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Flats not calibrating properly

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#1 rgenier

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Posted 30 November 2021 - 11:30 PM

I'm using DSS to stack my frames. When I stack the Lights by themselves, everything stacks (relatively) well. However, when I add my Flats, I then get all sorts of gradients / dust motes on the stack. It's almost like the Flat calibration is adding the defects to the Lights, rather than removing.

 

Below is a link to the lights (20 from my session, I actually stacked 170 total) and the flats. I would appreciate it if someone else was able to try and stack these to see if they get the same issues.

 

Flats - https://www.dropbox....NIBV9WOW7a?dl=0

Lights - https://www.dropbox....YaBexfg_Ga?dl=0

 

Thanks!

 

 

Attached Thumbnails

  • Ghost_NoFlats (Medium).jpg
  • Ghost_Flats (Medium).jpg


#2 TelescopeGreg

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Posted 01 December 2021 - 12:49 AM

Darks and Dark-Flats?  You need the full set for the pixel math to work out.

 

The Darks are used with the Lights (take them at the same exposure and temperature, but with the cover on the scope).  Dark-flats similarly with the Flats.  Then DSS takes the calibrated flats and applies them to the calibrated lights to give a proper stack.



#3 Jared

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Posted 01 December 2021 - 01:06 AM

Flats won’t work properly unless you subtract the bias from both the flats and the lights. There is more than one way to do that. The first option is:
- Subtract a master bias from your lights
- Subtract a master bias from your flats
- Integrate your flats
- Divide your lights by your master flat
- Register and Integrate your lights

The second option is
- Subtract a master dark from your lights (which would include your bias signal as well as fixed pattern noise from dark current)
- Subtract a master bias from your flats
- Integrate your flats into a master flat
- Divide your lights by your master flat
- Register and Integrate your lights

The third way is:
- Subtract a master dark from your lights
- Subtract a master flat-dark from your flats
- Integrate your flats into a master flat
- Divide your lights by your master flat
- Register and integrate your lights

In all of these approaches the bias signal and any pedestal is removed from both the lights and the flats before the master flat is applied to the lights. Otherwise, the flat correction will create as many problems as it solves since the math won’t quite be right.
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#4 sharkmelley

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Posted 01 December 2021 - 02:55 AM

Below is a link to the lights (20 from my session, I actually stacked 170 total) and the flats. I would appreciate it if someone else was able to try and stack these to see if they get the same issues.

It won't be possible for anyone to test this because you forgot to provide the darks and bias frames (or flat-darks instead of bias).

 

Mark



#5 rgenier

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Posted 01 December 2021 - 03:07 AM

Huh

 

So, it turns out I was fundamentally misunderstanding the process for processing calibration frames.

 

I was ready to jump into this thread and comment that Darks aren't required, and that I was only testing with Flats... Turns out, that's the whole reason it wasn't working in the first place.

 

For the last year, I've been using my DSLR and taking Flats and Bias frames no problem. Calibrating and Integrating my stacks worked no problem (I dropped darks due to the unreliability of taking them at the same temp as the Lights). However, when I moved to the cooled camera, I stopped taking Bias frames (since I had heard anecdotal evidence that it causes issues in some ZWO cameras). I took Darkflats with the Lights, and everything was good for a few weeks. But I didn't realize that I had to include Darks / DarkFlats as a baseline for the Flat calibration to work. I was under the impression that the Dark / DarkFlat subtraction was an optional step. How wrong I was!

 

Thank you so much to everyone who commented. I've been struggling with this issue for the last few weeks, wondering why my stacking was not working where it worked in the past. Thank you!!


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#6 TelescopeGreg

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Posted 01 December 2021 - 01:42 PM

Yeah, we sometimes do the overall community a disservice by commenting on what we can "get away with" (e.g. "The new cameras are so low noise we don't need darks."), without completing the statement with explanations and caveats. 

 

There certainly are shortcuts one can take, under the right conditions.  But unless and until one has the understanding and tools for fully manipulating the pixels, it's best to stick with Lights and Darks (we really should call them Dark-lights), and Flats and Dark-flats.  That set always works, and symmetry has its advantages in keeping one out of trouble even if it might be a little more work.


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