Hi, I am new to all this and have submitted a few observations. I wanted to get into filtered photometry with my CMOS camera,(ZWO ASI 2600mm), but despite having a mentor I am really struggling to work out transform coefficients for my V and B filters. If I give up on this for now, is it still worthwhile doing the photometry with no filter, or just the untransformed V filter. Is an L uminence filter any use?
Thanks
Hi. I was totally clueless about how to do the transformations until I had taken the Intro Photometry and the Vphot courses offered tis year. At this point, if you wish to pusue photometry, taking those courses when next offered might be a good investment of time. The instructor for the Vphot course, Ken Menzies, posted a very detailed set of instructions for using Vphot and the separate software, available from AAVSO, called Transform Generator, or TG. This can be found in the Aug 27 2024 post in the Vphot forum.
Good luck.
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Shoot for about half maximum linear range of your camera. If you are using a CCD camera the safe linear range might be 40% of maximum ADUs. As for CMOS cameras, I’ll show my ignorance here and discussions I see in chats here. I think you can go to over 95% of maximum ADUs.
Ray
The CMOS astro cameras I have used are linear until close to saturation. However, if you take test exposures and aim too high, even if the max ADUs in the test image are OK, you could find that saturation occurs later, particularly if you take time series over a few hours. One of the things I do with the raw photometry data is to plot peak ADU against JD for the var and all comp stars. Useful and instructive. It’s a good idea from my experience to do what Ray suggests, and keep peak ADUs around 50% or maybe just a bit more, even with CMOS cameras.
Mike,
I second Richard’s recommendation to take the VPhot and CCD1,2 course. I’d suggest taking the VPhot course first. VPhot is very helpful in the CCD courses.
Although the AAVSO prefers transformed magnitudes, untransformed measurements can still be useful. Using your B and V filters you can get pretty close to the transformed B and V magnitudes if you avoid red targets and pick a comp star which has a color index (B-V) which is close to color index of the target.
Phil SPP