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ASI294MM -- Simplifying Flats

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#1 sternenhimmel

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Posted 16 April 2024 - 02:29 PM

I'm starting to get a hang of taking flats for my ASI294mm, but it's still a bit of a chore and I'm wondering if there are ways to improve the process at all. 

 

Because the ASI294mm sort of requires exposures to be >= 3s, I can't use the autoexposure feature to take my flats. My current procedure looks something like this:

 

  • Take white pillow case folded X times and place atop aperture. 
  • Take tracing pad with adjustable brightness set to Y brightness on top of the aperture. 

I then tune X and Y depending on what filter I'm shooting through. For Luminance, I usually need 8 or 12 folds in the pillow case and the brightness setting to be at it's lowest to get something reasonable. For narrowband, usually 4 folds and the brightness somewhere in the middle. RGB somewhere between these settings. 

 

Last night I shot, L, RGB, and some narrowband, so taking flats required a lot of hands on adjusting between sequences, rather than just being able to walk away. 

 

I don't think there's an easy way out here, but thought I would ask. 



#2 dswtan

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Posted 16 April 2024 - 03:30 PM

I have multiple years on 2 concurrent ASI294MMs and haven't needed to be strict on the >3s "rule". 

 

I've previously posted numbers using an "Amazon cheap tracing panel", and a folded white t-shirt through f/4.8 and f/7, using the NINA wizard and auto exposure: 
 

https://www.cloudyni...ats/?p=13091167

 

0.03s and 0.11s on L

up to

2.8s and 13.53s on SII

 

It does appear though that "YMMV", perhaps with panel and filter variables. Also t-shirts. :-)

What I'm saying is -- have you just tried with shorter numbers and seeing how it goes? It works (two different cameras) for me...



#3 Drothgeb

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Posted 16 April 2024 - 06:14 PM

I have an adjustable tracing panel, and shoot 0.5 sec dark flats. Then just make sure the flats are 0.2 to 0.8 sec. Works for my 294 MM and MC.



#4 Oort Cloud

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Posted 16 April 2024 - 08:20 PM

https://www.amazon.c...ef=nb_sb_noss_1

#5 psienide

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Posted 16 April 2024 - 08:38 PM

I just take my flats against a dimly lit wall. Works fine. I tried flat panel and found that it was way worse.



#6 sternenhimmel

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Posted 17 April 2024 - 11:29 AM

Interesting. I actually don't know in what ways the 294mm behaves badly with short exposures, I just heard that it did. If I can shoot down to 1s, that would actually alleviate most of the need of adjusting the pillow case. 



#7 CharLakeAstro

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Posted 17 April 2024 - 06:53 PM

Review the linked thread below...

 

JohnnyBravo collected 294mm data which shows more than 1.5sec, gives linear response. Less than 0.8sec is slightly linear. Safest bet is 2 sec for flats. Using 1 sec would be the worst area for non linearity. 

 

https://www.cloudyni...elp/?p=11040561

 

gallery_347158_15202_437.png

 

Interesting. I actually don't know in what ways the 294mm behaves badly with short exposures, I just heard that it did. If I can shoot down to 1s, that would actually alleviate most of the need of adjusting the pillow case. 


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