[Aavso-photometry] What are transformation coefficients??

[Tim Crawford]

Trupti,

I think the shortest-best answer to that question
should be answered by our Director:

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 To quote Arne Henden (Jul 26, 2007):

"A Transformation coefficient generally tells how your
instrumental value differs from the true value as a
function of star color.  This is usually a small
correction; important if you need high accuracy but
unimportant if your target and comparison stars are
similar in color."

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Transformation Coefficients are mathematically
computed by looking at the relationships of your
systems filters data  vs the standard magnitudes and
then computing an adjustment to make them equal.  Once
computed they do change over time and it is
recommended that they be re-computed on an annual
basis with at least three nights of data from the
standard field selected.

With my own system these differences generally in a
area of a few hundredths of a magnitude, or less. 
Other systems may vary more or less.  Transforming
BVRI data is a bit time consuming, both in the
gathering and analysis of the standards data and then
in applying the TC's to BVRI observations.... They can
not be accomplished for a single filter observation,
to the best of my knowledge; at a minimum, as I
understand it, at least two filters are required.

For the most part, these differences are not that
significant in the generation of a typical light curve
and I consider their use to be a more advanced topic
and do not really recommend that begning photometrists
be to concerned about them.  Once you become
comforable with doing differential photometry with all
the filters you may pocess then you might want to
given serious consideration to venturing into this
aspect.

As previoulsy mentioned the following links should be
examined by those with an interest:

This HQ link will take you to two most excellent
papers and is where everyone should begin with an
interest in transformation coefficients.

www.aavso.org/observing/programs/ccd/ccdcoef.shtml

Another appropriate read is a paper by Arne "THE M67
UNFILTERED PHOTOMETRY EXPERIMENT" which will aid in an
over all understanding of the topic.

www.aavso.org/publications/ejaavso/v29n1/35.pdf

While I prefer the M 67 Standards field for computing
my own Transformation Coefficients NGC 7790 is a good
alternative when M67 may not be available as well as
possibly M11 or SA110-503 (aka SA1105); I do not have
links for the last two)

http://binaries.boulder.swri.edu/binaries/fields/m67.html

http://binaries.boulder.swri.edu/binaries/fields/ngc7790.html

Per Ardua ad Astra

Tim Crawford CTX
Arch Cape Observatory

http://homepage.mac.com/windwalker1/ACObserv.htm