Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Optimal number of light sub frame?

  • Please log in to reply
17 replies to this topic

#1 Stargazing)

Stargazing)

    Lift Off

  • -----
  • topic starter
  • Posts: 15
  • Joined: 05 Mar 2021

Posted 18 June 2021 - 01:24 PM

Hi guys,
Is there an online calculator that computes the optimal number of light Subs for a specific  dso magnitude using a specific dslr or astro camera?
My believes is that after a certain number of  light subframes the results of the final stacked  image can not get any better... Please correct me if i am wrong.
Thanks



#2 don314

don314

    Explorer 1

  • -----
  • Posts: 95
  • Joined: 10 Apr 2017

Posted 18 June 2021 - 01:44 PM

This subject has been discussed in “Beginning Deep Sky Imaging” and “Experienced Deep Sky Imaging” forums.  The Moderator may move the question to one of these, or you can do it yourself.



#3 fewayne

fewayne

    Apollo

  • *****
  • Posts: 1,413
  • Joined: 10 Sep 2017
  • Loc: Madison, WI, USA

Posted 18 June 2021 - 02:01 PM

"Optimal" is a pretty slippery concept here. You want the sub-exposure to be long enough that the signal swamps the read noise, but even that depends on your definition of "swamped". Other things being equal, you want the longest possible exposure...but other things are never equal! The longer the exposure, the more vulnerable you are to tracking errors, satellites, airplanes, gusts of wind, fireflies landing on your objective and going "MROOOM" with bright green light...and none of these are very easy to quantify. Or, for that matter, how much disk space and processing time you're willing to devote to kajillions of sub-exposures, going the other way,

 

The sky transparency and light pollution also figure into the mix, adding noise while stealing some dynamic range.

 

So the answer I'd give you is "as long as you can do without too many weird things happening, and without blowing out any bright detail". The real key to the kingdom is total integration time.

 

As for the swamping, there are some good links in this thread.



#4 rj144

rj144

    Viking 1

  • -----
  • Posts: 764
  • Joined: 31 Oct 2020

Posted 18 June 2021 - 02:10 PM

It always helps to have more integrated time (not number of subs).  Whether the difference is discernible to you is another question.



#5 fewayne

fewayne

    Apollo

  • *****
  • Posts: 1,413
  • Joined: 10 Sep 2017
  • Loc: Madison, WI, USA

Posted 18 June 2021 - 02:11 PM

Bracken has a good intro to these concepts, and Woodhouse goes into it in greater depth.


  • psandelle likes this

#6 Stargazing)

Stargazing)

    Lift Off

  • -----
  • topic starter
  • Posts: 15
  • Joined: 05 Mar 2021

Posted 18 June 2021 - 02:14 PM

Thanks for your reply..

Your right Optimal might be a wrong world to use.. I am shooting a 30s sub frame with my dslr  and wanted to know  if tgere is an online calculator that comoutes after how many light subs you start getting dimishining return.. 



#7 rj144

rj144

    Viking 1

  • -----
  • Posts: 764
  • Joined: 31 Oct 2020

Posted 18 June 2021 - 02:41 PM

Thanks for your reply..

Your right Optimal might be a wrong world to use.. I am shooting a 30s sub frame with my dslr  and wanted to know  if tgere is an online calculator that comoutes after how many light subs you start getting dimishining return.. 

Diminshing returns are quite subjective though.



#8 17.5Dob

17.5Dob

    Fly Me to the Moon

  • ****-
  • Posts: 7,211
  • Joined: 21 Mar 2013
  • Loc: Colorado,USA

Posted 18 June 2021 - 03:27 PM

Thanks for your reply..
Your right Optimal might be a wrong world to use.. I am shooting a 30s sub frame with my dslr and wanted to know if tgere is an online calculator that comoutes after how many light subs you start getting dimishining return..


Don't worry about your sub count. Worry about your total exposure time. I try for a minimum of 4 hrs a target at my mag 21.7 dark site. Some targets still require 2-4x that much. If you're shooting in a location with a lot of light pollution, it might take 30-40x more than that. Trying to quantify exposure by sub count is meaningless.
  • psandelle likes this

#9 Peregrinatum

Peregrinatum

    Surveyor 1

  • *****
  • Posts: 1,811
  • Joined: 27 Dec 2018
  • Loc: South Central Valley, Ca

Posted 18 June 2021 - 05:27 PM

It depends on how dark your skies are... you are aiming for total integration time.  Every jump up in the bortle scale requires 2.5x more total integration time to have equal signal, all other factors being equal.  Look at pics on astrobin for your Bortle skies and find out how long the total integration is, that will give you a head start.  My skies are 19.5 sqm (6/7 Bortle), and I determined that I needed a minimum of 10 hours for an acceptable image, I usually end up with 15 to 20 hours though.



