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Using Vizier catalogs to select comparison stars

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RMH
Using Vizier catalogs to select comparison stars

Hi all,

I'm using c-munipack to analyse a collection of 25k images of the same star region (about 50 arcminutes, 10k stars per image).
This resulted in finding a few unknown variables (hurray).

C-munipack only gives differential magnitudes, and I'd like to calculate absolute mags for AAVSO reporting. Given the fact that these are variables not known to VSX, there are no official comparison stars.

Now for my questions:

- does anyone have any python scripts (e.g. astroquery) to collect magnitude and color index for any star? APASS catalog looks interesting. I have Simbad queries but seems to be missing B-V entries, correct or my error?

- does this procedure sound right:
-- get the Magnitude and B-V data of the suspected variable
-- take all stars in the star field, filter on correct magnitude range (+/- 1 ?)
-- take filtered stars, filter on B-V (range?)
-- take top 10 stars with lowest std dev (to exclude other variables)
-- Use these 10 stars as comparison stars with c-munipack

Any pointers to documentation, software, etc are appreciated

regards,
Mike

pox
pox's picture
I can see a lot of 'variables

I can see a lot of 'variables' here. First off, you need to know the constraints of the individual catalogue you are querying, how the data was acquired and so on. Some things can be performed by algorithms that you clearly want to use but you first need to know that you are using the algorithm on a viable data set. Then I think you need to personally select comparison stars (for instance, you suggest 10 stars, but what if your viable data set contains either far more or far less than this number?) You also need to take into account who will be using your results. If intended for (say) visual observers, stuff like position angle effect has to be taken into consideration. Sometimes you may find that there are no suitable comp stars based on your intial filtering methods. This again relies on human intervention.

RMH
Hi,

Hi,

Thanks for your response, I'll try to clarify my request: I have 25.000 CCD observations of one part of the sky (47' x 47'). The goal is to able to produce absolute magnitude lightcurves for all variables in this field. There are a number of known variables in that field, so for those I could use the 'official' AAVSO comparison stars.

Suppose I find one other star in this field which is clearly variable but unknown to AAVSO. How do I go from differential photometry using the check stars which c-munipack automatically chooses to a set of 'acceptable' comparison stars? Manual would probably be best, but there is not one but hundreds of 'candidate' variables for whom I'd like the best possible comparison stars to increase the quality of the data.

If the algorithm doesn't find a good comparison set, I can always fall back to the automatic selection of c-munipack

I agree that the quality of the catalog is important, my hope is that someone here has experience with this kind of problem and could guide me to a suitable catalog. My best bet seems the AAVSO commisioned APASS catalog, but I'm not sure if this catalog is ready and approved for use.

regards,
Mike

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