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Shooting darks and flats

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#1 James Paulson

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Posted 20 July 2021 - 05:38 PM

I am just curious what DSLR users are doing in terms of darks, flats, and bias frames? I just read a post on Facebook that says it is a waste of time to shoot darks, just shoot flats and bias frames. I have never shot either flats or bias frames, I just shoot my lights, at completion I put my lens cap on, shoot about 10 darks and that is all I use. I am presently shooting with a T7i but I also have an old XS as well as a T2i. I also at times will turn on and use in camera dark frames (because I can).

 

Now I am not saying that I get awesome results or anything, but the objects sort of look not bad and most of my issues are not related to stacking anyways at this stage LOL.

 

Your thoughts......



#2 Jim R

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Posted 20 July 2021 - 06:21 PM

Interesting.  I saw a website (ianmorison.com) that said bias frames are not needed if dark frames are same exposure, iso and temperature. Not sure which is correct.



#3 KLWalsh

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Posted 20 July 2021 - 06:49 PM

I’ve read that shooting darks isn’t necessary for most Nikons, yet I still do them occasionally. Tbh, I haven’t seen much difference with/without them.
Flats are extremely important. I shoot ‘sky flats’ at dusk, then shoot bias frames. It doesn’t take long to shoot them, so why not go ahead?
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#4 AstroBrett

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Posted 20 July 2021 - 07:38 PM

Interesting.  I saw a website (ianmorison.com) that said bias frames are not needed if dark frames are same exposure, iso and temperature. Not sure which is correct.

Jim,

 

Both are correct. The bias signal is included in any dark frame, and by integrating the dark frame for the same length of time as your light frames, you have captured both the bias signal and dark noise all at once. A separate bias frame is only necessary if you wish to scale your dark frames to a shorter exposure length light frame.to do that, the bias signal must be subtracted from the combined bias/dark before scaling the dark noise.

 

I hope that helps.

 

Brett 


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#5 bignerdguy

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Posted 21 July 2021 - 08:32 AM

Jim,

 

Both are correct. The bias signal is included in any dark frame, and by integrating the dark frame for the same length of time as your light frames, you have captured both the bias signal and dark noise all at once. A separate bias frame is only necessary if you wish to scale your dark frames to a shorter exposure length light frame.to do that, the bias signal must be subtracted from the combined bias/dark before scaling the dark noise.

 

I hope that helps.

 

Brett 

The bias frame can also be used for things like Flat calibration as well, though most people prefer to take a dark-flat image instead.  There are other scenarios where a Bias frame can be used instead of a dark for shorter exposure images, say like planetary and such.  So having a library of Bias and Darks available can help you with all sorts of imaging needs where just one type wont work.



#6 galacticinsomnia

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Posted 22 July 2021 - 01:06 AM

I don't use any calibration frames and consider them a waste of time and resources.  With modern processing techniques they shouldn't be necessary, and that is my rule.  There are exceptions though, and when there are, I will shoot bias and flats, but that is all.  I can't remember a time when calibration frames were helpful for me.  There has been a lot of failure of calibration frames working.  Then when it doesn't come out right, well, you just didn't do this right or that right, your temp was wrong, you shot flats with panel, or other silly gadget. 

I'm sure they "can" be of use and "can" be helpful, but for me, I don't even bother unless totally necessary and don't think my images have suffered at all, and I'm still improving.  Some people do it, and find great success, but a part of me wonders, if they can be just as successful without them. 

Here are a few posts of images and specs. 

https://www.cloudyni...bula-61-edphii/

https://www.cloudyni...fid-61-edph-ii/

https://www.cloudyni...bula-happy-4th/

 

Clear Skies !!



#7 BQ Octantis

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Posted 22 July 2021 - 06:41 AM

As an avid stock DSLR APer, I find calibration to be exceptionally important, for very specific reasons. My goal is usable data at 100% scale, so that is my point of view.

