[Aavso-photometry] Differential transformation

[Chuck Pullen]

Geir - in an ideal world, your approach would be perfect.  However, for
determining transformation coefficients, you want a wide range of color as
well as stars of about the same magnitudes so they have about the same
signal to noise across the color range.  And, of course, the stars need to
be non variable, and calibrated against a known data set, such as Landolt
fields.  So, the odds of finding all of these requirements in your 15' by
15' image for a given part of the sky are pretty slim!

The good news is that there are fields that do meet all these requirements,
such as M67 and NGC 7790.  No extinction calcs are necessary, and you can do
your BVRI images on the same pointing.  And yes, you can get better
statistics by calculating your coefficients over several nights, and
averaging them together.  This is because transformation coefficients don't
change all that much over time, unless you change something significant in
the optical path or in your CCD. Data for these fields is available from
Arne Henden's FTP site at
ftp://ftp.nofs.navy.mil/pub/outgoing/aah/sequence/.

They can be crowded for amatuer focal lengths, so pick your stars carefully.
And note that some are variable, so look for ones on the edge of the fields,
with the most observations and least associated error.

By the way, if you have an I filter, you might get better color data with
V-I rather than V-R.

Chuck Pullen (PCH)

----- Original Message -----
From: "Geir Klingenberg" <geir.klingenberg at gmail.com>
To: <aavso-photometry at mira.aavso.org>
Sent: Tuesday, December 28, 2004 2:23 AM
Subject: [Aavso-photometry] Differential transformation


> Hi
>
> The methods I have seen for estimating the transformation coefficients
> involves using instrumental magnitudes in a LS solution. But - if one
> is only doing differential photometry and don't need to be concerned
> with extinction - is there anything wrong in using a differential
> approach? More specifically, if we ignoring extinction the relation
> between the stars magnitude, instrumental magnitude and color can be
> written in the usual form as
>
> V = v + a*(v - r) + b
> V - R = c*(v - r) + d
>
> This is for the V and R band - capital letters are magnitudes and
> small letters are instrumental magnitudes. So, subtracting magnitudes
> of star 1 and 2 and rearranging we get
>
> (V1 - V2) - (v1 - v2) =  a*( (V1 - R1) - (V2 - R2) )
> (V1 - R1) - (V2 - R2) = c*( (v1 - v2) - (r1 - r2) )
>
> which can be used to estimate a and c.
>
> Since all instrumental magnitudes in these equations are on
> differential form  between stars on the *same* image, won't the
> solution be more robust against sneaking cirrus clouds and changing
> atmospheric conditions? And can measurements from different nights be
> used to improve the coefficient estimates?
>
> Geir Klingenberg
> _______________________________________________
> Aavso-photometry mailing list
> Aavso-photometry at mira.aavso.org
> http://www.aavso.org/mailman/listinfo/aavso-photometry
>
>