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astrophotography controversies: subexposure time

funny to find two different approaches in light polluted environment:


1) Roger Clark use short exposure times that is 1/4 to 1/3 histogram, and use more subs:


Once one reaches a sky histogram of 1/4 to 1/3 histogram on the camera LCD, exposure time is long enough and longer exposures will not improve given a total exposure time. Simply saying sub exposures are not long enough without knowing other parameters is insufficient and could actually be detrimental. Exposing longer so that the sky histogram peak rises above the 1/2 level would decrease dynamic range, saturate more stars and bright areas losing color. For example, below is an image that goes at least as faint as your image, yet is ONLY 1-minute subs and only 9-minutes total exposure time. In another thread you said you do longer than one hour total exposure time and chided me for not going as long. How long is your exposure time? (Note too you have very little h-alpha and lost a lot of red).

Exposure is collecting light.  Two factors collect light from the subject: aperture area and exposure time.  

Sub exposure should be such that signal from subject and sky glow raises the histogram above the read noise.  For modern DSLRs that occurs at about the 1/4 to 1/3 histogram level.  Once you reach that level, read noise is insignificant and increasing sub-exposure time won't improve the image.  

For example, if histogram is ~1/3 level, then 100 1-minute exposures = ten 10-minte exposures in terms of reaching the faint stuff.  But if 1-minute exposures reaches the 1/3 histogram level, a 10 minute exposure is simply blowing out (blooming) more stars and brighter nebula.  The better image will be made from the set with histogram reaching no more than about 1/3 histogram level.

 or

2) Jon Rista

use long exposure times, and less subs:

...You could overcome that by stacking considerably more sub frames, but because of the way read noise compounds through an integration, you really want to take longer subs to pull out the background details best.

Your longest were 120 seconds, it seems? You only had two of those, which just isn't enough, but even if you had say 16, you probably wouldn't have quite enough to eliminate the banding. You probably need to go longer than that. At f/5, you probably want 360 second subs for your longest subs. I say that, because I used 270 second subs at f/4 with a 150mm aperture on this image.

dpreview link here

the argument is:
"100 subs x 1 minute per sub"  is same as "10 subs x 10 minute per sub"?


John Smith has made a online calculator for many CCDs in CCDware for sub-exposure

based on this, Steve Cannistra  and then Neil Fleming has developed their spreadsheet here

Steve also explained the Signal-to-Noise ratio as:

SNR (Tot) = sqrt[K*tsub]*(Obj) / sqrt[(Sky+Obj)*tsub + R2]
  



From Gibraltar Astronomy Society, they(based on Kayron Merciehgca) have made a Excel spreadsheet for this .

In case you don't know the characterisics ( read noise, ADU, dark current...) of your CCDs,

you need to make the calibration first.

David Haworth introduced on this, and used AIP4Win ( but seems this SW needs update )

John Smith has also made an excellent introduction on SNR and related stuff

Craig Stark has in depth discussion on SNR in cloudynights, fishing for photons column, and in his own personal website
 

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