VPHOT comparison stars

American Association of Variable Star Observers (AAVSO)
Fri, 09/28/2012 - 16:47

I am looking at my VPHOT display for a 30 second unfiltered image of CV variable LL AND taken with the Sierra Stars 24" in CA.  Comparison stars go down to and include the 170 comp star.  Yet when I look at the sequence for LL AND, there are three additional comp stars (176, 179 and 182) listed which are not displayed on the VPHOT image.  These three comp stars are shown when plotting a VSP genereated finder chart using a DSS image and a magnitude limit of 19.0.  Is there an arbitrary or an SNR based magnitude limit imposed on the comp stars that are displayed in VPHOT? 

Kevin Paxson - PKV 

American Association of Variable Star Observers (AAVSO)
I'm not sure if this is

I'm not sure if this is affecting your comp stars load but when looking at your image, up on the right under Tools, Settings there is:

Sequence file and catalog import
Only import stars with SNR >         20      that being the default setting.

I'm extremely new to VPHOT myself but in the video tutorials it was suggested your comp stars should have an SNR >= 100 if possible.

If you figure out what the problem is please post back to this forum thread.


Wayne Westlake

American Association of Variable Star Observers (AAVSO)
The "problem" may be in your

The "problem" may be in your perception.  It IS recommended that you USE only comp stars with a good S/N (>100).  But sometimes you can't, right?  When you upload an image into VPHOT and ask it to show you the AAVSO comp stars, it will show all of them in the catalog IF they have a S/N in that particular image greater than the limit you choose in the other drop down box.  So, if your VSP chart shows fainter comp stars than your image actually has in it, what else would you expect?  If you save the sequence, VPHOT will remember them but the next image you upload that has too low S/Ns for some (or all) of the stars, they won't be displayed.  Kinda makes sense, doesn't it?

YOU choose which stars you will use in the sequence for comps.  You should choose comp stars that area about the same S/N (and color) as your target star.  If your target star and all the available comp stars have low S/Ns, then you should consider taking longer exposures (and/or stacking) before attempting to measure them. 

If your comp stars are well-exposed and your variable has dropped down to a really low S/N (not uncommon) your variable measurement will be plagued by the uncertainties due to its low S/N but it is still a  worthwhile effort.  VPHOT will report an uncertainty commiserate with that low S/N but the uncertainties in the comp star will be negligble by comparison.