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Mon, 08/20/2012 - 21:01
I recenty entered transformation coefficients for my telescope definiton. I noticed that one get different results depending on which of the two sequences one chooses to use. For instance for a pair of images one through V and the other through R produces different results for the R sequence and the V sequence. Is there a procedure for deciding which sequence of a pair to use? Or is the difference considered slight enough that it doesn't matter.
..george
I guess I don't understand your question. What different sequences are you talking about? In the first place, both images are of the same field, right? Just two different filters? If so, there is only one VPHOT sequence which would identify your target, comp and check stars, right? The comp stars will all have photometry for the filters of interest. When VPHOT processes a single filter, it uses the comp photometry for that particular filter. When it does a transformation of two images of different filters it uses the appropriate comp star photometry on both the target star and comp star.
Now if you have more than two filtered images, say, V,R and I you have the possibility to transform the VR pair and the RI pair (also the VI pair). You might well get different results for the R* derived from the VR transform and the RI transform. I don't know what to say about that. Maybe Arne could comment. On the other hand, if you do a VRI transform (not implemented in VPHOT (yet)) you would get only one value for each filter, thus no confusion.
Many variables at play here.
It'll be easier to troubleshoot if you provide more specifics (telescope definition, sequence(s), images, etc.)
Feel free to share stuff to my VPHOT account: KTC.