Stacking images vs. Longer exposure time

14 years 6 months ago #24554 by Eirikg
Im wondering what is prefered, pros and cons for theese methodes? I know longer exposure will give more noise, what are the negative sides of stacking images?

And do you collect equal amount of light with 8 sec eposure and 4sec x 2 exposures?

I have been testing some with piggybacking my cheap digital camera. But seems i have to stack ALOT of images compared to as haveing one long exposure?

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14 years 6 months ago #24555 by dmcdona
Replied by dmcdona on topic Re: Stacking images vs. Longer exposure time
As well as collecting more noise, a single exposure collects more signal!

The preferred method is to take as long a single exposure as you can. Then repeat and stack them.

In general, one 8 sec exposure will be better than 4 x 2 sec exposures. Of course, these exposure lengths are more for deep sky than planetary imaging.

One final note, the longer the exposure the better but at some point, your exposure length will reach a limit as sky-glow starts to interfere. So there is an optimal limit, depending on where the scope is pointing...

HTH

Dave

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14 years 6 months ago #24557 by Eirikg
ok i was thinking for Deep sky

So best is expose as long as equpiment, guiding, ect can handle then stack

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14 years 6 months ago #24560 by dmcdona
Replied by dmcdona on topic Re: Stacking images vs. Longer exposure time
You got it...

Also, beware. Your mount will track at differing rates depending on where its pointed in the sky. You may get 1minutes exposures with no trainling in one part of the sky, but less than 30 seconds in another part of the sky...

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14 years 6 months ago #24562 by dave_lillis
Replied by dave_lillis on topic Re: Stacking images vs. Longer exposure time
Eirik,
Its an interesting question which is worth spending abit of time thinking about.
If you bring the comparison to an extreme, is 60 1minute images stacked the same or better then a 1 hour exposure ??, the answer is an emphatic NO.

The longer the exposure, the fainter the detail in the image, I've found that stacking deep sky images really brings out and sharpens the detail, but doesnt significantly increase the limiting magnitude. I have Orion nebula images on this site that show this.

In order to get very faint detail you need a longer exposure, there is just no getting around it.

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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14 years 6 months ago #24564 by Eirikg
that is what my tests showed also

Deep sky isnt my aim with what i got now, but im thinking of buying LPI.

So then the question is, i see wind is a big factor at high magnification, would it be better to then take loads of short exposures?

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14 years 6 months ago #24566 by dave_lillis
Replied by dave_lillis on topic Re: Stacking images vs. Longer exposure time
The LPI is the "Lunar and Planetary Imager", so its not going to be of any use on deep sky stuff. There are reviews on the web that indicate that the philips toucam pro webcam is actually a better planetary imager.

ALL of the best images on the planets on this site are all taken using the toucam.

The dsi is what you want in starting off in deep sky, as for the wind, go buy yourself some good wind breakers, like ones you'd use on a beech. 8)

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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14 years 6 months ago #24567 by dmcdona
Replied by dmcdona on topic Re: Stacking images vs. Longer exposure time
Wind is a big factor if you have a mount that isn't particularly stable. Don't forget, the big fellas on Mauna Kea regularly work during heavy winds (well, heavier than we are used to!).

I guess if you want to really get into deep sky imaging in a way that doesn't involve:

1. Chucking more than 75% of your images away due to trailing
2. Chucking all of your images away due to wind
3. Being disappointed at your M42 image of 1000 stacked 3 second exposures

etc

then you're gonna have to spend a bit of money.

The best advice that I personally took and that I'd give to anyone wanting to get into deep sky imaging is spend your money in this order:

1. Mount
2. OTA
3. CCD

You can have the best OTA and CCD in the world - stick them on a flimsy mount and you will always be disappointed....

Anyway, back to your question - yes, to avoid losing too many images due to wind, you'll need to shorten the exposure length and stack them. But the more you shorten the exposure, the less detail you will get - no matter how many you stack.

Its a bummer, I know, but there's always trade-offs in this game. Depends how much you want to trade for quality... :cry:

Cheer and HTH

Dave

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14 years 6 months ago #24579 by Eirikg
The ETX-70 isnt good for Deep sky so i dont even going to try that. But i want to try planets.

Dave that sounds good, ill go for the webcam. I have a quickcam from before, is that any good?

I found the toucam pro on a site posted here some time ago, with adapter, do you know if you get the standard lens too? And how easy is it to change between adapter and lens if i want to use the webcam as normal?

I would have wanted to try deep sky, but for a good setup its to expensive. Ill try something that should be posible with my etx first.

