Back to forums

Is there a way to calculate the needed exposure time at a given f-ratio at which more data will not lead to a better picture anymore?

Acquisition Optics Deep Sky
10 replies•1.4k views
Anderl avatar
Anderl avatar
is there any way i can calculate or at least come close to the needed total exposure time under a given f-ratio and bortle scale at which i won’t get a better image if i image any longer?

I also want to bring another question as well. 

is there a point at which a slow scope will never be able to deliver the same amount of detail and depth as a fast scope? To be concrete is a f12 scope able to deliver the same amount of information as  fast scope like a rasa as long as i get enough exposure time?
Arun H avatar
Arun H avatar
  1. I don't believe the first question can we answered in any definitive or meaningful way, simply because it is dependent on the brightness of the object you are trying to image. And even then, very few or no objects are of uniform surface brightness. You will want to image longer to bring out the dim parts of an object, so a single formula is not going to be useful.
  2. In answer to question 2, fast versus slow is not enough information in itself. So long as the object of interest fits within the field of view of both imaging systems, a larger aperture scope (as opposed to simply a fast one) will give you better object signal to noise ratio when viewed at the same image scale simply because it is capturing more photons from the object. For example, a 35 mm focal length f/2 lens is "faster" than a 14" aperture f/12 scope, but the 14" will vastly outperform the tiny lens on dim objects.
Well Written Helpful Insightful Concise
Wim van Berlo avatar
Wim van Berlo avatar
I find that the practical limit is determined by other factors than achievable signal to noise ratio. Whenever I use absurdly long integration times, I find that if I stretch my image hard enough, I eventualy end up seeing the remains of amp glow or the faintest hints of dust bunnies, even though I consider my image calibration process to be good (but not perfect obviously).
Helpful Insightful Concise
glennb44 avatar
glennb44 avatar
Yes.
SharpCap Pro will do the calculation for you. In the histogram hit the brain function.
GlennB
Christian Koll avatar
Christian Koll avatar
See this excellent talk of Robin Glover here:

https://www.youtube.com/watch?v=3RH93UvP358&t

CS
Chris
Robert Winslow avatar
Robert Winslow avatar
Sharp cap does a very nice job of this.  Very pleased with how well it does.
Daniel Arenas avatar
Daniel Arenas avatar
With the permission of the owner of this post I'm going to reformule the question…

How do you know when to stop acquiring data for an object?

I mean, most people will answer "the most, the best" (the most data you can collect, the best results you'll have) but, when to stop?. Is there any amount of collected time after which you won't notice any upgrade? I'm not asking about the quantity of hours in general terms (I know that will depend on the object) but how to know when to stop.

Daniel
Engaging
Christian Koll avatar
Christian Koll avatar
Daniel,

for me its 100-120 subexposures.
More than that yields no visible improvement.

That is for subexposures of 3-5 minutes duration.

CS 
Chris
Concise
andrea tasselli avatar
andrea tasselli avatar
Daniel Arenas:
With the permission of the owner of this post I'm going to reformule the question...

How do you know when to stop acquiring data for an object?

I mean, most people will answer "the most, the best" (the most data you can collect, the best results you'll have) but, when to stop?. Is there any amount of collected time after which you won't notice any upgrade? I'm not asking about the quantity of hours in general terms (I know that will depend on the object) but how to know when to stop.

Daniel

If you double the exposure time and the background noise shows no sign of improvement then you know you better stop.
Arun H avatar
Arun H avatar
What Andrea said is a reasonable rule of thumb. What you're looking for is improvement in signal to noise ratio.

In cases where read noise is not a factor (for most of us it isn't), you need to quadruple the exposure time to get a 2x improvement in signal to noise ratio. So if you are not satisfied with your 10 hour image, imaging a couple more hours is not going to get you much. Imaging 20 hours only improves SNR by ~40%. You'll need to image for 40 hours to double SNR. If you are unhappy with relatively long integration time images (eg. 20 hours), the best solution is to increase aperture to collect more photons so long as your object fits in the FOV.
Well Written Helpful Insightful Concise
Daniel Arenas avatar
Daniel Arenas avatar