Taking dark flats?

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Astrogate
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Taking dark flats?

Post by Astrogate » Mon Jan 28, 2019 6:54 pm

Hi Robin

I want to try out dark flats and I was wondering if this is possible? I would first figure out my flat time for e.g. 1 sec for my flats then cover the scope and use the dark capture to take 1 sec dark's for a master with the same brightness, gain and temperature as the flats . I would then load this SharpCap master dark in the pre-processing subtract dark section and leave the subtract flat section set to none. Next I would use capture flats feature and select mono, but since a flat dark replaces a bias in calibrating astro-images should I not select the bias feature or select it? I would then load this Sharpcap master flat/bias in the pre-processing subtract flat section and a master dark (with the same brightness, gain, duration and temperature as the light sub-images) in the pre-processing subtract dark section . * This would be my capture and calibration method for live stacking*.

Thank you for your feedback Robin

Chris A
Astrogate

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Re: Taking dark flats?

Post by admin » Tue Jan 29, 2019 5:17 pm

Hi,

The SharpCap bias frame option when capturing flats takes advantage of the fact that there is no need to subtract a true dark frame for flats – it's perfectly reasonable to subtract a shorter exposure light frame. So for instance you could take your flats at 100 ms exposure and the bias frame at 1 ms exposure with the flat panel still illuminated and subtract the two. What you get is a well constructed flat frame where the pedestal level and any dark level variations have been eliminated by the subtraction. This will work well as long as there are no hot pixels in the flat frame, which is unlikely considering the exposure time is usually used.

If you use the bias option in the flat creation tool then there is no need to take separate dark flats, although you should use dark frame subtraction along with flat frame correction when live stacking, otherwise you end up applying the flat correction to the pedestal level as well as the actual image levels and that gives incorrect flat correction.

Cheers, Robin

Astrogate
Posts: 71
Joined: Wed May 24, 2017 6:52 pm

Re: Taking dark flats?

Post by Astrogate » Tue Jan 29, 2019 6:16 pm

Hi Robin

Thank you for the helpful information! I will then take 25 flats with white balance set to auto (C9.25 @ F7.2 flats are 1/60th of a sec long at 350 gain using my ASI294MC) with mono and bias selected for my SharpCap flat master. I tend to set brightness/offset, gain, and cooling using the same settings as my dark's and lights, so it this the correct method for flats? I also take 5 dark's using the same duration, brightness/offset, gain and cooling as my lights SharpCap dark master.

Thank you!

Chris A
Astrogate

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