Why do we need dark frames?

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turfpit
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Why do we need dark frames?

Post by turfpit » Fri Aug 02, 2019 12:45 pm

Light frames may carry noise which has been generated by the camera sensor and associated electronics. This may appear as:
  • amp-glow or starburst
  • hot pixels
This is a dark frame from an Altair 183C captured at 60s exposure and gain 200. The frame has had a histogram stretch applied in Siril. The starburst can clearly be seen on the right hand side. This artefact would be present in all light frames unless removed.

_dark-frame-showing-starburst.jpg
_dark-frame-showing-starburst.jpg (156.65 KiB) Viewed 401 times


This is a dark frame from a ZWO ASI120MC captured at 60s exposure and gain 50. The frame has had a histogram stretch applied in Siril. The hot pixels can clearly be seen. This artefact would be present in all light frames unless removed.

_dark-frame-showing-hot-pixels-copy.jpg
_dark-frame-showing-hot-pixels-copy.jpg (218.13 KiB) Viewed 401 times


These defects can be removed by collecting dark frames - cover the camera or sensor and capture frames with the exact settings used for the light frames. Ideally the temperature for both lights and darks should be the same or very close. A temperature mismatch can result in 'raining' across the image or an effect like running paint or a noisy background.

The dark frames can be captured in the same way as light frames. A master dark frame would be created and applied to the light frames during post processing via software such as Deep Sky Stacker, Siril, PIPP or Autostakkert. Alternatively dark frames can be captured and a master dark created automatically by SharpCap via Capture > Capture Dark . This is particularly useful for those using the Live Stack facility.

The use of dark frames, although not essential, will greatly enhance the appearance of an image.

Some cameras can mitigate the above problems by having in-built cooling systems where the user can control the sensor temperature (expect to pay ££££'s for this facility). With this type of camera, it becomes practical to build a dark frame library which can be re-used, thus removing the requirement to take darks frames during every imaging session.

Dave

nexusjeep
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Re: Why do we need dark frames?

Post by nexusjeep » Thu Aug 08, 2019 7:50 am

Hi Dave,
Good write up the only thing I would add is even with a more expensive thermally controlled camera you would still need to build a dark library the plus side is that you only need to do it once for the exposures / gain you plan on using as afterwards you have a constant reference temperature that you can control in my case I built a large reference library at -20c from 5 secs up to 5 mins as you can just leave them running and go do something else.

Were as on my previous 183c I had to plan for these on the same night as imaging. I did keep libraries of these as when the night was the same temperature as a previous one I could reuse them. The other problem as you say is when to do them on an uncooled camera as during the evening the camera alters temperature quite significantly. Also before I knew about the amplifier glow pattern looking like a starburst I went next door and complained about there garden light :roll:

Cheers
Nick

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turfpit
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Re: Why do we need dark frames?

Post by turfpit » Thu Aug 08, 2019 8:16 am

Thanks Nick. I edited in your comment about dark libraries.

Dave

nexusjeep
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Re: Why do we need dark frames?

Post by nexusjeep » Thu Aug 08, 2019 8:23 am

Are you planning do a post on Bias (Dark Flats / Flat Darks) and Flat frames as well.

Nick

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turfpit
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Re: Why do we need dark frames?

Post by turfpit » Thu Aug 08, 2019 9:32 am

Nick

Yes, probably called Why do we need flat (and bias) frames?

I have some fine examples of a dirty sensor & other mucky optics which show graphic representations when stretched.

I don't get involved with Dark Flats / Flat Darks as to me the terminology is confusing and this is just creating complexity. I follow the steps for processing calibration frames as detailed in the Siril manual.

These postings in Tips & Tricks are really targeted at beginners to the hobby and will provide an easy link to answering questions which are going to be asked repeatedly.

Dave

vichman
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Re: Why do we need dark frames?

Post by vichman » Sat Aug 10, 2019 6:51 am

Hi Dave,

you could also add a sentence about dark frame scaling.

This is very powerful, as one needs only one master dark (with an exposure time longer than that of any lights), and - of course - a master bias, to compensate the dark current in pretty much any light frame (of any exposure time or temperature).

CS
Stefan

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turfpit
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Re: Why do we need dark frames?

Post by turfpit » Sat Aug 10, 2019 9:55 am

Stefan

Thanks for reading my post and for your comment. The answer is no. The reason is this is a post intended to help beginners with this often asked question and will provide a quick link to get people started.
one needs only one master dark (with an exposure time longer than that of any lights), and - of course - a master bias, to compensate the dark current in pretty much any light frame
This statement introduces bias frames as well and will not really help those starting out in imaging.

To be honest, I have never heard of dark frame scaling until I read your post. I have had a quick Google for the phrase and a read - turns up a lot with references to PixInsight and Astro Pixel Processor.

Can I suggest that you create your own post describing the information you wish to share. That way, you receive the recognition and will be able to assist others if they have any questions on this topic.

Dave

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