I create flats using an electroluminescent panel of fixed intensity. Target ADU is 25,000, and my filters are Astrodon Tru-Balance LRGB I-Series CCD Filters. Exposure times for each filter in seconds are as follows: L 1x1 3.2 R 2x2 17.0 G 2x2 4.7 B 2x2 5.4 Can this be right? 17 seconds for Red? Is the camera that insensitive to Red compared to the other colors? If so, then is the advertised R:G:B: ratio for these filters still 1:1:1? My telescope is a Tak FSQ-106 f/5. Thank you for your assistance. Alan
Hi Alan. Your exposure ratios are very similar to mine with a QSI camera and Astrodon Tru-Balance LRGB filters when using an Aurora electroluminescent panel for a source as EL panels tend to emit highly in the green-blue region and very poorly in the red. To do a true comparison you need to use a broadband light source for the flats. To test, try a white board illuminated with a halogen lamp, or sky flats, you should see much closer to a 1:1:1 ratio. I gave up using the Aurora EL panel some years ago as I could never eliminate interference banding in the flats and as my observatory is automated I just take sky flats (or rather, the automation program takes the sky flats while I am still sleeping!) HTH William.
The KAI-11002 CCD has good sensitivity to red, though it is a little higher in green. What you are seeing is more about the color curve of the EL panel, not the sensor. Bear in mind that what looks "white" to a human won't necessarily be the same as the camera. The human spectral response has a lot of overlap between red and green, and also the human visual system tends to adapt to the prevalent illumination color (it auto-color-balances!).
Thank you both very much. Makes a lot of sense. I am switching to sky flats as William suggested. William, in this regard, how do you handle NB flats?
Alan - the NB filters will take much longer to get flats. You’ll maybe need to set up multiple dusk and dawn runs to get all of your flats. The flat sky doesn’t last that long ...
No problems with NB sky flats as the filter pass bands are all part of the natural sky spectrum. You will see a difference between the OIII and Ha-SII ratios but nothing dramatic and there are small differences in the respective ratios between dawn flats and evening flats as dust that rises on thermals during the day tends to favour the red end of the spectrum while dawn flats are slightly bluer. William.
Depends quite a bit on your latitude and how close to the opposing horizon you can get, my dawn flats period lasts around an hour and a half during prime imaging season, trying to take sky flats down in the mediteranean last summer we had just barely thirty minutes! Good thing too now that I’m mainly imaging with a permanently setup refractor is that the closed optical path stays clear of dust for ages and a good set of flats will last many months or longer.
You may find the EL works okay for narrowband. In my experience, sky flats are always the best, if you can do them.
