[Aavso-photometry] Transform coefficients and two-filter measurements

[Michael Newberry]

Hi Tom,

Are you asking if it is possible for the graph of b-v vs B-V to have 
curvature? It so, the answer is yes---it depends on the filter/detector 
bandpasses. The transformations are linear over the range of spectral types 
if the bandpasses are an exact match to the standard system. But as the 
filters move off the standard bandpasses, in wavelength and shape, 
nonlinearities creep in. A graduate student of mine and I did some 
pioneering work in 1988 using synthetic photometry to design a filter system 
which, in combination with a target CCD response curve, would produce linear 
transformations over a broad range of spectral type. The study considered 
both giants and dwarfs. You can download the paper from the Astrophysics 
Abstracts server here: http://adsabs.harvard.edu/abs/1989PASP..101..849B
Click "Send PDF" in the middle of the page to get a copy.

Michael Newberry


----- Original Message ----- 
From: <tom_krajci at tularosa.net>
To: "Michael Koppelman" <michael at slackerastronomy.org>
Cc: <tom.krajci at gmail.com>; <aavso-photometry at aavso.org>
Sent: Wednesday, July 22, 2009 8:12 PM
Subject: Re: [Aavso-photometry] Transform coefficients and two-filter 
measurements


>> You don't care about B or V you care about B-V and it is simply either
>> redder or bluer. B-V is a single quantity, in a sense.
>
> B-V can be considered a single quantity, but it takes two different
> measurements...in two different filter bands.
>
> These two filter bands are independent of each other.  (In other words,
> one filter bandpass may be rather blue compared to the catalog
> values...and the other filter bandpass...can be anything from red to blue
> of catalog values.)
>
> I'm trying to get a deeper understanding, at a conceptual level, of what
> is going on here with two independent measurements to arrive at one
> quantity.
>
> Is the deviation of instrumental b-v versus catalog B-V...always a
> straight line?  Can there be considerable curvature?  How is it
> governed/controlled/determined by the two measurements?
>
> Tom Krajci
>
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