[Aavso-photometry] B-V, etc.

[Greg Crawford]

Hi Keith,

 

>So my question is how can I get B-V values from B and V filtered images without
>the use of a comp star with known B & V values?

 

This is a need which asteroid photometrists usually have because those darn
asteroids are never obliging enough to wander through a star field with a good
set of values, (I think the jargon is a "sequence"). It is possible to take
images which will enable you to extract the values needed for the overall
equation. However, it helps to make a distinction right from the start by
distinguishing between standardised filter values and instrumental filter
values, the former being represented in upper case and the latter in lower case.

 

So "B" is the standardised B filter magnitude, while "b" is what you got through
your b filter on the night.

 

As I understand it, using the equations in B.D. Warner's book, "Lightcurve
Photometry and Analysis",

 

CI = (ci - k'X) +ZP

 

Where CI  standard colour index for given star

      ci  instrumental colour index

      k'X first-order extinction coefficient multiplied by air mass  

      ZP  zero-point offset

 

This can be expanded, again using Warner's equations, to:

 

CI = (((m1 - k'1X) - (m2 - k'2X)) * TCI) + ZPCI

 

Where CI    standard colour index for (B-V), or (V-R), or (V-I)

      m1    instrumental magnitude of first colour of index

      k'1   first order extinction for first colour

      X     averaged air mass of target field

      m2    instrumental magnitude of second colour index

      k'2   first-order extinction for second colour

      TCI   hidden transform slope  

      ZPCI  hidden transform intercept

 

This is simplified to leave out second order extinction.

 

Basically the routine at the scope is to take a standard field at high altitude
(low air mass) and a standard field at low altitude (high air mass) to calculate
transformation coefficients and first order extinction; then do your target
field in the two colour filters of the colour index you want.

 

You could design a spreadsheet to feed all of your data from AIP4WIN and crunch
the numbers, or you could get Warner's software Canopus, in which it is all done
internally and automatically. In Canopus, you simply measure the images from the
at-the-scope routine above and click on the routines for crunching the
calculations.

 

And like Tim, I'm open to correction. :-)

 

Greg Crawford

 

 

-----Original Message-----
From: aavso-photometry-bounces at mira.aavso.org
[mailto:aavso-photometry-bounces at mira.aavso.org] On Behalf Of Keith Graham
Sent: Thursday, 26 July 2007 10:09 AM
To: AAVSO Photometry Group
Subject: [Aavso-photometry] B-V, etc.

 

Hi All,

 

I have a basic question regarding color indices determination. I have been
trying (without success) to match sequence B-V calculations and am wondering if
I am missing something in my computations. 

 

If I simply use the B and V comp star mags as reference and then take B & V
values of other sequence comp stars from B & V filtered images, I am pretty darn
close ( to within a few hundreths). My problem lies when I attempt to get B-V
without the use of comp stars. I assume I must use instrumental mags to arrive
at the color index values, so I have been merely extracting the B instrumental
mag from a B filtered image and a V instrumental value from a V filtered image
and subtracrting the two values. I know I am missing something here because the
software (AIP4WIN) does not know that these images are B & V filtered. It merely
extracts what it "sees". So my question is how can I get B-V values from B and V
filtered images without the use of a comp star with known B & V values? 

 

Cheers,

 

Keith Graham  

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