 

1. Flats. Or better yet, flat—as RawTherapee uses just one flat. I don't have a lot of dust motes in any of my setups, so the lens vignette model is the most critical part of the calibration. It's not that I can't create a synthetic flat. It just saves me hours of processing, especially in tricky sky gradient conditions. Better to fight one largely linear light pollution gradient than a convolution of it and a radial vignette gradient. And even a bad flat—such as a dark in the wrong orientation from my C5/750—is more helpful than no flat. And I've successfully used the same flat for each configuration for over a year now.

 

2. Darks. I've joined the dither or die bandwagon, and indeed, I could get away with just a handful of bias frames and no darks if I dither, let the mount drift, and use sigma-reject stacking. This eliminates hot pixel paths and averages out the residual fixed pattern noise quite nicely. But in going back through some old data from back when I didn't know to dither, I paradoxically found that cherry picking the lights and darks to where their average temperatures matched to within ±0.1˚C produced better results than just adding more data—especially in the red channel. Because of the mount drift, the full stack was walking noise limited from the residual dark noise error getting walked across the image. But with the reduced-but-matched stack, I had less residual fixed pattern noise and a shorter distance any noise got walked. And I found that at f/2.8, my images were rarely shot noise limited—so less subs was actually quite viable.

 

3. Bias. I have a library for ISO800 and above, but I only use them for short exposure subs (such as for the moon, assuming I'm imaging at high ISOs). For longer exposures, I use my darks library.

 

4. Flat darks. I find them to be unnecessary, and I've purged the library. And RawTherapee has no way to use them. But I've been told that it's wrong, so not to tell anyone. So I'm not tell you here.

 

There are more opinions than DSOs in the sky. So find what works for your workflow for the output you're aiming for.

 

Cheers,

 

BQ


Edited by BQ Octantis, Yesterday, 12:18 AM.

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#8 belliott4488

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Posted 22 July 2021 - 07:08 AM

I don't use any calibration frames and consider them a waste of time and resources.  With modern processing techniques they shouldn't be necessary, and that is my rule.  There are exceptions though, and when there are, I will shoot bias and flats, but that is all.  I can't remember a time when calibration frames were helpful for me.  There has been a lot of failure of calibration frames working.  Then when it doesn't come out right, well, you just didn't do this right or that right, your temp was wrong, you shot flats with panel, or other silly gadget. 

I'm sure they "can" be of use and "can" be helpful, but for me, I don't even bother unless totally necessary and don't think my images have suffered at all, and I'm still improving.  Some people do it, and find great success, but a part of me wonders, if they can be just as successful without them. 

Here are a few posts of images and specs. 

https://www.cloudyni...bula-61-edphii/

https://www.cloudyni...fid-61-edph-ii/

https://www.cloudyni...bula-happy-4th/

 

Clear Skies !!

Well, that's not an opinion you see every day on this forum! wink.gif

 

I'm curious - how dark are the skies where you usually do your imaging?


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#9 galacticinsomnia

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Posted 22 July 2021 - 04:35 PM

Well, that's not an opinion you see every day on this forum! wink.gif

 

I'm curious - how dark are the skies where you usually do your imaging?

I'm in Bortle 6-7 depending on sky conditions as I'm in the valley.

This is a good night, and just a 20 second exposure at iso 100 of my imaging setup taken July 11.
2021-07-11_162648.jpg

I could be my own worst enemy when it comes to calibration frames, but I'd just rather not when I consider the costs of camera bodies, and the lifespan, I just don't want to waste the life of my camera making my images maybe 10% better, when improvement in signal processing and data capture techniques can easily compensate.

Dither or Die, is definitely where it is at, and I'd rather spend 20 to 30 seconds every 5 or so frames to remove the unwanted noise.

There are times when I put my adapters or whatever in a bad order that gives me a bunch of vignetting, and there are times when I get major color casts from stray light due to neighbors and or traffic, and on those occassions, I will reach for what I have in a library of stored flats and bias.

I do try to make things as accurate as possible.  As BQ said, having a goal or usable data at 100% scale, is a good goal to have.  I think it is possible without calibration frames. 

Clear Skies !!


Edited by galacticinsomnia, 22 July 2021 - 04:40 PM.



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