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Dark Flats anybody?


TerryMcK
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An interesting topic came up on another forum specifically talking about dark flats of which I don't have any opinion. However I personally don't use them as I just use darks, flats and bias (bias needed for APP but not for PI) when integrating images. I know a lot of us imagers do use dark flats to great results.

 

Here is the person who wrote the article take on using, or not using, dark flats.

 

https://siril.org/2021/12/enough-with-dark-flats/

 

Interested to see your points of view.

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For me it's really a question of do you really need bias frames? If you consider them important then imo  flat darks don't help as the resulting bias will be the manufacturer bias plus additional bias from the noise of the longer exposures you have for flats. If you are concerned about removing the absolute value of the bias from a light it is going to be tricky. You really do need to take the fastest subs you can but make sure your camera is at the same temp as for your lights as even then there will be some thermal noise added. 

 

I don't worry about bias for my CMOS camera it has enough dynamic range that the bias won't swamp the final stack. I am led to believe the bias is a reasonable constant across the frame so effectively adding a constant illumination. It's there preventing noise introducing black clipped signal. 

 

I can adjust the black point later to remove that grey. 

 

If you take enough dark frames then your master dark will just contain the bias plus the smoothed thermal noise for that exposure. Subtracting the dark from the light should just leave wanted signal. 

 

A master Flat contains bias + an averaged noise + the much larger illumination signal. The first two are a small constant which isnt important as the flat is used to normalise the light frame adjusting the "gain" across the image to create even illumination. 

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My calibration frames consist of darks, dark flats (taken at the same exposure as flats) and flats. I stack the dark flats in Pixinsight to make a master dark flat, then calibrate the flats with the master dark flat. After they are calibrated, they are stacked into a master flat. Never used bias frames with my ZWO 1600, and I have never had any problems with calibrating light frames with this method.

Edited by AstronomyUkraine
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Yes I followed that thread; overall, I'll continue to use dark flats, flats and darks as that process generally works well for me with the ASI1600 and APP. With the Flats Wizard in Nina, taking dark flats really is no hassle.

 

APP never gives me grief about not using bias - I wonder is it camera specific, or does it just want either DFs or bias?

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I've used APP and now PI, I never had an issue with APP wanting Bias frames. I've used Dark Flats for a very long time now with no issues.

 

I'd like to see Cyril Richard and Adam Block have this debate live.

Edited by Greg M
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On 12/22/2021 at 12:18 PM, TerryMcK said:

An interesting topic came up on another forum specifically talking about dark flats of which I don't have any opinion. However I personally don't use them as I just use darks, flats and bias (bias needed for APP but not for PI) when integrating images. I know a lot of us imagers do use dark flats to great results.

 

Here is the person who wrote the article take on using, or not using, dark flats.

 

https://siril.org/2021/12/enough-with-dark-flats/

 

Interested to see your points of view.

Bias isn't needed by APP.  It's just a warning.

 

Some CMOS cameras do not like very quick speeds, so Bias frames are a problem - they cause more issues than they solve.  The fix to this is to use Dark Flats, which are taken at the same settings as Flats.  If you use NINA, you do them at the same time.  It takes me 5ish minutes to do 50 flats and 50 dark flats at the end of a session.

 

I did once try a 2s shot as my Bias, but that didn't work either... I think it is to do with the integration algorithms. However, I've had no issues with shooting 0.3s flats, dark flats and integrating these in PI, either via WBPP or manually.  (Manually is actually pretty easy, just more administrative than other software!)

 

 

Regarding the post, they've failed to acknowledge CMOS cameras... in the world of CCD, I probably agree that BIAS is ideal.  but as I said, certain CMOS do not do well with a very quick shutter exposure.

My 26c does actually seem to cope with bias frames better than my older 183c did.  I'm now in the process of testing integration of lights, bias and flats (no master dark either, due to the long exposure time and low read noise of the 26c)

 

Edited by Astroarg
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to confuse things even more, this article suggests you shouldn't have a master dark and a master BIAS!

Dark Frames and Bias Frames Demystified - Sky & Telescope - Sky & Telescope (skyandtelescope.org)

 

I've signed up to Adam Block's PixInsight videos.  Apparently his latest stance is if you have CMOS (including DSLR) then use Dark Flats and not Bias.

Only use Bias frames is the sensor is CCD.

 

I'll confirm once I find which video he details it in!

