Re: NGC 7000 with CMOS camera

Dale Ghent

The only goal with flats is to get an illuminated frame from an evenly-lit light source, where the histogram is a nice hump centered at the %50 point. It doesn't have to be perfectly nailed at 50%. in truth, a flat is alright so long as no pixels are clipped one way or the other, but a perfect flat will look like that.

Obviously, getting that kind of result with an correct (or, adequate) exposure time is going to be specific to every setup, from optics speed and sensor settings, to the intensity of the flat light source - as well as considerations for any annoying interactions such as the rolling shutter one I mentioned. So there really isn't (and shouldn't be) a "rule" when it comes to exposure length and flats.

To take the fiddling out of arriving at this kind of result, we put a Flat Wizard into the app I contribute to:


On Aug 25, 2019, at 2:53 AM, Robert Chozick [ap-gto] <> wrote:

The 3 second rule I have seen is from multiple posts as I go through comments on issues. I generally try to get the benefit of common experiences and comments whether I totally understand them or not. I don’t get as much chance as I would like to image in dark skies and don't want to take any chances. I regularly used .3 sec flats for my STF-8300 CCD without issue.

On Aug 25, 2019, at 12:46 AM, Dale Ghent [ap-gto] <> wrote:

You may need to have a longer exposure time for flats because LED light panels adjust their brightness via PWM. If the shutter speed of the camera is quicker than what the panel is being driven at for its level of brightness, you will get banding across the image due to how PWM rates interact with rolling shutter sensors.

This need for a longer exposure can lead to an issue where the dimmest setting of a panel is still too bright for that exposure length and the flat is blown out, and one must resort to attenuating the panel light with ND film or other methods.


On Aug 24, 2019, at 10:16 PM, Mike Dodd [ap-gto] <> wrote:

On 8/24/2019 9:31 PM, Robert Chozick [ap-gto] wrote:

The problem is that you cannot use bias on a CMOS camera. Any exposure
less than 3 seconds is not advisable (not an engineer so not sure why).
So if you cannot use a bias frame on your flats you have to use flat
I don't understand what's magic about 3 seconds. ZWO specifies an
exposure range of 32μs-2000s for the '1600 and the '294. I have seen no
issues with flats exposed for less than 1 second with my '1600.

--- Mike

Posted by: Mike Dodd <>

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