Understanding “Dark Flats”

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Ken_nneth
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Understanding “Dark Flats”

Post by Ken_nneth » Tue Oct 24, 2017 7:17 am

Trying to get a better grip on calibration files and want to understand “Dark Flats” and how they work. I understand that they should be taken with cap on at the same temp, gain and exposure as normal flats. But what about filters, my flats are taken with different exposures depending on filter choice. Do I take “Dark Flats” for all filters”? How do I use my subs to create a master or masters? How do I then use these masters, where do they fit in the calibration workflow? Does anyone have a good tutorial or instructions to help me with this.
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Re: Understanding “Dark Flats”

Post by Doug » Tue Oct 24, 2017 8:26 pm

I've never taken dark flats before. So I'm in no position to tell you either way. Personally, I take Bias, Darks, and Flats. Zach also mentioned a new method he was trying, that is to eliminate darks all together because he believes with "dithering" it eliminates the need for darks.

There are a lot of different positions out there about calibration frames, if you think Dark Flats are worthy of your time, then by all means try it!

Hopefully someone else has an opinion too.
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Re: Understanding “Dark Flats”

Post by Rudeviewer » Tue Oct 24, 2017 8:43 pm

I've seen the term dark flats but have no clue what they are. In my case of the last 2 images, I use extreme dithering and settle at 1.0 pixels and without darks the image has less noise. Now that could just mean that I need a new dark library.

Dark flats I would assume they are the same length of the flat but with the lens cover on. Maybe to help create a cleaner dark file???
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Re: Understanding “Dark Flats”

Post by Ken_nneth » Tue Oct 24, 2017 10:13 pm

Rudeviewer wrote: In my case of the last 2 images, I use extreme dithering and settle at 1.0 pixels and without darks the image has less noise.
This is something I have to try, how does this effect the total imaging time?
I live in an area where the weather gives me only short periods of clear skies, so total time is important.
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Re: Understanding “Dark Flats”

Post by Rudeviewer » Tue Oct 24, 2017 10:29 pm

Dithering only takes a few extra seconds per frame. Whileb guiding it moves the image around just a tad so when you sigma stack the images it can basically average out the read noise.
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Re: Understanding “Dark Flats”

Post by h2opolo » Mon Nov 06, 2017 6:55 am

Basically a dark flat is pretty much what it says. What you do is take a flat shot with the cap on in order to further assist with noise removal. I personally think it's overkill. I'm old school as far as darks go. I know that people don't use them with the sony sensors like I have but in my book what the big deal. I have a dark library so it's just a matter of choosing the master dark and adding it in to my calibration. If it even does a small amount of noise removal then it's worth it to me.
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Re: Understanding “Dark Flats”

Post by seti_v2 » Wed Nov 08, 2017 6:39 am

Agree with Sean. Dithering is great for things like "walking noise" in your images (as you image throughout the night flexure and inconsistencies lead to a few pixel drift sometimes and noise can be seen walking).

Darks remove all sorts of noise including amp glow.

Flats help correct vignetting in the system

Bias removes fixed pattern read noise and any consistently high read noise pixels

Dark flats remove any amp glow and column or row defects from your flats so your flats can actually calibrate your lights correctly.

If you have them all, the process kinda goes:
subtract bias from everything, subtract dark flats from flats, subtract darks from lights, divide lights by flat, stack all them lights

Now will anyone notice a different if you don't have bias or dark flats with new low read noise sensors on most objects? probably not for most objects. People like Sean are shooting extremely narrowband extremely faint objects with huge integration times. That one or two electrons or few ADU of photonoic shot noise from that faint whisp may get missed if you can't get the noise floor as low as achievable.
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