I'm sure this question has been asked before, and I was hoping to find a search utility on the forum web page. Since I could not find anything, here is my question. I've been doing CCD photometry with Johnson B and V filters for about a year. These are AstroDon intereference filters purchased used through Astromart. I recently added a Schott glass RG9 filter as an analog for an Ic filter. Measurements of the standard cluster NGC 7790 yield a very nice transform from instrumental magnitudes with the RG9 filter and standard Ic-band magnitudes. Baadar Planetarium is still offering Rc and Ic interference filters, and if their US distributors ever get any in stock I might be able to buy some. My question is this. What can I do, or what projects can I undertake, with the Baadar Planetarium Rc and Ic filters that I can not do with my current setup? Will there be a significant advantage with the Ic as compared to the RG9? What type of projects make use of Rc data?
Mon, 02/21/2022 - 20:32
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The benefits range from small to large. For eclipsing binaries, the added baseline helps resolve ambiguities as far as temperatures of the components. For pulsating variables (RR Lyraes, Cepheids) there is structure in the lightcurves that varies as a function of wavelength that allows information about the pulsation mechanism to be investigated. (The times of maxima also shift later at long wavelengths.) Both these come from analyzing colors, that is, color-indices, not the separate B,V,I magnitudes. For most other types of variables, where the variation is largely from (simple) temperature changes, there is not much to be gained: whatever happens at I (say) parallels the variation at shorter wavelengths but with smaller amplitude.
My _opinion_ is that the R passband by itself is relatively useless; the R-I color-index, on the other hand is useful because it is relatively free of effects of differing metal abundances in stars. (So this is a special case.) I wouldn't bother observing at R (alone) unless there is good astrophysical reason to do so --- and would probably go with Sloan r' because of the availability of reference stars just about everywhere in the sky so that zero-points can be made consistent.
The Schott RG9 is acceptably close to the nominal Ic passband, assuming the detector falls off in sensitivity longward of ~9000A, that there is no obvious reason to have a separate special filter. The plot you posted shows that your system response is linear as a function of magnitude. But the interesting plot is the magnitudes as a function color-index (V-I, say), which tells you how close the RG9 + detector is to the standard system. Don't be surprised if the color-term is a quadratic or cubic when very red stars are included in the standard-star mix.
\Brian
Thanks so much for…
Brian,
Thanks so much for your detailed response. I've been trying to learn how to do CCD photometry for just over a year now. You may know Kevin Anton. He has been helping me as my mentor. I started with HADS pulsating variables using only B and V filters, and Kevin and I just had a paper on B417 Boo released in the JAAVSO. More recently, I've been looking at EW overcontact binary systems, and that's why I added the RG9 filter. I was really surprised that the RG9 filter is parfocal with the AstroDon B and V interference filters. I took some data of NGC 7790 and processed it with VPhot, and then I ran the results through a program called Transform Generator V6.6. The program made some plots, Tbv.png, Tbi.png, and Tvi.png. I won't pretend to fully understand what these mean about my system. I can send them to you if you want to see them.
I'll probably replace the RG9 with one of Baadar Planetarium's Ic filters if their US dealer ever gets any in stock. I'm just wondering if adding an Rc filter is worth the extra cost.
By the way, I think we met a few years ago at the Texas Star Party. I hope that you and those close to you are doing well. Thanks again for your help.
Allen