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Old 12-04-2015, 03:55 PM
guggle (Michael)
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A question about darks

Hi everyone,

I've seen some photos here where people have said that they have stacked photos of differing exposure times. My question is what length dark frames should be used: the shortest, longest, or a mixture of both?

Cheers, Michael.
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Old 12-04-2015, 06:29 PM
kosh
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Michael,

you would stack the subs of differing lengths separately, subtracting a dark of matching length. then you blend them in your processing software of choice like photoshop, PI etc.

Goran.
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Old 12-04-2015, 07:50 PM
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Ryderscope (Rodney)
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I agree with Goran, Michael. My preference is to match the darks to the lights. In Pixinsight it is possible to use a masterdark which is longer than the light subs as it will optimise the dark by scaling it to match. Having said that I admit that even though I am a PI user I still process by matching the darks to the lights.
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Old 12-04-2015, 09:07 PM
guggle (Michael)
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Hi Goran/Rodney,

Thanks to both of you for your thoughts! Much appreciated and will attempt this in future. Is blending a difficult and/or time consuming process?

Cheers, Michael.
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Old 12-04-2015, 09:14 PM
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Amaranthus (Barry)
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Michael, a more general tip - you should never need to have more than two different exposure lengths for your subs, and only then when you are seeking a balanced exposure across a HDR object (e.g. bright M42 trapezium and faint surrounding nebulosity). Anything more than a shorter and longer exposure stack is wasted effort.
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Old 12-04-2015, 09:17 PM
guggle (Michael)
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Thanks Barry!
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Old 12-04-2015, 11:53 PM
kosh
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Quote:
Originally Posted by guggle View Post
Hi Goran/Rodney,

Thanks to both of you for your thoughts! Much appreciated and will attempt this in future. Is blending a difficult and/or time consuming process?

Cheers, Michael.
This might be a good place to start.
https://m.youtube.com/watch?v=THlwDf4zDDo

I Dont know what you process with but it explains the concept well enough.

Goran.
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Old 13-04-2015, 01:23 AM
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jsmoraes (Jorge)
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In DSS you can group differents sets of files and proccess them together.

Some times my master group has the main photos. And I use another group with very few photos with more ISO and/or time of exposition. Each group receive the appropriate dark, flat and bias.

And some times... this procedure result in better informations, mainly with nebulas or faint clouds. Others times... the result is worst.
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Old 13-04-2015, 05:07 AM
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ZeroID (Brent)
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I've normally used the longest darks for all exposures taken when processing with DSS. Once a dark is applied to kill the hot pixels or whatever it can't go any darker regardless.
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Old 13-04-2015, 06:15 AM
glend (Glen)
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If your willing, there are ways to virtually eliminate the need to stack darks, and this can make the workflow less hassle. How? Get a camera with deep cooling applied to the sensor. Most quality CCD cameras use snsor cooling, and a few DSLRs designed or modified for astrophotography also cool the sensor. You will still need offset/bias frames to map any hot pixels in the sensor but dark 'noise' drops off considerably when the sensor
temperture is held below zero.
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Old 13-04-2015, 09:38 AM
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rustigsmed (Russell)
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Quote:
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If your willing, there are ways to virtually eliminate the need to stack darks, and this can make the workflow less hassle. How? Get a camera with deep cooling applied to the sensor. Most quality CCD cameras use snsor cooling, and a few DSLRs designed or modified for astrophotography also cool the sensor. You will still need offset/bias frames to map any hot pixels in the sensor but dark 'noise' drops off considerably when the sensor
temperture is held below zero.
yes, I've actually just started experimenting not using darks myself. I just use a DSLR that isn't cooled, as long as you dither between subs and get enough data and enough subs (I am aiming for a minimum 16) it seems to take care of noise.

Applying flats have been the most useful addition to my pics, they make the biggest difference to an astrophoto in my view. if you haven't much data though applying darks is a good option.
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Old 13-04-2015, 10:41 AM
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Dithering will deal effectively with hot pixels, but not dark current (and the noise associated with this unwanted signal). As noted above, if you have a low-noise sensor coupled with deep cooling, then dark subtraction is probably not worth it. But for uncooled DSLRs, for instance, it's a vital calibration.
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Old 13-04-2015, 12:22 PM
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Quote:
Originally Posted by Amaranthus View Post
Dithering will deal effectively with hot pixels, but not dark current (and the noise associated with this unwanted signal). As noted above, if you have a low-noise sensor coupled with deep cooling, then dark subtraction is probably not worth it. But for uncooled DSLRs, for instance, it's a vital calibration.
here are my recent efforts without darks.
https://www.flickr.com/photos/803366...ream/lightbox/

https://www.flickr.com/photos/803366...ream/lightbox/

while definitely not noise free, I wouldn't claim that missing out on doing darks was a vital error.
I would perhaps have a different view if I were in warmer weather or had evident sensor banding, used a higher ISO or had an object with less signal, or as I mentioned earlier had a lot less integration time or total sub numbers.
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Old 13-04-2015, 12:34 PM
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Applying flats have been the most useful addition to my pics, they make the biggest difference to an astrophoto in my view.
I totally agree. From my limited experience, sufficient number of dithered subs treated with flats(*) and complemented with skilful processing will result in achieving quality images, of course given accurate guiding, sharp focus and good optical alignment of all components.

I feel that dark-subtraction is often overestimated in its impact on improving the final result. For this very reason I only subtract a master bias frame from my subs and I usually try to collect about 25-30 subs through each filter.

The best thing however is to personally and objectively investigate these matters, since there are so many different setups for astrophotography and thus there is no one definite answer regarding dark-subtraction.

(*)If using narrowband filters flats become pretty much unnecessary.
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Old 13-04-2015, 01:22 PM
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The best thing however is to personally and objectively investigate these matters, since there are so many different setups for astrophotography and thus there is no one definite answer regarding dark-subtraction.
Yes, indeed! If you're chasing faint data (e.g. narrowband) with a noisy sensor (e.g. TrueSense/Kodak) then you'll almost certainly benefit noticeably from full calibration even with dithering. With brighter targets and a low noise sensor (like many of the Sony chips) then darks may be optional.

Cheers,
Rick.
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Old 13-04-2015, 04:57 PM
guggle (Michael)
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Quote:
Originally Posted by Amaranthus View Post
Dithering will deal effectively with hot pixels, but not dark current (and the noise associated with this unwanted signal). As noted above, if you have a low-noise sensor coupled with deep cooling, then dark subtraction is probably not worth it. But for uncooled DSLRs, for instance, it's a vital calibration.
I'm not sure what dithering is. Is it moving the scope by small amounts every few frames?

My camera is an unmodified Nikon D3100, and chances are, I won't be modifying it anytime soon..
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Old 13-04-2015, 08:50 PM
kosh
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I'm not sure what dithering is. Is it moving the scope by small amounts every few frames?

My camera is an unmodified Nikon D3100, and chances are, I won't be modifying it anytime soon..
Correct. Dithering moves the mount my a few pixels ( give or take ) which then stops the same pixels being used all the time for the same part of the image, and also uncovers camera artefacts and noise because they will move with the dither, the image will not.

Or something like that

Goran.
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