Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Does stacking increase the need for calibration frames?

Astrophotography
  • Please log in to reply
6 replies to this topic

#1 belliott4488

belliott4488

    Vostok 1

  • *****
  • topic starter
  • Posts: 152
  • Joined: 05 Oct 2020
  • Loc: MD, US

Posted 04 March 2021 - 06:31 PM

I understand that stacking multiple exposures and using calibration frame will produce better final results than leaving out any one of those ingredients, but my question is this: If I don't have, for example, flat frames, will my stacked images suffer more than if I had taken a single exposure?

 

The reason I ask is that I attempted my first imaging session with multiple exposures and calibration frames two nights ago. Unfortunately, I messed up the flat frames by finding out too late that I should have retained my focus setting and camera orientation. I attempted to reproduce them from memory, but that was unlikely correct, and my results weren't very appealing - lots of vignetting.

 

I then decided to try stacking without the flat frames, guessing that they were doing more harm than good and assuming that my results would be at least better than a single exposure with no calibration at all. I was surprised to find that I still had noticeable vignetting, which I had not seen the week before with a single exposure.

 

Is there anything about the stacking process that exaggerates whatever vignetting might be occurring?

 



#2 imtl

imtl

    Gemini

  • *****
  • Posts: 3,487
  • Joined: 07 Jun 2016
  • Loc: Down in a hole

Posted 04 March 2021 - 06:35 PM

Vignetting is not going to go away with stacking. You're only going to emphasize it more because you are improving SNR as you stack more.


  • belliott4488 likes this

#3 joeytroy

joeytroy

    Messenger

  • *****
  • Posts: 438
  • Joined: 14 Aug 2020
  • Loc: Belen, NM, USA

Posted 04 March 2021 - 06:38 PM

I am not sure what software you are using to stack with, this article on CN may help a little.

 

https://www.cloudyni...adient-problem/



#4 petercoxphoto

petercoxphoto

    Vostok 1

  • -----
  • Posts: 183
  • Joined: 26 Apr 2020
  • Loc: Cork, Ireland

Posted 04 March 2021 - 06:38 PM

Stacking increases the signal in the image, whether good or bad. Stacking will reduce noise, as it's random and is averaged out in the stacking. However, vignetting is going to be the same in every frame, so it'll be amplified as well. 

 

That being said, if you stretch a single frame to match your stacked result, the vignetting should appear about the same in both, but the image will be overall cleaner with more detail in your target.

 

So you definitely do want to stack your frames. There are things you can do in post to reduce vignetting - there are tools for that in Photoshop and Lightroom, and plenty of ways to do it in Pixinsight as well. But in general, if I didn't have flats to match my lights, I'd be inclined to discard them and recapture the target with appropriate flats.

 

Cheers,
Peter


  • imtl and belliott4488 like this

#5 belliott4488

belliott4488

    Vostok 1

  • *****
  • topic starter
  • Posts: 152
  • Joined: 05 Oct 2020
  • Loc: MD, US

Posted 04 March 2021 - 07:32 PM

I am not sure what software you are using to stack with, this article on CN may help a little.

 

https://www.cloudyni...adient-problem/

I should have mentioned that I'm using DSS.

 

Thanks for the link. I'm brand new to all this, so every bit of additional information helps.



#6 belliott4488

belliott4488

    Vostok 1

  • *****
  • topic starter
  • Posts: 152
  • Joined: 05 Oct 2020
  • Loc: MD, US

Posted 04 March 2021 - 07:40 PM

Vignetting is not going to go away with stacking. You're only going to emphasize it more because you are improving SNR as you stack more.


Stacking increases the signal in the image, whether good or bad. Stacking will reduce noise, as it's random and is averaged out in the stacking. However, vignetting is going to be the same in every frame, so it'll be amplified as well. 
 
That being said, if you stretch a single frame to match your stacked result, the vignetting should appear about the same in both, but the image will be overall cleaner with more detail in your target.
 
So you definitely do want to stack your frames. There are things you can do in post to reduce vignetting - there are tools for that in Photoshop and Lightroom, and plenty of ways to do it in Pixinsight as well. But in general, if I didn't have flats to match my lights, I'd be inclined to discard them and recapture the target with appropriate flats.
 
Cheers,
Peter

 Thanks! That makes sense. It kind of reminds me of the situation where increasing magnification also magnifies problems with seeing conditions.

 

Next time I'll know to capture my flat frames while the camera is still set up. I need to figure out how to do that without a convenient light source, but that should be solvable.

 

Thanks again.

 

(As an update, I've been stretching and otherwise adjusting the stacked image, and it's getting much better.)



#7 joeytroy

joeytroy

    Messenger

  • *****
  • Posts: 438
  • Joined: 14 Aug 2020
  • Loc: Belen, NM, USA

Posted 04 March 2021 - 07:48 PM

I use a white tshirt and my iPad to fake out the light. Trevor from Astrobackyard waits to the morning and shoots his flats and leaves the mount all the same. What ever works best for you is the way to go.


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: Astrophotography



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics