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Hi Robin
A question based on a chart you had in your presentation "Deep Sky Astrophotography with CMOS Cameras"
In the chart "Optimum SUB exp time/secs

As general rule of thumb:
So If I am in Bortle 7 urban area, with F/10 scope, using a colour camera, with a narrow bandpass filter
from the chart, notes.
would I take 18 sec times 3 ( for colour camera) then times 100 ( for 3nm bandpass)? 5400 secs?

And yes, I have done the sensor analysis, so I need to let your SMART HISTGRAM software do its work for "optimal" exposure time. (using a ZWO asi 2400MC pro camera on a SCT 9.25,f/10, no FR)

Thanks for great support and a great program.
Take care
Steve

Hi Steve,

don't think it is quite that bad, but narrowband always does require longer exposures.

You can pop the figures for your camera into the calculator at https://tools.sharpcap.co.uk. Bortle 7, f/10, 6 micron pixels, 80% QE and 3nm filter (choose mono, since everything that passes your 3nm filter will be in a single colour channel - the other two will be black) gives 0.14 e/pix/s.

Now, if you run at minimum gain, the rule of thumb calculation (10 * read noise squared / sky background rate) gives about 3200s for read noise 6.5. However move up to 140-150 gain where the read noise drops to 1.5 due to the switch from LCG to HCG mode and you are down to a much more reasonable 170s or so. That's probably the best way to use your camera with a narrowband filter.

cheers,

Robin