[Aavso-photometry] Calculation of Transforms, K' and ZP

[Jeff Hopkins]

Hello again Steve,

If your main interest is exoplanet transits, I think just the R 
filter is what you should be using. Ted Dunham from the Lowell 
Observatory gave a presentation on this last week at our local SAC 
meeting. He indicated that seemed to be the best band for such a 
project project.

The zero point should not change night to night. The zero point is a 
function of your equipment sensitivity. For the extinction, just 
image a standard star or one you are confident with the published 
magnitude, when the star is close to the zenith and when it is 30 
degrees or more from the zenith. Several in between images are good 
too. Then you can determine the extinction for that night. If you can 
find a comparison star of known magnitude in the same field as your 
exoplanet search image, just do differential photometry and don't 
worry about the extinction.

Good luck.

Jeff

At 12:02 -0700 09/01/2007, Steven Orlando wrote:
>Hello All,
>
>A couple of nights ago, I imaged two Landolt fields at different 
>airmasses. I took 5X1 min of BVRI, for the two fields. I used MPO 
>PhotoRed to analyze the data. This are the results:
>
>V-R:
>B: Transform = 0.328, k' = 0.458, ZP = 17.724
>V: Transform = -0.145, k' = 0.525, ZP = 19.062
>R: Transform = -0.196, k' = 0.635, ZP = 19.266
>I: Transform = 0.007, k' = 0.411, ZP = 18.406
>
>B-V:
>B: Transform = 0.173, k' = 0.428, ZP = 17.567
>V: Transform = -0.078, k' = 0.538, ZP = 19.132
>R: Transform = -0.105, k' = 0.383, ZP = 19.361
>I: Transform = 0.002, k' = 0.410, ZP = 18.403
>
>V-I:
>B: Transform = 0.17, k' = 0.421, ZP = 17.531
>V: Transform = -0.073, k' = 0.541, ZP = 19.148
>R: Transform = -0.101, k' = 0.387, ZP = 19.383
>I: Transform = 0.005, k' = 0.410, ZP = 18.402
>
>Are these good ballpark figures, I think the k' are way too high. I 
>really don't know about the trans and the ZP.
>
>If you're capturing a timed event like an exoplanet transit, how do 
>you transform the mags if you only shot the transit with one filter. 
>Do I just take some images of the transit star with another filter, 
>like R for example? I ask this because I don't understand how you 
>can calculate k' and ZP each night, if you are using one filter.
>
>Steve

-- 
Jeff Hopkins
HPO SOFT
Counting Photons
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