#10 Voska

Voska

    Vostok 1

  • -----
  • Posts: 177
  • Joined: 28 Oct 2019
  • Loc: Beach Park, IL, USA

Posted 18 June 2021 - 07:12 PM

I believe the correct answer to this is always going to be N+1


  • psandelle, 17.5Dob and Phishin_phool like this

#11 Phishin_phool

Phishin_phool

    Viking 1

  • *****
  • Posts: 573
  • Joined: 31 May 2020
  • Loc: Ohio

Posted 18 June 2021 - 07:46 PM

42 

- Douglas Adams


  • jonnybravo0311 likes this

#12 antsalan2002

antsalan2002

    Sputnik

  • -----
  • Posts: 44
  • Joined: 09 May 2009

Posted 18 June 2021 - 08:44 PM

This is a video From one of the the makers of SharpCap Dr Robin Glover that I thought was good( there is also a follow up video)
https://youtu.be/3RH93UvP358
  • psandelle likes this

#13 antsalan2002

antsalan2002

    Sputnik

  • -----
  • Posts: 44
  • Joined: 09 May 2009

Posted 18 June 2021 - 08:44 PM

This is a video From one of the the makers of SharpCap Dr Robin Glover that I thought was good( there is also a follow up video)
https://youtu.be/3RH93UvP358

#14 bobzeq25

bobzeq25

    ISS

  • *****
  • Posts: 25,024
  • Joined: 27 Oct 2014

Posted 18 June 2021 - 09:24 PM

Hi guys,
Is there an online calculator that computes the optimal number of light Subs for a specific  dso magnitude using a specific dslr or astro camera?
My believes is that after a certain number of  light subframes the results of the final stacked  image can not get any better... Please correct me if i am wrong.
Thanks

More is always better.  But it's a square root relationship.  4 times as many subs gives you twice as good an image.

 

The key parameter is total imaging time.  That (and the optical speed of your scope) sets how many photons you capture.  You want a lot.  <smile>

 

My rule of thumb.  One hour minimum.  Two is better, four is good.  My longest is 7 hours.  Serious people do 10 or more, imaging the same target over multiple nights.

 

For you (30" subs) 4 hours would be 480 subs.  Gives you the idea.

 

If you want to know more, the first couple of pages of this are excellent.  Look at the graphs.  Note the curves rise steadily with no real stopping point.  They do tend to level off after a while, that's the square root deal.

 

https://jonrista.com...duction-part-1/


Edited by bobzeq25, 18 June 2021 - 09:33 PM.

  • psandelle likes this

#15 don314

don314

    Explorer 1

  • -----
  • Posts: 95
  • Joined: 10 Apr 2017

Posted 18 June 2021 - 10:12 PM

Keep in mind that 1 hour exposure in a dark Sky does not equal 1 hour exposure in a light polluted sky. 
A way to compare such exposures you need to know the SQM of the background sky.  The relationship is 2.512 (the fifth root of 100, or one magnitude) raised to the power of the difference in SQM values.
Example:   I measure an SQM of about 18.5 typical at home.  If I go to a dark site the SQM is 21 typical.  The difference is 2.5 SQM, so you get 2.512^2.5 = 10.  This translates to "I need 10 hours exposure in my light polluted sky to match 1 hour exposure under my dark sky location".
Charles Brackens book “The Deep-sky Imaging Primer – Second edition” (mentioned in post 5) explains this well.  I strongly suggest getting this book, I would have been lost without it when I started imaging.


  • Phishin_phool and bazookaman like this

#16 Stargazing)

Stargazing)

    Lift Off

  • -----
  • topic starter
  • Posts: 15
  • Joined: 05 Mar 2021

Posted 19 June 2021 - 02:55 AM

Thank you guys for your answers..they were very helpful..i Appreciate it.
  • bobzeq25 likes this

#17 Stargazing)

Stargazing)

    Lift Off

  • -----
  • topic starter
  • Posts: 15
  • Joined: 05 Mar 2021

Posted 19 June 2021 - 02:55 AM

Thank you guys for your answers..they were very helpful..i Appreciate it.
  • bobzeq25 likes this

#18 addos

addos

    Sputnik

  • -----
  • Posts: 45
  • Joined: 09 Mar 2021

Posted 21 June 2021 - 02:18 AM

also need to factor local light pollution and the brightness of the target, as this will drastically impact the amount of total integration time you need to get a decent image, unless using light pollution filters.  im bortle 6/7 and without a light pollution filter, on less than say a mag 4 target, the numbers start getting scary.  


Edited by addos, 21 June 2021 - 02:18 AM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics