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Andy R

Darks, Flats & Bias Frames

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Evening all,

Just set the equipment up, framed the image, set the intervalometer and booom, the clouds come rolling in, however there are clear patches in between them so all is not lost. I should be used to it by now, anyhow what I usually do is take dark frames when this happens. I imaged a couple of nights ago with exactly the same lens on the same DSLR camera,  same 18mm focal length same ISO, exposure etc. I know you can re-use the dark frames but can I use the flats and bias frames as they have exactly the same lens orientation (lens locks into place). the focus may be ever so slightly different thou. Would this make a difference when stacking?

 

Thanks in advance.

Andy

 

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My SWAG (Scientific Wild-A** Guess) is that focusing won't. Vignetting is unlikely to significantly change with tiny focus moves.
 

Changes in dust on the sensor will affect the result both ways (new specks since flats were taken, old specks that got knocked off since). Dust on the glass will be diffracted into invisibility anyway.

You can definitely reuse the bias frames. In fact you should only have to redo those periodically, no need to run them every time.

Edited by rickwayne
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You can most definatly make a Dark and Bias library.    Both of these types of images, can be taken anytime, as they only need the camera, and no light leaks.    To help you get the best from them, you should make sure that you match the temperature of the darks and bias, so the temperature of the image frame.    i.e. if you shoot the image when the camera is at 5°C, then shoot the darks and bias at the same temperature.    Having a library of darks and bias for each temperature that you shoot will make like a little easier over time.

 

Flats on the other hand are transient.  As has already been mentioned, dust bunnies come and go, so you should make new every time.  The things that change are the focus position, and the specs of dust in the sensor.  The differences will have a profound effect on the image.  Specifically a slight shift in the focal position will change the vignette pattern.   Each time you attach and detach the camera, the optical train gets opened to dust, which may enter or exit.  Hence it changes.   So take these every time, they only take a few minutes, depending on your settings.

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Thank you for the replies fellow members @cjdawson @rickwayne  . A bit of a disaster last night with cloud and wind, dew heater went on the blink, my dog chewed the polar scope EP cover and I knocked the lens out of focus after changing the camera battery. I didn't realise (my fault for not checking as I usually do) resulting in 1.5hrs of blurred subs. Im using a small strip of masking tape in future to hold focus.

I didn't take any flats last night so I'm just about to as it was 5.30 before I finished and rather angry with myself.

Im thinking some is better than none perhaps (I will stack images with & without to see) so I will cool the camera outside for a bit first as roughly same temp.

Andy

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"The dog ate my astrophotography session."

THAT'S new.

[Sorry, man.]

I don't recheck focus nearly often enough myself, but even if you have it nailed and don't touch the equipment, temperature change can still affect it.

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18 minutes ago, rickwayne said:

"The dog ate my astrophotography session."

There's nothing she wont try eating once lol. 

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I happily re-use flats, even taken months previously. The only issue is if there are significant dust spots, but my Canon does sensor cleaning whenever you switch it on and off, and there are very rarely any visible dust spots. If there are they can be fixed in post-processing.                

NigelM

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Ok here I go showing how very, very little I know about AP. As someone who is looking at possibly getting into astro photography can you please tell me about flats, darks and bias. How are they taken and how are they used. (Told you I don't know anything lol) Thank you in advance for the information

 

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35 minutes ago, Older Padawan said:

Ok here I go showing how very, very little I know about AP. As someone who is looking at possibly getting into astro photography can you please tell me about flats, darks and bias. How are they taken and how are they used. (Told you I don't know anything lol) Thank you in advance for the information

 

I'll have a go...

My comments are based on CCD type cameras.  There are some differences with CMOS type sensors.

If you take an image with the lens covered ( a dark), then there will be some variations in pixel values, and also some/many hot pixels.  These will appear as very bright Red, green, or blue dots in your final image.  However, they tend to appear in exactly the same spot in every image, so if you know where they are, then you can subtract them, or cancel them out.  The effect varies with temperature, and exposure duration.  So you take a bunch of darks at the same temperature and exposure to make a master dark.

If you take an image of a uniformly illuminated white surface, then the image that you get will have imperfections caused by optical issues and by dust, dirt etc in your optical system. Processing software will use these flats to balance up your image to cancel out the imperfections.

The bias is the signal that is present when you just read the dark sensor in the shortest possible exposure.  This will be in every image that you take, including your flats and your darks.  So, to get a true flat that can be used with any length of exposure, then you should remove the bias before producing the final flat.

 

I remember that when I first read about this, it seemed awfully confusing.    Focusing, tracking and simply just finding the target are all more important IMHO.

 

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1 hour ago, Older Padawan said:

Ok here I go showing how very, very little I know about AP. As someone who is looking at possibly getting into astro photography can you please tell me about flats, darks and bias. How are they taken and how are they used. (Told you I don't know anything lol) Thank you in advance for the information

 

Couple of links to Trevors astrobackyard website for DSLR imaging, I found useful too along with this smashing forum:-

https://astrobackyard.com/bias-frames-astrophotography/

https://astrobackyard.com/how-to-take-dark-frames/

https://astrobackyard.com/how-to-take-flat-frames/

Failing that look for a copy of "Making Every Photon Count" (available from FLO) I found it useful to start with but had to put it down as I became a bit lost and overwhelmed with the more technical side of things, so I practised what I learnt from the book upto that point. Now I shall finish reading it (after misplacing the book), as I have gained more experience and would be able to understand everything a little better.

 

Good luck

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Thank you Don and Andy. I appreciate the information. I can tell I have a lot to learn. I think I need to get out, shoot, read, make mistakes and keep going.

 

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3 minutes ago, Older Padawan said:

Thank you Don and Andy. I appreciate the information. I can tell I have a lot to learn. I think I need to get out, shoot, read, make mistakes and keep going.

 

Lol sounds familar, good luck with your journey 👍

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