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#26 unimatrix0

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Posted 23 July 2021 - 09:41 AM

I have recently tested my camera with narrow band filters.  Both N.I.N.A and Sharpcap recommended me to take 80 second subs at unity gain. 
I did

-300 second 
- 240 second

- 120 second
- 90 second

-60 second 
subs at various emission nebulae.  80 second sounds small for an osc camera with a duo-narrow band filter, but it does work. 

my 4x80s (320 second) subs stacked looked very much like a single 1x300s, with the benefit of better stars.  Both versions shot at the same temp/gain/offset. 
Last night I was waiting for Saturn to approach the Meridian and to pass time, I was live-stacking the Eagle Nebula  with 4 second subs and 2x the unity gain.
All that on an alt-az mount and not even using a flattener or cooling. At 10 minute integration, with sigma clipping enabled  the image looked suprisingly nice, beside the amp glow and some elongated stars near the edges, which is expected.

So, it can be done, I also confirmed by doing this, that my flattener is junk, because the stars look good (no waterdrop like shape) without a flattener. 

 

Also, easy way to check if you blown the stars: 
- Also I learned something.  Check your subs you're taking in both stretched and un-stretched. In un-stretched view  if you see a lot shining stars, those are all over saturated. Just turn the stretching on/off in NINA after taking a sub. 
There always be a few, but if you get a lot, you're over saturating your image. The less stars shown on an unstretched image (some will be blown, no matter what) the better chance for a nice image. 


Edited by unimatrix0, 23 July 2021 - 09:48 AM.


#27 bobzeq25

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Posted 23 July 2021 - 11:13 AM

I appreciate the healthy discussion so far. I've been thinking about this lately as I have standardized at 5min subs at FL around 1000mm, f/7, CEM60 guided, but my keep rate is 50-60%. I'd like a better keep rate, but fear going down in exposure time I will lose out on finer detail in my image. I guess I will have to run a test to be sure.

Changing subexposure time (down to a point, eventually read noise gets you) is unlikely to  cost you dim detail, _provide_ you maintain total imaging time.  What counts most is the total number of photons, not how you break the total into subs.

 

Yeah, it's unintuitive.  But it's why we stack.


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#28 bobzeq25

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Posted 23 July 2021 - 11:16 AM

- Also I learned something.  Check your subs you're taking in both stretched and un-stretched. In un-stretched view  if you see a lot shining stars, those are all over saturated. Just turn the stretching on/off in NINA after taking a sub. 
There always be a few, but if you get a lot, you're over saturating your image. The less stars shown on an unstretched image (some will be blown, no matter what) the better chance for a nice image. 

Bingo.  <smile>  I'd alter a bit.  No stars (unstretched) is also a sign you're out of the ballpark, maybe substantially.  A few is best, then you're close enough.
 


Edited by bobzeq25, 23 July 2021 - 11:40 AM.

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#29 idclimber

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Posted 23 July 2021 - 11:46 AM

Several weeks ago I moved to Voyager for my automation software. I believe their fits viewer is free and can be used by anyone with windows PC. It uses PI algorithms to stretch the subs and has a really cool metric besides the typical average ADU and FWHM/HFD. It gives you the number of pixels in your image that are saturated.

 

In the example sub from last night you can see my average ADU is right around 1,100 as well as that their are 593 pixels that are fully saturated at an ADU value of 65,535. If I turn off the stretch you can see that is the cores of about a dozens stars around the image. I would probably benefit from reducing the exposure from 600 to 300 in this case.

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#30 Tapio

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Posted 23 July 2021 - 12:10 PM

Strange that you have so low keep rate.


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