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Any problem with 10-second flats?

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#1 murray8144

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Posted 17 September 2021 - 05:40 PM

I'm about as lazy as folks get, so it's taken more than a year of AP to finally make it a habit to consistently make flats part of my imaging routine.  I'm using the ASIair to run the ASI1600mm-Pro, shooting through ZWO 7nm narrowband filters in case that matters.  There's an option for "auto flats", where the ASIair determines the appropriate flats exposure length.  I doubled up a t-shirt and rested an iPad on top of my scope, used "auto flats", and the resulting exposure length was 10s.  In my newb AP experience, it seems that most flats exposure lengths are 1-2 seconds...any problems/issues/drawbacks to my flats being 10s?

 

thanks



#2 idclimber

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Posted 17 September 2021 - 06:05 PM

I never used a t-shirt when I used an iPad. I just used the iPad by itself. This was with my 1600mm before I upgraded to the 2600mm. You could change to a piece of vellum or even a piece of white acrylic. 

 

I routinely take my flats at 3-10 seconds using the program in Voyager (sky flats). I would not expect a problem with the 1600mm as long as you properly calibrate the flats with flat darks of approximately the same exposure.


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#3 SnowWolf

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Posted 17 September 2021 - 06:07 PM

Shouldn't be a problem, but you could remove the t-shirt and just use the iPad for a shorter exposure.

 

With the ASI1600, you probably want to use dark-flats instead of bias frames.  In fact, the 10 seconds plays into the ASI1600.  It doesn't like really short exposures.


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#4 zakry3323

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Posted 17 September 2021 - 06:24 PM

I get results I like with my 1600mm when I make sure I'm exposing for longer than 1 second. As long as the exposures aren't shorter than a second, they calibrate well.
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#5 murray8144

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Posted 17 September 2021 - 06:31 PM

thanks all - much appreciated.  I am indeed also shooting dark flats @ whatever exposure length matches the flats, in this case 10s.  didn't know I could just use the iPad w/out the shirt...many thanks for that tip...


Edited by murray8144, 17 September 2021 - 07:12 PM.


#6 dswtan

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Posted 17 September 2021 - 06:47 PM

I routinely take ~10s flats with the 1600. I use t-shirts and a drawing panel (cheap). While others posting above seem to be using (naked!) iPad screens without issue, I will just point out that empirically, I've noticed the vast majority of posters posting here saying "my flats don't work" are using iPads. And possibly naked. :-)


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#7 SnowWolf

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Posted 17 September 2021 - 08:16 PM

The idea for the t-shirt is to blur the pixels from the iPad.  But, when the screen is so far from the infinity focus, it won't be any where near focus anyway. I've made good flats with a laptop screen perched on the dew shield.


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#8 ChiTownXring

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Posted 17 September 2021 - 09:01 PM

I use NINA's flats wizard with a double layer of t-shirt over my scope and the flats average 2.15 sec and my Dark Flats the same..


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#9 dswtan

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Posted 17 September 2021 - 11:14 PM

The idea for the t-shirt is to blur the pixels from the iPad.  But, when the screen is so far from the infinity focus, it won't be any where near focus anyway. I've made good flats with a laptop screen perched on the dew shield.

If that is what people think the t-shirt is for, I agree it’s completely ridiculous and not why I use a t-shirt with my panel at all. (Which doesn’t even have pixels.) The shirt is to dim the light from the panel in my case. For a 1600 camera, I *want* multisecond exposures, to avoid sensor nonlinearities with short exposures. 


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#10 SnowWolf

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Posted 18 September 2021 - 12:44 AM

If that is what people think the t-shirt is for, I agree it’s completely ridiculous and not why I use a t-shirt with my panel at all. (Which doesn’t even have pixels.) The shirt is to dim the light from the panel in my case. For a 1600 camera, I *want* multisecond exposures, to avoid sensor nonlinearities with short exposures. 

Well, yeah.  It's also to dim the light from the panel. 

 

But there are better ways to do that - Lexan sheets or sheets of paper.  That's what I used before I got the USB controller for the Spike-a-flat.  A t-shirt wasn't even enough across the frame or consistent from session to session.

 

I ended up with several sheets of paper sandwiched between 2 translucent Lexan sheets. It wasn't particularly cheap, but it worked consistently. And, by changing the number of sheets of paper or just using the Lexan sheets, I could dial in different light levels for broadband and narrowband filters.




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