Affiliation
Variable Stars South (VSS)
Sat, 09/07/2019 - 07:06
Hi
I have been having trouble with VPhot not regognising the target star in my images.
The images are of the recurrent nova V3890Sgr. I have attached a screenshot of the problem. The images are plate solved and the comparison stars are recognised. It is as if the target has the incorrect position as the star is clearly visible and well exposed. It has happened with images through all VRI filters so not specific to one image. I can share the images of needed.
Any suggestions?
Terry
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V3890problem.PNG633.23 KB
Terry,
I occasionally have the same problem. I find it interesting that it always seems to be the target. I don't know if the target's coordinates somehow have been incorrectly reset, or perhaps these stars have a large proper motion.
My solution is to zoom in on the star enough so that when I click on the star I get "Star 1" in the list. I can then change it to a "Fixed target" in the information box and give it the correct name. The important next step (which I sometimes forget) is then to Save the user defined sequence. This has worked for me, and the problem doesn't seem to come up again for that star.
Phil
Hi Phil/Terry:
Good workaround although one would hope that it was not necessary!
Worth comparing the RA/Dec of the center of the visible star in VPhot image and the quoted RA/Dec of the variable in VSX/VSP, if you want to? Would be interesting? Also, worth checking to see if you are using the default centroid settings (5/5) in VPhot or something smaller?
Ken
Ken,
I my case the default (5/5) centroid settings were being used when this happened to me most recently. I did try enlarging the search radius (...don't remember how much), but that didn't seem to work.
Yes, it would be interesting to compare the RA/Dec from the image with the VSX information. I did also consider this, but when the workaround was successful I just wanted to get on with business. Next time I'll look at that. Maybe Terry could still do that.
Terry, if you use the workaround, could you please then give us the coordinates of the target centroid for comparison with the VSX position.
Phil
Thanks guys.
I have played with it a bit. Even zoomed in it won't find the centroid unless I change the settings to 10/10. It then finds it with no problem.
The measured centroid position is about 2arcsec off from the catalogue on Dec but the RA is almost identical.
Interestingly I took more images a few days later with the exact same equipment and there was no problem measuring them. I'm still not sure what causes this.
Terry
Terry, Ken,
I wonder if the problem may be in the GCVS. When you loaded the variable star catalog to make your sequence is it possible that you used the GCVS catalog and not the VSX? In my case, most recently with GU Sgr, I think I probably did use the GCVS.
In my BSM_Berry field for GU Sgr there are 14 variable listed in GCVS, but the variables listed in VSX are just too numerous to count, I'd guess more than 100. When I made my sequence it was a lot easier to weed out the unwanted targets from the GCVS data rather than trying to do it from the VSX variables.
Phil
Nope.
The VSX got the position information from the GCVS. The current position in the GCVS is identical to the position given in VSX.
VPhot probably needs an additional, third option for importing variable stars, something like "Variable Star Index (reportable only)". When I grab VSX stars, I often have to delete a dozen or more of asterisked stars that have no AAVSO/AUID code. I would love an option to just ignore them--I can't report data for them anyway.