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Do you stack images when you are using autoglider?

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#1 bigdirtyfly

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Posted 22 April 2019 - 11:53 PM

Do you stack images when you are using autoglider? 

 

Let say I am planning to expose for 10 minutes straight. Is it better to stack 5 images shot at 2 minutes each or take 1 image exposed for 10 minutes? 

 



#2 einarin

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Posted 22 April 2019 - 11:58 PM

It depends.

If your mount can handle 10 minute exposures.

If your light pollution situation doesn't allow 10 minute exposures.

If you have faint object and you are using narrowband filter you must take 10 minute exposures.

If you take 1 10 minute exposure and you have satellite trail it's hard to remove but if you take 10 2 minute exposures then much easier to remove.

 

PS Guess you mean autoguiding ?



#3 bobzeq25

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Posted 23 April 2019 - 12:06 AM

An eternal question.  This is an issue worthy of study and understanding.  DSO imaging first, then a note on nightscapes.

 

The big thing for DSOs is this.  Assume you want to do a stack of subexposures, which you will.  Assume, say, one hour of total imaging time (which is really not much).

 

If you do too many subexposures, say 240X15", read noise (which accumulates with each sub) becomes a problem.  Too few, say, 4X15', and your dynamic range is shot, many stars will be saturated, and star color will be horrible.

 

Where's the sweet spot?  It depends on your light pollution level.  You want to expose long enough so the read noise, which you can control, is a lot less than the sky noise, which you can't. 

 

If you're using a DSLR, a good approximation is to expose long enough that the histogram on the back of the camera (which shows the sky noise) is 1/3 of the way over from the lefthand edge.

 

There are many threads about this on this site.  It gets really complicated.  The best discussion I've ever seen (the above leaves out a lot of details) is in The Astrophotography Manual, by Chris Woodhouse.

 

One thing to _really_ keep in mind.  Total imaging time is far more important than getting the subexposure "perfect" (which is impossible).  Get that in the ballpark, and shoot more subs.  My rule of thumb is one hour is a minimum, two better, and four good.  That also depends on light pollution, imaging in light polluted skies requires more total imaging time.

 

I see you want to do nightscapes.  They're a thing unto themselves, since you need to balance out the sky and the landscape.  Nightscape photographers often don't stack, since that blurs the landscape part.  The alternative is to do two images, the sky and the landscape, and combine them.  The stuff above applies to the sky part.


Edited by bobzeq25, 23 April 2019 - 12:16 AM.

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#4 ButterFly

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Posted 23 April 2019 - 01:03 AM

One stacks to improve the signal-to-noise ratio from sources such as atmodpheric seeing and light pollution.  Any improvement depends on the SNR of individual subs.  If your SNR for subs does not improve appreciably going from 100s to 300s, then you are only risking 200s of birds, planes, and supermen, and losing two stacks.  The optimal sub-exposure time depends on your skies, your equipment, and which filters you are using.

 

Here is a link to a lecture by Dr. Robin Glover (developer of SharpCap) discussing the optimal sub-exposure time.  It will give you a background in which questions to ask yourself to get better results.


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#5 kathyastro

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Posted 23 April 2019 - 09:48 AM

Yes, you should always stack images whenever possible.  Stacking multiple images removes random noise.  The stacking process typically also includes calibration, which removes non-random "noise".  I always stack at least eight frames of each type, since that many gives me access to windsorized sigma clipping, which can remove satellite trails.

 

If you only have 10 minutes of sky time available in one session, then you will have to decide which is more important: having a potentially noisy image with decent dynamic range in dim parts of the target (1x 10m), or a cleaner image with less dynamic range (10x 1m).

 

Personally, I try to go for an hour and a half of total exposure time, which gives me the best of both worlds.



#6 17.5Dob

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Posted 23 April 2019 - 11:41 PM

Do you stack images when you are using autoglider? 

 

Let say I am planning to expose for 10 minutes straight. Is it better to stack 5 images shot at 2 minutes each or take 1 image exposed for 10 minutes? 

 

10 minutes is not enough exposure for anything. You need to start thinking in hours of time, not minutes.

Longer subs are better, but your mount has to be able to handle it, and you need absolutely pristine skies, with no LP, to support a long exposure.

My "average" subs are 6 minutes and I normally stack 25-40 of them. YMMV


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#7 Alex McConahay

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Posted 24 April 2019 - 08:35 PM

Hey, Bigdirtyfly….

 

What are you planning on taking pictures of? What are you using to take those pictures? What kind of camera and mount? And what size lens?

 

The answer may change when we know that.

 

ALex


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#8 bigdirtyfly

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Posted 26 April 2019 - 02:20 AM

Hey, Bigdirtyfly….

 

What are you planning on taking pictures of? What are you using to take those pictures? What kind of camera and mount? And what size lens?

 

The answer may change when we know that.

 

ALex

Sony a7r3 with 16-35 gm lens on sky guider pro 

 

I am doing Astrolandscapes. I am planning to create composite images with one exposure for the sky and one for the landscape foreground. 



#9 bobzeq25

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Posted 26 April 2019 - 11:03 AM

Sony a7r3 with 16-35 gm lens on sky guider pro 

 

I am doing Astrolandscapes. I am planning to create composite images with one exposure for the sky and one for the landscape foreground. 

You'll get better sky images if you stack, but it's not an absolute requirement.

 

The difference between terrestrial photography and astrophotography is that astrophotography of deep space usually involves a _much_ lower signal to noise ratio.  That brings a lot of things into play that are just not issues in terrestrial, like the thermal noise of the camera.  One also needs to shoot RAW, and to "stretch" or expand the dynamic range, of the data you gather.

 

It's all quite complex, there's far more to it than you can get from any short posts here.  For what you want to do, there are specialized websites.  This book is aimed at people just imaging the sky, but it might be useful.

 

https://www.amazon.c...d/dp/0999470906


Edited by bobzeq25, 26 April 2019 - 11:04 AM.


#10 james7ca

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Posted 26 April 2019 - 12:07 PM

You should always register your images before you stack. That said, here is a good overview of image capture including a section on determining the minimum (or "proper") exposure that you should use given your particular sky conditions and your camera and scope. In case others haven't seen this, I definitely recommend it, it's a March 2019 lecture by Dr. Robin Glover the author of SharpCap.

 

  https://youtu.be/3RH93UvP358?t=62



#11 Alex McConahay

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Posted 26 April 2019 - 03:50 PM

Register and Stack the Sky.

 

Then register and stack the earthly portion.

 

Cut the sky out of the earthly portion, and paste the sky into it. 

 

Alex




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