Exposure time vs number of exposures?

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Larry 1969 United States of America
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Exposure time vs number of exposures?

#1

Post by Larry 1969 »


Someone pleas correct me if I'm wrong.

My understanding is that 20 100 sec exposures has the same amount of data as 100 20 sec exposures.
If so, why would one want to take long exposures? I would think the chances of something going wrong ( air plane, satellite, etc ) would be much greater with longer exposures.

I'm taking 200 20 sec exposures of the Deer Lick Group right now....

Thanks!

Larry
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Re: Exposure time vs number of exposures?

#2

Post by JayTee »


Your supposition is not exactly true. There is a trade-off. The signal-to-noise (SNR) for the longer exposures is better and they will give you a lower noise final image. It's not a lot, but it is noticeable. BUT, if all you can take are 20-second images then that is what you do.

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Re: Exposure time vs number of exposures?

#3

Post by bobharmony »


There are a couple of practical reasons for taking longer exposures that have to do with disk storage and stacking your images.

From the storage side, 100 20 second exposures take up 5 times as much disk storage as 20 100 second exposures. Whether that is an issue depends on how much space is available, and how much you want to allocate to your astrophotos.

From the stacking side, stacking 100 exposures takes longer than stacking 20 exposures. It isn;t necessarily 5 times as long, but it is a longer period and puts more of a load on the processing device.

Finally airplanes and satellite trails can be removed from the final image by using a Kappa-Sigma stacking algorithm. That algorithm rejects pixels from an image that lie outside the norm for all the images averaged together. A plane flew through one of my subs the other night when I was imaging the Iris Nebula. After stacking, there is no sign of the plane lights going through, but I was able to use the data from that sub along with all the others.

On the other hand, taking longer subs exposes you to more risk from tracking issues, and may require you to autoguide your mount, adding some complexity to each session. There is no free lunch in AP, just like there isn't any anywhere else.

Bob
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Re: Exposure time vs number of exposures?

#4

Post by Larry 1969 »


Well put.

Thanks guys!

Larry
For visual:
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Re: Exposure time vs number of exposures?

#5

Post by yobbo89 »


more dynamic range if you're going with longer subs with lower iso (you get range back in stacking anyway),more lp can get soaked in but can be processed it out.A 60 x 30 sec image will basically make very similar results as a 6x5 min image with a very small differences between the two ,doing short exposures is ok given that there's a min exposure length with your scope/camera to lift the skyfog, other then that processing time is where you suffer as others have explained.

deepsky stacker needs a minimum amount of frames to process out artifacts from satellites ,i think the recommended is about 25 frames, so there's a down fall for doing longer subs, example at 5x 30 min exposures .
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Re: Exposure time vs number of exposures?

#6

Post by Baurice »


I quite often stack while doing other things.
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Re: Exposure time vs number of exposures?

#7

Post by OhNo »


I started a thread over at the old place that asked a similar question. http://www.astronomyforum.net/astro-ima ... -much.html After digesting the replies I found that with my equipment better results/image came from longer exposures. Even better was longer exposures with a lower ISO. Attention to getting my guiding was essential, I like to think of that as "Preparation for the future".

Prior to making the changes I made on ISO and exposure I learned the hard way that with my system and sky environment lot of short exposures didn't really yield me with a better finished image. Bob spoke the truth, there is "no free ride" in AP.

Don't be afraid to experiment, it's not like the boss is going to fire you for not meetin' your quota. It a hobby not a job, it's supposed to be relaxing and enjoyable!

Brent
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Re: Exposure time vs number of exposures?

#8

Post by jrkirkham »


Thanks everyone. This is an interesting conversation.
Rob
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Re: Exposure time vs number of exposures?

#9

Post by depratnt »


The longer the exposure the better the s/n ratio will be. You will get better results with 20, 100s subs than the other way around.
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Re: Exposure time vs number of exposures?

#10

Post by KenGS »


In term of SNR, if your 20s exposures are not shot noise dominant (i.e are read noise dominant) then stacking 100x20s expsoures will have worse SNR than stacking 20x100s exposures. If your 20s exposures are shot noise dominant over read noise then the SNR difference between stacking more shorter subs vs fewer longer subs is small. Then the other factors mentioned - tracking, dynamic range, processing time etc come into play.
--Ken
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Re: Exposure time vs number of exposures?

#11

Post by goldstar »


I am currently processing 356 X 30 secs, yes 356!, of the Heart and Soul nebulae in DSS.
Already I can see the moon and LP was creating a sky background of 26 % in all frames as well as a FWHM of 6 pixels.
I do not expect a great result but I will see in the morning since while I sleep DSS will spend nearly 4 hours beating it's way through stacking.
I'll let you know....
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Re: Exposure time vs number of exposures?

#12

Post by Star Dad »


One thing to keep in mind about long exposures... I lived near a VOR (aircraft beacon) and aircraft are constantly in my images if I'm shooting to the North or North East. I'd rather ruin a 1 minute exposure than a 20 minute one. And don't forget those pesky satellites that show up just about anywhere.
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