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Transformation Coefficient

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joemmonster's picture
Transformation Coefficient

Good day to all. Im currently doing an undergraduate thesis to observe and determine the period of a dwarf cepheid variable star AD CMi by DSLR photometry. I read on many forums on how to calculate the transformation coefficient by creating a plot of V-G vs B-V and getting the slope which will serve as the transformation coefficient but it's not indicated how often should I recalculate and apply my transformation coefficient to my target variable star.

I'm doing a time series photometry of my object which has a period of about 3 hours and will observe it for about 7 hours. I'm assuming that the atmospheric condition will vary (since it is also a part of my optical system right? and could affect the coefficient). Is it enough to determine my transformation once and use it for the entire observation period or should I get my coefficient once every hour? thanks..

Mark Blackford
Mark Blackford's picture
Transformation frequency

Hi Jonah,

if you don't change your lens or camera the transformation coefficient will be ok to use for up to a year. However, when I started DSLR photometry I repeated the calibrations every month or so until I was confident the value was not changing significantly.

You are right that atmospheric conditions can change throughout a night but these will not alter the transformation coefficient, however the amount of atmospheric extinction can vary due to the star's elevation, thin cloud, smoke or sea haze, etc. That requires an extinction correction coefficient which is different from night to night (or even from hour to hour). 

I'll contact you later today off-line to discuss your project and how to apply transformation and extinction corrections to time series observations. Cheers,


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