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VPHOT ID'd wrong star as the variable

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Lew Cook
Lew Cook's picture
VPHOT ID'd wrong star as the variable

I recently reported on photometry of V1032 Oph. I have retracted the data because there was confusion in VPhot on which was the variable. In developing my sequences for V1032 Oph, I used the 138 and 139 as the check and comp star. VPhot originally ID'd the 130 star as the variable, and gave me  garbage results, naturally. Even after I revised the sequence and used the 130 star as the comp star, VPhot would get confused with the 130 star, and occasionally substitute IT as the variable. VPhot was also using the 130 star (which has V = 13.002) as the comp star, so I got 13.002 or 13.003 as the CV result. Has anyone else had a similar experience when a much (2.5 mag) brighter star is very close to the variable?

Thanks go to Josch Hambsch for pointing out my erroneous photometry.

MZK
MZK's picture
Centroid Difficulty

Hi Lew:

Yes, this is similar to the difficulties which occur in the "double trouble" stars. IF two stars are very close to each other (as in overlapping or almost) the target centroid may move to the brighter star. This problem is not unique to VPhot but perhaps a bit more prevalent. I'm not sure about this. Changing the centroid search settings may help BUT in a time series if you use the default FWHM it may occur unexpectedly. It is a known issue and I do not have a perfect answer to fix it. Gordon Myers uses the fixed offset tool in AIP4Win to help with this issue in his images.

Ken

 

Lew Cook
Lew Cook's picture
VPHOT ID'S WRONG STAR

Ken, that's what I ended up doing. Only trouble is iTelescope sends images in zipped files. To get them separated and in a file and unzipped required ~500 clicks and mouse movements for a set of ~180 images. Took 2 hours just to get the images free and in one file.

That's why I like VPhot. If anyone has a secret to getting all the individually zipped images into a single file, unzipped, please let me know.

Lew
 

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