 

 

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It certainly looks like dark Flats are a compromise that addresses issues with CMOS cameras. Also, I understand CCD bias can be more variable and perhaps that informs a lot of the more established processes.

 

There is a lot of received wisdom in this hobby, some of it is self-evidently good advice which I quickly take on board. Sometimes though when I do my "does this add up" thing I'm left puzzled (usually because the "why" doesn't come with the advice). 

 

I also stumbled on the AB video, until now I have resisted buying into the learning packages but there is enough of an issue here and with this whole hobby to make me dip my toe. Then maybe I can weigh up the pros and cons better. 

Edited by paul
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I've been toying with the idea of splashing out on his videos and content for a while, and decided I might as well.  I am now looking to get data from other members and even remote telescopes (inc. hubble!) whilst the UK weather is so poor so I can continue learning PI.  As I add more of my own data into my sets, then I can slowly improve on what I've already produced (which since April and house move, hasn't been a lot...)

 

the $180 certainly gives a lot, and also lifetime access and all future updates.  You can start with the basics for $60 and then if wanting to upgrade they only charge the $120 difference

Edited by Astroarg
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OK so from one of his FastTrack videos, the suggestion is that if the sensor gives ampglow, then flats should be calibrated by dark flats.

Once the MasterFlat has been created, this has subtracted the dark flat data, so when using the masters with WBPP again (e.g. reprocess or another target done on same night with no rotation changes) you only need

*lights

*master flat

*master dark OR master bias  (the new WBPP doesn't allow for MD and MB.  If you select "Dark" then it removes Bias from the calibration diagram, if you unselect "Dark" it uses MB)

 

 

What was interesting was his notion that for Flats, because its not evaluating noise, 7-8 subs are enough, 15-20 for the darks.

Edited by Astroarg
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I've been looking in to this and think I have is sussed. Dark Flats are only needed for certain CMOS sensors (e.g. ASI1600) and then only if using long exposure Flats (qualified below).

 

Traditionally, a Master Bias frame is used to remove the Offset(bias) and FPN (fixed pattern noise) common to all sensors. This is done at the shortest exposure possible to make the dark signal as small as possible. The Bias frame is subtracted from the Flat frame so the Flat frame now contains just dark signal (proportional to the Flat exposure time) and the illumination signal. 

 

The ASI1600 doesn't produce good results for very short exposures (0.1-0.3 sec) so they don't make good traditional Bias frames. The solution was dark flats. A dark flat is a short dark exposure.  The dark Flat has to be longer than the problem exposure (say 0.3s) and less than or equal to the Flat exposure time. Then it performs like a Bias frame with a small benefit of removing more of the dark current and some FPN. In principle if your Flats are always short duration then you could have a Master Dark Flat that is used for all imaging (per gain). (I suspect a dark Flat made this way would need to classed as a Bias frame in PI WBB.)

 

The exception to this is when Flat exposures are long enough to introduce a significant "amp" glow. In that situation you need dark Flats that match the Flat exposure to ensure the glow is removed from the flat. This may be required for some NB setups. ...To avoid having to test if you suffer "amp" glow it is easier to produce a set of dark Flats for each exposure and gain you use for you Flats. But once created you should not need to make them again. Dark Flats should be taken with the same camera temperature, gain and offset as your Flats. (PI's WBB has a default 10 second tolerance on exposure so if you are consistent taking the Flats you won't need to retake dark Flats each session).

 

You still need to produce traditional Master Darks with exposures the same as your Lights. These Dark frames are needed to remove the Offset, Dark signal, "amp" glow and FPN from the lights before the Master Flat is applied to them.

 

Currently I don't use Bias or dark Flats so theoretically some of my NB images may show amp glow or FPN but so far, I haven't seen any evidence of amp glow and only once did I see FPN. In future I will try creating Master Dark (Flats) for 1 second and 15 seconds at each gain to remove potential amp glow bias and FPN from my flats. 

 

 

 

 

 

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Very interesting topic Terry. I got my first dedicated astro camera in early December (Zwo 294 mc pro) which definitely shows quite a bit of amp glow. I've been using APP for stacking then Pixinsight for processing. APP recommends dark flats for this camera so my calibration so far has been to load Lights, Darks, Dark flats and Flats into APP which from the two images I've done seems to work well. Flats & Dark flats calculator in the asi air plus was coming up with 6 sec durations which was way more than my old dslr so was a bit concerned initially but seemed to work just fine. So for the time being sticking with recommended dark flats but interesting subject none the less👍   

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