[Aavso-photometry] What are transformation coefficients??

[Jeff Hopkins]

Hi Tim,

While Arne gives good advice, there are other considerations.

Many times there are fair color differences between comparison and 
variable stars. Many times this can't be helped especially with 
brighter stars where few choices for comparison stars exist.

Variable stars sometimes change color significantly during their cycle.

If an observer has many observations and all the data stand on their 
own then transformation may not be of utmost importance. However, as 
is the case most of the time with AAVSO data, there are many 
observers submitting piecemeal data to be combined into a single set 
of data. While data can be massaged, it is far better if everyone is 
on the same page by having transformed their data. This also becomes 
very important when doing multi-band filtered photometry. Knowing 
accurate color magnitudes at various points of the star's variation 
can reveal much about the star system.

A side note:
Tomorrow is the deadline for submitting abstracts for the May meeting 
of SAS in Big Bear Lake, California. Papers are due 20 April. Last 
year's meeting saw many great folks, including Arne, Ron Kaitchuch, 
Richard Berry and a fair number of AAVSO members. For information 
see: http://www.socastrosci.org/

Clear skies!

Jeff

At 23:43 -0700 03/12/2008, Tim Crawford wrote:
>Trupti,
>
>I think the shortest-best answer to that question
>should be answered by our Director:
>
>------------------
>
>  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."
>
>-------------------
>
>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
-- 
Jeff Hopkins
HPO SOFT
Counting Photons
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