Lucky Imaging Theory

The Enemy

In the UK the atmosphere wobbles around so much that when we look into space it blurs everything we see by 1 to 3 arc seconds (depending on the weather). This isn’t a problem for small telescopes.  A perfect small 72mm refractor can’t resolve anything smaller than 1.6 arcseconds anyway and most refractors aren’t perfect. But when the fatness of your telescope grows and its potential resolution increases then  assuming your mount isn’t wobbling all over the place then the atmosphere becomes your biggest source of blurriness. INCREDIBLY us nerds have found a way to reduce the atmospheric blur by employing a technique known as lucky imaging.

Below, the atmosphere giving Jupiter (as seen through Big bertha with a green filter) a good wobble 

By choosing just the lucky less wobbly frames and stacking them (and adding the data from the red and blue channels) the above produced this much sharper image of Jupiter.

Lucky Imaging

Lucky imaging relies on the fact that the amount of blurriness the atmosphere delivers varies from one moment to the next. So over the course of a minute you might get a few patches of relative calm. This can help us.  If during that minute you shoot 60 x 1 second exposures rather than say a single 60second exposure  then you will discover that  the shots taken during the calm periods will be crisper than the shots taken when the atmosphere is more wobbly. If you then only stack the sharpest images which were shot at a lucky moment of atmospheric calm and throw away the blurry images your resulting stacked image will be much sharper than the single 60 second exposure. The shorter your exposures and the more subs you throw away the sharper your final image will be. Lucky imaging makes it possible for  us amateurs  to resolve details smaller  than 1 arc second (and maybe even smaller than that). So lucky imaging is really exciting. Of course because we are throwing away a lot of frames lucky imaging requires us to image for longer to catch the same number of photons. So the sharpness comes at a price and that price is noise.

Lucky imaging has been used for 20 years on the bright planets but now thx to very sensitive low read noise CMOS cameras we’re able to lucky image dim deep space too. 

Solving the noise problem

To reduce the noise in images shot with the lucky imaging technique you need to collect more photons. You could simply throw less subs away but that’ll reduce your sharpness. You could buy a bigger, fatter scope so you collect more photons per second but that costs more money (see best lucky imaging rigs). Or you could collect more photons by simply shooting the target more often but in the UK clear skies tend not to last. I decided that the best way to reduce the noise was to persuade as many amateurs as possible to shoot the same target and share their data with me 😜

Best Exposure Length

This remarkable graph taken from a physcis paper by N.M. Law, C.D. Mackay, and J.E. Baldwin tells us how much sharper our images will get when we reduce our exposure time AND how much sharper our images will get when we keep 1/10/50 and 100% of the subs.

The graph above shows us that even  at a comparitively long 0.3 frames per second (3.33 second exposure) we will start to see the benefits of lucky imaging (if you were to stack  just 10% of your 3 second subs you should see a 40% improvement in sharpness). This is the kind of sub length I would recommend for deep space lucky imaging. If you have one of the new low read noise cmos cameras, and if the target you are shooting is bright enough, you may be able to go much shorter. If you are able to shoot 0.1second subs (and still successfully stack them) then by throwing away 90% of them you may be able to double the sharpness of your image.

Best Percentage of Subs to Stack

From the graph above i would suggest that stacking around 10% of your subs is a good place to start. But this isn’t just theory. Bat member MrCrazyPhyscist has run his own real world tests with his C14 and ASI1600MM and come out with similar results.

Best Gain Setting

This is an easy one. Check out the read noise vs gain graph for your particular camera and choose a gain setting that gives you near as damn it the least read noise. For my ZWO asi2600mm its a gain of 350 (out of 400). You don’t need to think twice about all the dynamic range we’re loosing with these high gains bc our lucky imaging  shots are so short we’ll never fill our wells up anyway.

Best Targets

The best targets for lucky imaging are bright ones!!! That’s because we loose so many photons through throwing away 90% of our subs  we need to make up for it by having lots of photons to begin with.

Like planetary photography it also makes a huge difference shooting targets that are directly overhead. This is so important that its almost not worth bothering to shoot any target that is lower than 70 degrees.

And the final consideration in my opinion is contrast. This isn’t directly to do with lucky imaging but sharpening techniques such as wavelets of deconvolution allow us to pull out signifinatly more detail and these sharpening techniques work very well on high contrast targets.

Best Wavelengths

Short wavelengths of light like Ultra violet get disturbed by the atmosphere far more than longer wavelengths of light like red or infra red. It’s clear that when lucky imaging we must block out the UV light. I have a UV cut filter which I’m planning on using. But is it enough. Is it possible that just shooting in infra red will actually give us the sharpest results. If you look at my shot of Jupiter you can clearly see how much sharper Jupiter becomes as the wavelengths get longer. I think this is a good area for experimentation. It maybe longer exposures in just red and ir yield better results than shorter exposures in luminance… when I find out I’ll tell you! If you are interested you should really be a member of The BAT already!

Jupiter shot with a blue filter

Jupiter shot with a red filter

Jupiter shot with an ir filter