How does VPhot determine object positions when doing astrometry - for example, when measuring separation and PA of 2 objects? Does it use UCAC-4?
VPHOT uses PinPoint to do the astrometry. PinPoint inserts WCS parameters into the FITS header that VPHOT can use when needed. Pinpoint is currently set up to use UCAC-3.
Thanks, Geir, for answering so quickly.
I like VPhot a lot, and find the interface particularly good. I have used it several times, successfully I think, to submit photometry measurements to the AAVSO.
I have worked through the video tutorials twice and read the embedded help material, but have not been able to develop a good understanding of how it works as yet - that is, how it works internally or algorithmically, as opposed to how to use it in a step-wise fashion.
Your short answer indeed tells me something that had escaped me until now: I use the iTelescope.com facilities, and tell their system to "plate solve" my images before sending to VPhot. It seems as though this command has the effect of plugging in certain WCS information into the FITs header. What is this information exactly? Is it simply the RA and Dec of the centre of the image as "solved" by PinPoint on the basis of UCAC-3 data?
Is there some accessible documentation available that might explain how all this works?
From looking at other postings in this forum, I presume you are the author of VPhot?
I hope you don't mind all these questions ...
Geir can correct
Geir can correct me if I am wrong but I believe I know the answers to some of your questions!
The WCS information is somewhat more extensive than just the RA and Dec. Usually at least the image scale and a rotation term are also found. But most WCS determinations allow even "higher-order" corrections like a radially-dependent plate scale, and RA/Dec "cross terms". Unfortunately,
there are a number of different ways that astronomers have adopted to deal with the higher-order corrections, so there is no single answer about the keywords you might see in your header.
VPHOT normally attempts to re-find the WCS, regardless of whether or not one exists in the header. That action ensures that VPHOT has the keywords it expects!
Since VPHOT uses PinPoint, you might want to visit the PinPoint website for more info: