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Vphot "Got You" - Transformations

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Ed Wiley_WEY
Ed Wiley_WEY's picture
Vphot "Got You" - Transformations

One of my CCD1 students was having trouble with calculating his transformations via a spreadsheet and contacted me. I suspect the problem was a "got you" that occurs when downloading comps. Here was my response to him.

"I don’t know if you are aware of this problem or not. If you have several comps of the same general magnitude 112_1, 112_2, etc., they are not always the same when you download comps. Once you have a particular sequence saved, then they are stable. The problem is matching them to the photometry table. Why? Because the photometry table has no trailing numbers, simply 112, 112, etc. So, if you had different sequences for each filter, there is no guarantee that the comps are the same for each (added in this post: even if they have the same label subscripts, i,e, 121_1 and 121_1 may not be the same comp).

So, you need to match the comps in your saved sequence with the comps in the photometry table and save those comps in Excel. To do this you must match by RA and Dec since there are not unique identifiers in the table or when you call up details in VPhot (that I can see).  I have attached a screen shot that shows a “first guess” of what may be suitable for your calculations. You can certainly modify if. I have also included the M11 photometry table. Steps

  1. Call up the M11 comp stars in VPhot and delete all but the ones on my document.
  2. Save this sequence with a unique name. You will use it for all filters, not just V.
  3. Make a copy of the photometry table on another sheet.
  4. Eliminate all stars in the new photometry table except those in your sequence. Do this by matching RA and Dec (you can find that on the details page of each comp.
  5. You now have your comp stars with correct information for BV and R."

Phil Sullivan relates that this may not be a problem while using TG. I would not know about that as I use Excel.



mgw's picture
TG works around VPHOT labeling

TG avoids the problem associated with VPHOT's labeling of comp stars.  It does this by ignoring the VPHOT label (e.g. 112_1).  It matches the comp stars based on their reference magnitude and B-V values contained in the VPHOT download report.

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