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BVRI data on N Sgr 2012 No. 4

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jrsquid3's picture
BVRI data on N Sgr 2012 No. 4

So, I've been taking filtered BVRI on PNV J18202726-2744263 (thread: in BVRI with standard Johnson filters from Sierra Stars.

Now since there are no I and R comp stars in the field, I'm trying to use VPHOT to create a time series to compare them. It certainly looks like I have something there. I'd like to "follow it down" as a project for myself, and possibly track it down to limiting mag for SSO's 0.61m.

In any event, I'd like to better calibrate my data, so that I'm not wasting my time on this. I guess I'm trying to find/create a V-I and I-R set for this field.

I see that I'm a newbie here, and I'm using this as a good path in.


KTC's picture
Rules of thumb - when there is nothing else available

"...since there are no I and R comp stars in the field...."

There are comp stars, but without published I/R mags.

Check this out:

...but my hunch is that other people have already requested I and R mags for this field because of this new object.

If you want to manually put in some *approximate* I/R magnitude data into VPHOT for those comp stars, then you can use some rules of thumb:

V-R = 0.6 * (B-V)  (again, this is approximate, and I don't know how much reddening will affect this rule in this particular piece of sky.)

As an example, if the 109 comp in this field (AAVSO unique ID:  000-BKN-952) is V mag 10.85, and B-V = 0.84, the V-R = (approx) 0.5.  therefore the R mag is approx. 10.35.  (Note, the source magnitude info for this star is Tycho...caution!...may have large errors!).

And V-I is approx. 1.15 * (B-V).  That makes the I mag for this star to be about 9.85 (because the approx. V-I color index is about 1.0)

This will get you in the ballpark.  But after proper comp star mags have been published for this object you will have to make adjustments to the results you've obtained...typically that means you'll add or subtract a small value to all of your results...and the correction amount will be different for each filter.  (Since these are Tycho mag's...I bet there will be corrections to all filter bands!)

Keep the questions coming.

I hope this helps.

jrsquid3's picture

It does help.  But I think I'll need to find some other fields with some known stars that are nearby andgo back and forth between them, preferably and different airmasses, so I can get the color corrections.  It'll burn up a lot of time on a scope like Sierra Stars, but it'll give some I/R standards to at least 4 other variables in the field.  I just have about a week or so to put it together.  That's no fun since Work intrudes, and I'm doing a big thing for the upcoming Curiosity EDL, and I'm getting oe a cold (do-dah do-dah day).

I know, cheese with that whine?, but anyway, I've done a SIMBAD search on nearby objects:

and it seems that if I'm willing to stray afew degrees away, which I must, then I can at least get a good handle with some nearby HD stars.  I could go all the way up to Landolt standards, but I need to prowl some more.  


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