Flat Field Calibrat...
 
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Flat Field Calibration issues - Canon 6Da  

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(@huerbsch)
White Dwarf Customer
Joined: 2 years ago
Posts: 13
August 14, 2019 19:01  

I'm having a hard time getting flat field calibration when using a Canon 6Da + 70-200 f/2.8 stopped down to f/4 or a 135 F/2 stopped down to 2.8 or even 3.2. 

I created a folder with a MD, MB, 15 lights and 20 flats if anyone can try and help me out plz!

Drop Box folder

Flat Field Not Calibrating
Flat Field Not Calibrating Gradient Removal


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(@vincent-mod)
Galaxy Moderator
Joined: 2 years ago
Posts: 725
August 14, 2019 22:10  

That looks really strange indeed. Exactly how are you taking you flats, do you use special software (like SGP) to take them, what are your settings etc. I'll have a look at your data as well.


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(@huerbsch)
White Dwarf Customer
Joined: 2 years ago
Posts: 13
August 14, 2019 23:15  

@vincent-mod

AV mode with BYEOS. I put the flats in the Dropbox link.


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(@huerbsch)
White Dwarf Customer
Joined: 2 years ago
Posts: 13
August 14, 2019 23:16  

@vincent-mod

Triple layer of diffusion fabric stretched over lens with rubber band. Lens pressed against flat white laptop screen. Same f/4 but lower ISO which shouldn’t matter.


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(@astrogee)
Main Sequence Star Customer
Joined: 1 year ago
Posts: 41
August 14, 2019 23:34  

Ah ha! I had the exact same problem. Let me guess, you have a light pollution filter screwed onto the end of your lens. The problem is created by stray light. I corrected it by making a lens hood out of black felt and extended quite a bit beyond the end of the lens - acts as a good dew shield too, when getting the lights. I take my flats by pointing to a dusk sky and one needs a lot of light baffle to prevent stray light. If you are using the t-shirt method, it may have something to do with where you put the t-shirt and if you have a filter or not. I found with the t-shirt method, you cannot have a dew shield and probably not a LP filter at the end of the lens.


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(@huerbsch)
White Dwarf Customer
Joined: 2 years ago
Posts: 13
August 14, 2019 23:37  

I am not using any filters  Best they can cause reflections on bright stars  but I am using the lens hood.  For flats I have to remove the hood. I’ll try and do some tests without using the lens hood. Good point thanks 


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(@vincent-mod)
Galaxy Moderator
Joined: 2 years ago
Posts: 725
August 14, 2019 23:56  

Ok, I'll get to your data tomorrow. Flats do indeed need to be taken in the exact same conditions as your lights so that hood difference might affect it. Keeping the focus exactly the same is important too, I can imagine the lens focusring shifting a little bit easily.

This post was modified 1 month ago by Vincent Groenewold - Moderator

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(@huerbsch)
White Dwarf Customer
Joined: 2 years ago
Posts: 13
August 15, 2019 01:02  

Good point on the focus, I’ll tape the focus next time. 


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(@vincent-mod)
Galaxy Moderator
Joined: 2 years ago
Posts: 725
August 15, 2019 11:01  

ps. I had a look at your data and it looks like the flats are working actually, I'm now suspecting stray light maybe in your lights. And this is more likely as I look closer, trying without flats and then to get the gradient gone, APP clearly is unable to do so as there is no normal gradient. This is almost always the case with light coming into your lens from e.d. a light somewhere in a corner or so.

This post was modified 1 month ago by Vincent Groenewold - Moderator

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(@mabula-admin)
Quasar Admin
Joined: 2 years ago
Posts: 2127
August 20, 2019 19:04  

Hi Robert @huerbsch (& @vincent-mod),

Were you able to solve your flat issue in the mean time?

I would think your issue is caused by either:

  1. stray light entering your light and your flats in different amounts...
  2. not shooting the flats with the exact same optical train as the lights. The circle illumination pattern actually might have to do with the lens hood being there or not in the optical train and/or the pressing as mentioned : "Lens pressed against flat white laptop screen"

Kind regards,

Mabula

Main developer of Astro Pixel Processor and owner of Aries Productions


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(@huerbsch)
White Dwarf Customer
Joined: 2 years ago
Posts: 13
August 21, 2019 00:50  

Thanks everyone, I will try your recommendations and report back. 


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