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Longer Subs to Save Time?

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#1 Sparks-M16

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Posted 20 April 2021 - 12:20 PM

There are a lot of discussions on shorter vs. longer sub-exposures  with a similar total exposure time.  And I don't recall a definitive answer that one is dramatically better than the other.  So if say 6 hours of exposure time with 150s subs should produce roughly a similar result as 6 hours of exposure time with 300s subs, and assuming there are no technical issues with guiding and acquisition, shouldn't I go ahead with the 300s subs just to reduce the computer calibration, registration and integration time?  My astro-dedicated computer isn't all that fast and with 40 or 50 lights, its takes a good hour (or more) to stack.  For the simple goal of reducing preprocessing time, wouldn't fewer lights be quicker?  Does it work that way, or do longer subs have more data in each sub, making the over-all process take the same amount of time?



#2 Midnight Dan

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Posted 20 April 2021 - 12:27 PM

The overall processing time will definitely be shorter with fewer subs.  But that shouldn't be your determining factor.  Coffee was invented to occupy your time while waiting for your computer. :-)

 

Seriously though, there are other factors that should determine your sub exposure length.  You want to expose long enough so that the background level is high enough to swamp the read noise of your camera.  But you don't want your exposures so long that too many of your stars are saturated and you lose star color.

 

-Dan


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#3 Reece

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Posted 20 April 2021 - 12:37 PM

I'm not sure what Astro equipment you have. Some of the cheaper mounts like I have are not capable of reliably tracking for 300 seconds so that is one possible reason to use shorter subs. Also if there are a lot of airplanes or very unpredictable weather. More experienced members than me will tell you like they told me that when starting out "total integration time" will generally make a much larger difference than getting the sub-exposure times just right. 

 

 You could have your computer do the stacking when you go to sleep at night and then just set the computer in "settings" to go into low power / sleep mode after 2 hours or whatever amount of time would be adequate for the stacking process. I have a 9 year old laptop and it handles 300 subs fine but it does take 1 full hour like you said. At 150s per sub, that would be only 24 subs per hour so 6 hours would only be 144 subs.

 

If you do a Google search for "Dr. Robin Glover video" (no quotes), there is a very helpful 53 minute video that should be at the top of the search results.


Edited by Reece, 20 April 2021 - 12:39 PM.

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#4 cybermayberry

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Posted 20 April 2021 - 12:42 PM

There are a lot of discussions on shorter vs. longer sub-exposures  with a similar total exposure time.  And I don't recall a definitive answer that one is dramatically better than the other.  So if say 6 hours of exposure time with 150s subs should produce roughly a similar result as 6 hours of exposure time with 300s subs, and assuming there are no technical issues with guiding and acquisition, shouldn't I go ahead with the 300s subs just to reduce the computer calibration, registration and integration time?  My astro-dedicated computer isn't all that fast and with 40 or 50 lights, its takes a good hour (or more) to stack.  For the simple goal of reducing preprocessing time, wouldn't fewer lights be quicker?  Does it work that way, or do longer subs have more data in each sub, making the over-all process take the same amount of time?

For the simple goal of reducing preprocessing time, wouldn't fewer lights be quicker? Yes, but that doesn't mean better results. 

Most users here are more limited by light pollution as to the length of time they can expose for then any other factors.

More subs gives you better SNR to a point, but it's always a balancing act on the physical characteristics of your equipment, and environmental factors at your location at any given time.



#5 unimatrix0

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Posted 20 April 2021 - 12:47 PM

There are a lot of discussions on shorter vs. longer sub-exposures  with a similar total exposure time.  And I don't recall a definitive answer that one is dramatically better than the other.  So if say 6 hours of exposure time with 150s subs should produce roughly a similar result as 6 hours of exposure time with 300s subs, and assuming there are no technical issues with guiding and acquisition, shouldn't I go ahead with the 300s subs just to reduce the computer calibration, registration and integration time?  My astro-dedicated computer isn't all that fast and with 40 or 50 lights, its takes a good hour (or more) to stack.  For the simple goal of reducing preprocessing time, wouldn't fewer lights be quicker?  Does it work that way, or do longer subs have more data in each sub, making the over-all process take the same amount of time?

Your enemy here is your sky brightness and light pollution vs how good your camera sensors are.  I could make 300 second subs, but I need either a strong light pollution filter or narrow band filter not to overspill the welldepth of the camera sensors. Otherwise the image is blown out and detail is lost. The other thing is why many people take short subs but many, due to satellite trails, airplanes, renegade clouds etc.  Another is stomping noise. many-many stacked images stomps the noise better than few but long exposure images. Also sharpness short exposure stacks have better sharpness.  All these have been demonstrated and explained over and over in youtube videos and posts here on the forums. 
BTW I'm not talking about people doing 10 minute subs with specific narrowband filters aimed at faint nebulae 




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