[Aavso-photometry] Determination and use of BVRI transformation coefficients with Maxim DL

[Tom Richards]

Brian Warner's Canopus program (see http://www.minorplanetobserver.com/) has
a routine called StarBgone for removing in-the-way stars from an image set.
It's pretty much necessary for crowded-field asteroid work (as I'm
discovering right now!), but I haven't tried it on a star overlapping a
variable. It would be interesting to hear comments from those who have.
Tom Richards

-----Original Message-----
From: aavso-photometry-bounces at mira.aavso.org
[mailto:aavso-photometry-bounces at mira.aavso.org] On Behalf Of Arne Henden
Sent: Sunday, 13 January 2008 03:35
To: Lionel Catalan
Cc: aavso-photometry at mira.aavso.org
Subject: Re: [Aavso-photometry] Determination and use of BVRI transformation
coefficients with Maxim DL

On Jan 12, 2008 10:46 AM, Lionel Catalan <lcatalan at lakeheadu.ca> wrote:
> Following the procedures outlined in the AAVSO CCD variable star
> manual, I determined the colour transformation coefficients (Tbv, Tvr,
> etc) and the terms Qbv, Qvr, etc for my system by taking sequential
> exposures of M67 with B, V, R and I filters. Since all the stars were
> in the same frame and the elevation was high, I did not include first
> order or second order extinction coefficients in my analysis. Maxim DL
> requires that I select a reference star and set its magnitude to
> calculate the magnitude of all the other stars in a given frame.
> Therefore, the determined numerical values of the terms Qbv, Qvr, Qri
> and Qvi are dependent on the particular reference star that I
> selected. If I now want to use my colour transformation coefficients
> for another star field, I assume that my values of Tbv, Tvr, etc would
> still be valid, but I wouldn't be able to use the same reference star,
> and therefore I would have to recalculate values of Qbv, Qvr, etc
> based on some standard star in the new field. Does this make Maxim DL
> innapropriate or at least inconvenient for using transformation
> coefficients?
>
It is clumsy, but ok.  You just select one of the sequence stars
for that new field as your "reference star", and use your Tbv etc.
with respect to the magnitudes that are returned by MaxIm.
In general, you can do the color-correction transformation on any
field, whether conditions are photometric or not.
When doing this kind of differential photometry, where you are
actually calculating something like

Vstar = (vins_star - vins_comp) + Vcomp

what happens is that the zeropoint is constant for both stars,
and the subtraction will remove it from the solution.  You can see
the full derivation in my old book, Astronomical Photometry.

Does MaxIm actually apply the coefficients, or are you using ancillary
software (like an Excel spreadsheet) to do the transformation?
Arne
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