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14 years 6 months ago #24580 by dmcdona
Replied by dmcdona on topic Re: Stacking images vs. Longer exposure time
Eirik - the ETX70 OTA is great for deep sky - just that the mount isn't up to it... With the ETX mounted on my AP1200, the deep sky images were great!

If you come by a bit of money, get a good mount but keep the ETX70...

Cheers
Dave

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14 years 6 months ago #24581 by Eirikg
but wont i still need a guidescope + guidecam?

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14 years 6 months ago #24583 by dmcdona
Replied by dmcdona on topic Re: Stacking images vs. Longer exposure time
If you get a good mount and have it accurately polar aligned, you'd get at least a couple of minutes worth of exposure with the ETX70 without any guiding.

I don't guide at all :D And that's with a C8!

Concentrate on the mount first - worry about guiding later. One thing I've learned is one step at a time...

Cheers
Dave

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14 years 6 months ago #24596 by dave_lillis
Replied by dave_lillis on topic Re: Stacking images vs. Longer exposure time
Eirik,
One trick I found doing 2-3 minute exposures with the etx, is to wait until the object is near or almost crossing the meridian, that way the declination drive is almost stopped, so one less drive to worry about.

I agree with Dave McD, the etx70 is a good deepsky starter scope due to its nice widefiled of view, I would not rank it as a good planetary scope, the skylux does a better job of them.

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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14 years 6 months ago #24639 by Eirikg
If you use a dslr camera, 35mm and etx 70 is only 350mm = 10x maginification. Is that good enough for deep space? Can you use barlow?

Say if i buy another mount, how much would i have to spend? And woulnt it be better to spend less on mount and then use guidescope ect?

dmcdona: the mount you got dont need guiding, but woulnt it be alot cheaper to use guiding with a cheaper mount?

Dave Lillis: So you dont use a wedge? How much better would that be?

Im thinking abit about a dslr camera as it could be used to other things to. But as with the etx mount i got it would thouch the body, so a mount (that maybe even could be used for later heavyer scope) sounds good. As long as we are not talking in the pricerange as dmcdona's mount! :O

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14 years 6 months ago #24700 by dmcdona
Replied by dmcdona on topic Re: Stacking images vs. Longer exposure time
Hi Eirik - I'm sure there are plenty of mounts out there cheaper than mine :D And of course, you shouldn;t have to spend that money. But again, do beware, even an autoguider will not compensate for a dodgy mount.

Additionally, if you go down the seperate guidescope and guiding imager, you are adding extra strain on the mount...

I'm sure if you are prepared to spend a thousand or two on a mount (possibly less if you get a secondhand bargain) then you should be OK.

For deep sky, magnification is not that important. In fact, for some objects, the less magnification the better - extended objects like nebulae; detaile milky way shots; large galaxies (e.g. Andromeda). Of course, small galaxies and paletaries usually need a good bit of magnification but usually less than solar system planets...

To get the extra magnification, yes, you can use a Barlow. A 2.5 x powermate would give you x25 in your example.

The thing about deep sky is the (usual) low brightness of the objects you are imaging requiring long exposures. You can expose planets in less than a second. But a lot of the M oejcts eg require 10 seconds plus. Sometimes into minutes worth....

As ever, hope this helps

Dave

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14 years 6 months ago #24704 by Eirikg
euros?

Then i think 1000 would be max what i could spend.

Up to 30x with 3x barlow sounds good, thats about same as 12mm eyepice. Now i havent watching any other nebulas other then m42, so i dont know the size of other m object. Beside others ive seen in pictures in the orion constalation.

Could you give some suggestion to mount?

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14 years 6 months ago #24719 by dmcdona
Replied by dmcdona on topic Re: Stacking images vs. Longer exposure time
If you had 1000 euro for a mount, I'd say you'll get a good second-hand one or possible a heavily discounted new one.

As for which mount, that's a tricky one and I don't really have any concrete advice for you. Try Cloudy Nights reviews - they have honest reviews of astro equipment - I'm sure they have mounts in there.

Or some of the folks here could comment...

Cheers

Dave

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14 years 6 months ago #24956 by Eirikg
just so i got this right

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

Have anyone tried a Nikon D50? How is it for astrophotography? I read it gives more noise?

As i also want to get in to photography in general im thinking of getting the camera first. And then maybe try a bit with the mount and all that i got.

As for mount i only found one that maybe is in my price range, CG6 ?

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14 years 6 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
Equipment List here

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14 years 6 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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14 years 6 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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14 years 6 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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14 years 6 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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