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Stacking images vs. Longer exposure time

  • DaveGrennan
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18 years 4 months ago #25018 by DaveGrennan
Replied by DaveGrennan on topic Re: Stacking images vs. Longer exposure time
Eirik,

Let me jump back to your original question. Are fewer longer exposures better than more short ones. Up until recently I would have totally agreed with exactly what the guys above said. But after seeing that image the guy took from spain using 150 x2 minute exposures I'm not so sure.

see here;

irishastronomy.org/boards/viewtopic.php?...8a43e9fa204b5bc2ee50

From reading some stuff by this guy, his belief is that the key factor is signal/noise ratio not specifically the strenght of the signal. If you can get the S/N high enough you can stretch the histogram a huge amount to bring out the detail. Its hard to disagree when you see the guys results.

There are a few more benefits of this approach which strike me about this approach. Not least is the fact that your far less reliant on your mount. My mount is a CG5. When properly aligned I can can get 2 minutes unguided about 4 out of 5 times. With autoguiding that is easily 19/20 perfect images. When the exposure times go up so does the number of bad frames. Even with autoguiding I can expect to lose maybe 1 in 5 autoguided 10 minute images.

In your case you have no choice but to keep it short. Even if the ETX performed like Dave McD's mount you have field rotation to contend with so really two minutes may well be around your max.

The second benefit is that light pollution becomes less problematic. Again looking at that image by Mike Hernadez really says it all. The problem with using light pollution filters is that you reduce the dynamic range of the captured data simplty because the filter is only passing a specific set of wavelenghts. I always wanted a way to image like this without the filter and I do beleive this may well be it.

I cant wait to put this guys theory to the test. The very next moon free clear night I plan to set up to take as many shorter exposures as I possibly can (at least 100) without the LPR filter and lets see what happens.

If I ended up with an image even half as good as that guys I'd consider it a major success.

Regards and Clear Skies,

Dave.
J41 - Raheny Observatory.
www.webtreatz.com
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  • DaveGrennan
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18 years 4 months ago #25019 by DaveGrennan
Replied by DaveGrennan on topic Re: Stacking images vs. Longer exposure time

Togheter with a t ring ect (prime focus) you can use barlow?


Trust me on this Eirik, you do NOT wanna use a barlow. Many objects in the sky are much bigger than you think. If you look around a few astrophotography site you'll see a common theme of guys trying desperately to reduce the focal lenght, not increase it.

By using a barlow to up the focal ratio you dim the target meaning much longer exposures required to acheive the same level of saturation.

With the size of the CCDs on modern DSLRs you will end up sizing DOWN a lot of images even with a short scope like the ETX.

Then there is flexure. You need to keep as few things in the photographic train as possible. Everything you put in there increases the chance of the system flexing,

I doubt if you would ever get an ETX to track accurately enough to use a barlow with the increased exposure times that comes with it.

Good Luck!

Regards and Clear Skies,

Dave.
J41 - Raheny Observatory.
www.webtreatz.com
Equipment List here

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  • Eirikg
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18 years 4 months ago #25025 by Eirikg
Im looking forward to see your results, then there is maybe hope for me :)

With a wedge, do you think i can expose as long as 1 min? Is there any software that can align and stack images?

I understand dmcdona, how he thinks, but i dont have the biggest goals.

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18 years 4 months ago #25039 by dave_lillis
Replied by dave_lillis on topic Re: Stacking images vs. Longer exposure time
Interesting points there Dave, that guy certainly got good results, its an experiment work persuing.

But, if you take 120 images and the exposure in each image is not sensitive enough to pick up mag 9 objects and fainter (for example) , then stacking them will not majically make them appear, if its not there, then its not there.

So, I think the trick here is to see how much exposure do you need to just about catch the object and then enhance it with stacking.

Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor

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  • Eirikg
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18 years 4 months ago #25049 by Eirikg
what i found also, its almost the same as just incresing the contrast (though with stacking you get less noise)

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