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Experimenting with sub-exposure length

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#1 Rickster

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Posted 17 November 2019 - 02:12 PM

Results summary for those who don't want to plow through the entire post -  Increasing sub-exposure length improved image contrast and reduced image resolution.  The optimum was in the 3 to 10 second range (with a total stack exposure of 5 minutes in all cases)  There are many variables, so results are likely to be different from rig to rig and night to night.  Please only use these results as a starting point for your own experiments.

 

This post stems from a recent post that suggested that longer sub-exposures are better.  https://www.cloudyni...r-even-for-eaa/

 

Two nights ago, I had an opportunity to run some sub-exposure length experiments.  I am posting the results in this thread.  But before I do, I want to be clear that these results are in no way definitive, nor do they apply broadly.  There are many variables.  I tried to eliminate as many variables as I could, but there were many that I could not eliminate.  As they say, your mileage may vary.

 

The camera was an ASI183MM Pro (mono cooled).  The telescope was a 16" f4.4 Newt on a German Equatorial Mount with autoguiding maintaining a total RMS error in the 1.5 arc second range.  1.5 arc-sec is a bit high, but not bad, and was limited by frequent light wind gusts which jostled the scope and made the skies slightly unstable.  On a calm night this rig is capable of 0.5 arc-sec.  Ambient temperatures were roughly 30 degrees.  The skies were clear with minor instability.  The camera cooler was maintaining sensor temp at -20C.  The location has a dark sky, but a 3/4 moon was overhead.

 

Six 5 minute stacks were taken; one each for sub-exposure lengths of 1, 2, 3, 10, 30 & 60 seconds.

Gain was readjusted for each sub-exposure length trial to obtain similar histogram peaks for each stack. Only minor adjustments were made to the histogram dark and mid points.  

 

An inexpensive UV/IR filter was used.  No darks or flats.  No coma corrector.

 

The images have been resized to fit the forum.  You may not be able to tell much difference, depending on your monitor settings.  And, even though using a 32in screen at full resolution, I found judging the images to be a little challenging.  I found that the most telling factor was in how easy/difficult it was to get a good image when adjusting the histogram.  Very little fiddling was required for the better exposure lengths.

 

I found that image resolution improved as sub-exposure time was reduced.  Conversely, contrast improved as sub-exposure time increased.  The best balance between resolution and contrast was in the 3 to 10 second range.

 

The images follow.  The target is the eastern veil. Make of them what you will.


Edited by Rickster, 17 November 2019 - 02:23 PM.

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#2 Rickster

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Posted 17 November 2019 - 02:14 PM

1 second

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#3 Rickster

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Posted 17 November 2019 - 02:15 PM

2 second

 

Edit to add: The background is brighter on this image because it was the second shot and I hadn't standardized the black point yet.  The black point for the 2 sec shot was behind the histogram peak.  In the subsequent shots, I set the black point right on the histogram peak, for the sake of consistency, and to cut off more of that pesky moonlight.

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Edited by Rickster, 17 November 2019 - 08:17 PM.

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#4 Rickster

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Posted 17 November 2019 - 02:16 PM

3 second

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#5 Rickster

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Posted 17 November 2019 - 02:17 PM

10 seconds

 

Edit to add: The background is brighter on this image because it was the first shot and I hadn't standardized the black point yet.  The black point for the 10 sec shot was behind the histogram peak.  After the 2 second shot, I set the black point right on the histogram peak, for the sake of consistency, and to cut off more of that pesky moonlight.

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Edited by Rickster, 17 November 2019 - 08:18 PM.

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#6 Rickster

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Posted 17 November 2019 - 02:19 PM

30 seconds

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#7 Rickster

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Posted 17 November 2019 - 02:20 PM

60 seconds

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#8 sg6

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Posted 17 November 2019 - 02:26 PM

Quick visual scroll through - prefer the 3 second and the 60 second.

More nebula detail in the 60 but also slight drifting/trailing.

 

Background in the 3 and 60 appears darker.


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#9 mikenoname

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Posted 17 November 2019 - 02:51 PM

Great test, Rick.


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#10 Rickster

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Posted 17 November 2019 - 03:29 PM

Quick visual scroll through - prefer the 3 second and the 60 second.

More nebula detail in the 60 but also slight drifting/trailing.

 

Background in the 3 and 60 appears darker.

Good eye sg6. 

 

I have added a note to the 2 second and 10 second slides to clarify that the background is brighter because they were the first shots and I hadn't standardized the black point yet.  The black point for those two shots was behind the histogram peak.  In the subsequent shots, I set the black point right on the histogram peak, for the sake of consistency, and to cut off more of that pesky moonlight.


Edited by Rickster, 17 November 2019 - 08:19 PM.


#11 GaryShaw

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Posted 17 November 2019 - 04:15 PM

Great test Rickster, thanks for sharing it!

 

This kind of research is great to see. It’d be great if the CN EEA folks could collaborate in developing a list of useful tests like this to conduct along with a few basic standards to ensure usability of the results. Then the testing could be carried out by volunteers and whoever has the right gear for each test. Common Image standards for presenting the results would be important as well I imagine....just a thought.


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#12 DSO_Viewer

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Posted 17 November 2019 - 09:43 PM

30 seconds

I like the 10 to 30 second range and notice that there is less noise than the 60 second stacked image. 

 

Steve


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