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DSO lucky imaging: What does it mean and how does it work?

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I have been recently coming across this term and had never heard of it before. I looked it up and read an article and my take away was that it pretty much is taking hundreds if not thousands of short exposure images. And then go on to stack them and throw away a lot of frames to avoid atmospheric aberrations for instance. A few of the examples I saw showed very detailed and impressive images. The head scratcher for me is how are people taking 1-2 second frames and pulling in enough data to produce anything, even with a massive stack?

I have no be confused on what it means, and the process though. 



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Usually they are pooling their resources within a group and you also need a large aperture scope in order to have the resolution to capture any fine detail for the DSO part of lucky imaging. It's not really any different from planetary imaging in that you take hundreds/thousands of frames and the software sorts through and you tell it to only stack a small percentage of the best frames to get the best detail.

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It’s not true lucky imaging as that requires exposures of a fraction of a second (to properly freeze the seeing) but it is kinda like a halfway technique.

Mostly used by owners of big scopes whereby the mounts required for normal long exposure imaging would be hideously expensive, so instead thousands of short (~1sec) images are captured and stacked.

These short exposures are too long to really freeze the seeing but the May help to reduce the seeing blur to a certain extent (depends on what the seeing is like - high frequency or low frequency blur) and also helps to avoid tacking errors, oh and guiding isn’t generally required.

The downside is you need a big fast scope  to get enough signal in such short exposures. Saying that though the release of the latest gen ultra low read noise cameras certainly makes it an attractive technique and certainly worth trying if you fancied having a go with the brighter targets

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Only difference between say 1h of total exposure made out of 10 minute subs (that is 6 x 10 minute) and 3600 x 1s subs - is in read noise.

All other noise sources add like with time. Signal also adds like that (take two 1 second subs and add them - you will get same amount of signal as one 2 second sub).

Only thing that does not add with time is read noise. It adds once per sub, regardless of sub duration.

Stack of 6 subs will have x6 read noise, and one consisting out of 3600 subs - will have x3600 of read noise.

(in fact - noise does not add like normal numbers - it adds like linearly independent vectors - square root of sum of squares).

This means that for very low read noise camera - one can get good image even if doing very short subs.

Only drawback of using short subs is ability to properly stack them. You need to have enough stars to perform alignment of subs, and with very short subs we run into SNR issue - signal in stars is not very big compared to noise and we have difficulty determining true star position. Sometimes stars are so faint that software simply can't detect them.

This is the reason why most of lucky type DSO imaging is performed with very large aperture scopes - they gather enough light for stars to show nicely in even short exposures.

As far as resolution increase - Lucky type imaging as term has some sense in planetary lucky type imaging. With DSO - it is just borrowing that term from planetary - but in reality it is far far less effective.

With planetary imaging we actually freeze the seeing and while image ends up being distorted - it does not suffer from motion blur (as image shimmers on order of dozen or so milliseconds - any longer exposure will be blurred by this motion).

Lucky DSO exposures are far too long to be able to do this - and best we can do is simply select subs with better seeing FWHM than most.

It is not different than discarding poor subs that is regularly performed in normal DSO imaging if something bad happens to that sub - like gust of wind or spell of very poor seeing that blows up stars.

Advantage of short  exposures is that you can still use "part" of long sub.

Imagine that you image with subs that are 5 minutes long and in one such sub - there is a gust of wind that shakes the scope enough to ruin stars and image. That shake usually lasts for 10-20 seconds but whole 5 minutes of that sub will be discarded as sub standard.

With short exposures - you get to keep 4 minutes and 40 seconds that were fine and just discard those 10 or so short subs that were actually ruined by wind.

Same goes for sudden spells of worse seeing - if they are short - they will affect whole sub - but with above technique, we get fine grained control of what data we keep and what we throw away.


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