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I just had a reply back from ZWO in my image problem
I’m surprised at such a quick reply
Very much appreciated
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With the ASI2600, ZWO turns on the Sony sensor's High Conversion Gain (HCG) mode at a gain of 100 or above. You should expect the signal to noise ratio of shorter exposures to be about 8 to 10 dB worse when you use a gain of 0 vs when you use a gain of 100. So, changing the gain to zero does make a huge impact on signal to noise ratio, in the case of the ASI2600.
Remember that by using a constant exposure time, your signal drops dramatically with a gain of 0 (0 dB), while the read noise has popped up by more than 3 dB. With a gain of 100 (10 dB, or factor of about 3.16) your signal has not only gone up by a factor of 3.16, but HCG causes the read noise to go down by a factor of more than 2.
If you use really long exposures (30 minutes to 60 minutes), the read noise contribution is less. But at Bortle 7, I suspect that you won't be able to do that, even with a narrow band filter.
You will loose dynamic range with a gain of 100 (but not by much with Sony's HCG).
If you really need to use gain of 0, try longer exposures (10 to 20 minutes?) to mitigate this read noise problem at gain of 0, especially since you are using a narrow band filter -- although you are probably limited by Bortle 7 skies that prevents you from using very long exposures, even with narrow band filters.
With Bortle 7 skies, you won't really need that much dynamic range for anything but something like M42 and M31. And for those, you can always use high dynamic range techniques (taking images at different exposures). Bortle 2 or 3 skies are different; you have really dark backgrounds, and can make use of the dynamic range.
My recommendation is to bump the gain to 100 to turn on HCG. That uses less time to collect the sub-frames, too.
Personally, I always use gain 100 with my ASI2600 and ASI6200. I usually use 180 second exposures (Bortle 5+ to 6-; different parts of the sky are brighter), but when I use any narrowband filter (Radian Quad for color 2600 and 6200, Chroma 5nm Ha filter for mono 2600), I lengthen the exposures to 300 seconds to mitigate the read noise.
Chen
“Unquote”
I really didn’t think using Gain 0 would be a “Huge” issue using the LExtreme filter
I’ve completed all my procedures listed in previous post so now just waiting for the skies to clear next week
More testing on Gain 100 under Sydney skies
Martin
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