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M51 a waste of time

Jessun

By Jessun
in Imaging - Deep Sky

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Davey-T

Davey-T

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Hi Jessun

In the April ussue of Astronomy Tony Hallas has an interesting article on the diminishing returns of stacking images basically coming to the conclusion that after about 25 frames the SN ratio shows no improvement.

Davey T

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Jessun

Jessun

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Wow! For anyone following this thread Harry got the stack TIFF a mere 20 minutes ago...

Well what can I say. Some colour and resolution in there that I simply never saw. I am pleased and gutted at the same time haha!

Cool magic Harry! Now dropbox that TIFF back so I can tell my mates I did it :)!

BTW care to elaborate on PI CC?

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Jessun

Jessun

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Davey T, I quoted that exact article in another thread - one about how many darks you should use! I just happened to read it after I did these three nights imaging....

I see HP doing absolute magic with this, but I persist that I could improve on my rather crude version with longer subs - given my lack of processing skills. A hole is dug...

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RobH

RobH

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Hi Jessun

In the April ussue of Astronomy Tony Hallas has an interesting article on the diminishing returns of stacking images basically coming to the conclusion that after about 25 frames the SN ratio shows no improvement.

Davey T

An interesting conclusion that is completely wrong in my experience....I've done stacking experiments increasing the number of subs and there is still a lot of improvement after 25 subs....TBH, 25 is about the minimum to use, and I am in Harry's camp...if I can get 100 subs, great :)

Rob

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Shibby

Shibby

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Very nice image Jessun! I have to say that I agree with those that say increasing your sub length wont help much. Increasing the sub count was the right approach because it reduces the noise present in the tiny section of the histogram you are working on. Like you, I am always struggling to separate faint signal from the sky background. I find the key is to make very small, iterative curve adjustments, making sure you are anchoring your curve to the darkest pixels in your image. Makes you long to move house, doesn't it :)

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Jessun

Jessun

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Makes you long to move house, doesn't it :)

It sure does, but there's so much to learn - even with brown subs!

Right now I can't wait to get some clear Moon free, calm nights to crack on with another M51. I did an M81 in 10 minutes subs, and it came out a lot better - well at least easier to process for me - even fun. True it's larger and mag 7 instead of 8ish...

Long term as you add more and more data - perhaps over several seasons, I think I'd rather have 100 (if that is a good number) long subs, than short ones.

Ah well, bare with me as I fight the smog like Don Quixote on a long sub quest. If it turns out rubbish, at least I learned something :-)

Thanks again everyone for your input and experience. Makes this forum truly great!

PS

I am in Harry's camp...

Hmm RobH I'm gonna need an army of trebuchets to even come close to invading your camp.... Bit of a proven strong hold. :-)

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Davey-T

Davey-T

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An interesting conclusion that is completely wrong in my experience....I've done stacking experiments increasing the number of subs and there is still a lot of improvement after 25 subs....TBH, 25 is about the minimum to use, and I am in Harry's camp...if I can get 100 subs, great :)

Rob

Hi Rob

Ive experimented with short subs, long subs ,lotsa subs and have to say that the improvement is only noticable with longer subs.

Given that I live in the lovely orange glow of SE London their is limit

to what I can achieve. One 10min sub will always be better than 10

1min subs providing you can track that long.

T Hallas probably images from somewhere nice and dark so has the

advantage of better subs to start with.

Davey T

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RobH

RobH

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I agree David....lots of short subs don't show the improvement of long subs.

The most I've done is about 100 or so x 10 minutes, and the image was very smooth and processing easy, but the difference between 80 and 100 was very slight, but it was there.

I suppose if you do 25x 30 minutes, then you probably won't need many more....Oh for the skies to do that!!

Cheers

Rob

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Davey-T

Davey-T

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Hi Rob

I guess there are so many variables that different equipment gets different results from the same subject. I have subs of M51 that the only difference between them

is the 12 months between taking them and they are not particularly similar when processed the same way.

Also I expect T Hallas camera cost more than my whole setup.

Yours Davey T

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ollypenrice

ollypenrice

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An interesting conclusion that is completely wrong in my experience....I've done stacking experiments increasing the number of subs and there is still a lot of improvement after 25 subs....TBH, 25 is about the minimum to use, and I am in Harry's camp...if I can get 100 subs, great :)

Rob

I totally agree. I image from a dark site and take longish subs. With Yves' scope 15 minutes is the shortest we use. The idea that there is no improvement after 25 subs is, I'm sorry, just plain wrong.

Olly

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dph1nm

dph1nm

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lots of short subs don't show the improvement of long subs.
Once you are dominated by Shott noise in the sky then there is no point taking longer subs. But when this happens depends very much on your local conditions and your camera/scope. So in a city there may be no point doing more than 1 min subs, whereas at truly dark sites this might be 15mins or more (and narrowband will be even longer!).

As for how many - well the improvement in signal-to-noise goes as the square root of the number of subs (or the square root of the total exposure time, which is the same thing), so to get, say, a factor 2 improvement you always need to quadruple your number of subs. Thats OK if you go from 10 to 40, but the next equal step would be 40 to 160, then 160 to 640, so you rapidly end up having to take ridiculous numbers of subs to get the same improvement. Of course, you will still get some improvement no matter how many you add, but eventually the only answer is a bigger scope!

NigelM

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Jessun

Jessun

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I'm with you there Nigel, and to my surprise at 5 minutes on a good night I haven't quite hit the roof yet. Fingers crossed for the revisit of M51.

Any excuse tho for going bigger... Not sure if my other half cares an awful lot about Shott noise but I'll try it whilst browsing for some Officina Stellares...

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