Star color indices greater than 1.2 present serious accuracy problems with DSLR photometry. Unfortunately, most variables and many comparison stars are red.
I have developed a method that works to minimize those effects and allow accurate TG measurements of all stars using DSLR photometry.
Using a green filter ensures that only green wavelengths reach the sensor. It reduces the exposure by a factor of approximately 2, but the results plot consistently with the light curve for visual and ccd observations. Outliers are effectively eliminated, even with carbon stars such as HK LYR and U HYA. I use standard green photographic filters, available from Amazon. Special photometric filters are not needed.
I’ve used this technique on many northern stars in the AAVSO Binocular Program over the last month. I’ve placed a comment noting the filter use in each observation, if anyone cares to review.
John Pickett
PKT
Have you determined the transform for your setup that would be equivalent to Tv_bv? I suppose it would be best described as Tg_bg.
Roy
Since there are no reference values for TB, TG and TR you cannot derive a transform like Ttg_tbtg (the naming convention would use the complete filter name)
The definition of TB is that B is used for reference, TG you use V and TR you use R
So the transforms you create will be like Tv_bv, which moves the data into the Cousins domain.
George
My…
Point taken, George.
My question was aimed at finding out if John's Tv_bv is close to zero (in other words if his V-g where g is the green instrumental magnitude is almost independent of B-V). It should be if his non-transformed green filter magnitudes across a range of target B-Vs are close to transformed V magnitudes found by observers using V filters.
Roy
I’m afraid I can’t answer your question Roy. I have no experience with transformation calculations. I’ve always submitted my results as standard DSLR observations, tricolor green. It’s a great question, and I would appreciate an answer, if you would be willing to check it out.
John
I don't have a green…
John,
I don't have a green photographic filter of the type you use.
You could check it out yourself if you image a field with several AAVSO comp stars having a range of colours (or even better, photometric standard stars). For each star plot V-g on the y axis against B-V on the x axis. A spreadsheet scatter plot makes this easy.
Roy