to line up the CCDs. So I decided to try ds9.
The handy command ds9 -mosaic filename*.fits
is intended to do just this: display several images at once and align them with their WCS information. However:
It looks to me like the WCS for each consecutive file that ds9 accesses is even worse than the previous one. So if you are just displaying a few CCDs at at time, you probably won’t notice, but if you do more than about 10, things start to get weird.
So far, I have tried this and gotten similar garbled patterns for a variety of DECam visits for both calexps and diffims. I have been able to reproduce the view exactly with lsst-dev’s ds9 version 7.5 and my own machine’s ds9 version 7.6b8. I have also done a cursory look at the WCS-related header values and compared them to what ds9 thinks it has read in; they seem to agree.
If you would like to examine the particular images I have shown above, they are the difference image files which live in /project/mrawls/hits2015/rerun/slurm6/deepDiff/v411420
.
I should note that the utility of the ds9 -mosaic
command was most recently brought to my attention by Shenming Fu and a colleague at PCW 2018. They found a similar issue when displaying DECam images in ds9 and originally thought their astrometry had failed in some way, but we did not have time to get to the root of the issue.
Any insights would be very welcome.