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determining sub exposure times

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#1 Orion64

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Posted 16 July 2014 - 05:13 PM

Are there any tutorials that will show me how to calculate good exposure times for my subs? I use Maxim Dl.

Are these methods to calculate sub exposure times critical to imaging or can one just check what the maximum time is before details are blown out?

#2 dawziecat

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Posted 16 July 2014 - 05:21 PM

Starizona Calculator

Dave Watson site

And google Steve Cannistra Calculator.

When all is said and done though, there always seems "wiggle room." You can never really nail it down definitively.

And, it always seems controversial too.

#3 Alex McConahay

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Posted 16 July 2014 - 07:32 PM

There are several calculators that assume you want to get signal up a certain percentage of background noise. And once there, stop that subexposure and start another. That is the basis of the Starizona calculators. they will do you well in general.

You won't find them too useful for narrowband. And if you are looking for star color instead (of nebulosity) , you may have to alter their recommendations.

Alex

#4 gezak22

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Posted 17 July 2014 - 12:12 AM

Are there any tutorials that will show me how to calculate good exposure times for my subs? I use Maxim Dl.

Are these methods to calculate sub exposure times critical to imaging or can one just check what the maximum time is before details are blown out?


For L, R, G, and B use the Starizona calculator to calculate the optimum exposure time. Cutting that exposure in half and then doubling the number of subexposures is a perfectly valid alternative (Last bold subsection at bottom of page). My guess is that your f/4 astrograph will get you a very nice signal with 3 minute exposures from dark skies.

For narrowband, you'll find that the optimum exposure can be prohibitive - > 30 min. For narrowband I just expose such that I can get 12+ frames for a decent sigma-kappa stack.

Most importantly, dither your frames.

#5 Orion64

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Posted 17 July 2014 - 03:59 AM

Thank you for the good info. I have previously had a look at the Starizona calculator. Problem is that one needs to be online to use it. Are there any apps that can be downloaded that will assist with exposure time calculation?

Also, do some of these calculators give an optimal total integration time estimate? I am assuming that there should be an optimal total integration time on any target. Of course, the more time the better, but as you know, there is a point where there are diminished marginal returns.

#6 dp297

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Posted 17 July 2014 - 05:11 AM

What I have trouble understanding with these calculators..is that u can..
1) Take A exposures of B secs...or
2) Take 2A exposures of B/2 secs
and u will get the same SNR.
However, SNR is completely different from getting a deep image. SNR is about clarity of information...no matter how much information u have (target). If I want to image really really faint details I need to go with the longest exposure possible. Do these calculators take this into account. When u enter Sky background and target signal....what target signal r u actually referring to?

#7 dawziecat

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Posted 17 July 2014 - 07:59 AM

I have come to the conclusion that there is theory . . . and there is the real world.

Theory is fraught with complexities, and it is good to have a grasp of all of them.

But "real world" is pretty simple and your optimum exposure for YOUR sky and YOUR equipment will become clear early in the game and there will be precious little reason to stray from a very narrow set of values.

In my case, 10 min subs in wideband and 30 min in NB works well except I often revert to 5 min WB for bright objects to prevent saturating stars when imaging at f/5.0 or faster.

As for needing an internet connection, Steve Cannistra's calculator available here is a spreadsheet that you can download to your own machine and use with no internet connection.

#8 Phil Sherman

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Posted 19 July 2014 - 03:19 PM

I've always used a couple of simple real world rules of thumb.

1. The minimum exposure is one that has the background sky "hump" of the histogram separated from the left edge. This means that every pixel has recorded something.

2. The maximum exposure is one where the item being imaged doesn't saturate pixels. For phometry, this rule is modified to mean that the item being imaged lies on the linear portion of the pixel's response curve, avoiding the non-linear portion where the well starts bleeding off electrons.

If the maximum exposure is shorter than the minimum, use the maximum unless highlight details are unimportant.

The overall objective is to get an image with as little noise as possible. There's a lot of sources of noise and if the largest source of it is from the arriving photons, longer expoures will have less signal noise as a precentage of the recorded signal. (ie. Assuming 100% efficiency converting photons to electrons, 100 photons will have 10 photons of noise or 10% of the signal. 10000 photons will have 100 photons of noise or 1% of the signal.)

A stack of 50x200seconds will produce results almost identical to a stack of 10x1000 seconds. The shorter exposures will be easier to guide and each image ruined by a satellite, plane or cosmic ray will be a smaller percentage of the total exposure time. The longer images may show a bit more faint detail than the shorter ones.

#9 Nicola

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Posted 22 July 2014 - 11:16 AM

PerfectExposure http://skymonsters.n...ectExposure.php






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