Deep Space Lucky Imaging
Lucky imaging gives us amateurs the possibility of shooting deep space targets at resolutions hitherto only seen in multi million dollar professional observatories. The basic premise behind lucky imaging is explained in the first five minutes of my video: 300 Amateurs take on NASA
- Can resolve incredible amounts of detail
- Might help negate a wobbly mount.
- Doesn’t have to cost that muchsee: cheap(ish) lucky imaging gear
- Only works with scopes that can resolve stars to better than 3 arcseconds FWHM (see a list of scopes that can do this here).
- You throw away more than half of the subs so you need to spend more than twice as long gathering data to catch the same number of photons
- Lucky imaging techniques struggle to resolve the dim bits. If you are interested in going ‘deep’ regular imaging techniques are prefereable.
Lucky Imaging Theory
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 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 😜
And that led to the biscuit setting up The Big Amateur Telescope (aka The BAT): an international band of amateurs who all shoot the same target and share their data. The BAT is also a place where fellow nerds delve deeper into lucky imaging theory. Below is some of what we have found out so far.
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.
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.
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
Lucky Imaging Gear
The most important bits of kit for lucky imaging is the telescope. If your scope isn’t sharp enough then there is not point in lucky imaging. That’s because of the way optics works. Basically whatever is causing most blurriness in your system dominates all other sources of blurriness. So your telescope’s optics are making stars blur by about 4 arc seconds and if you are shooting on a night when the atmosphere wobbling around is causing each star to blur by 2 arc seconds then your resulting stars are going to have a blurriness of not 6 but 4.4arc seconds [ sqr(42+22)]. And if you do lucky imaging with this telescope and halve the amount of blurriness caused by the atmosphere (which would be very impressive) your resulting blurriness will be 4.1 arc seconds. All the effort of Lucky imaging has made almost no difference to the sharpness of your image and whats worse it will have cost you a huge amount in terms of your images noiseyness. In The BAT we only encourage folks to try lucky imaging when their scopes create less than 3 arc seconds of blurriness.
The Sharpest Scopes
So when buying a telescope for lucky imaging the prime concern is that it is sharp. Although telescope Companies don’t tell you how sharp their scopes are WE CAN! In The BAT we ask our members to test their scope by sending our team pictures which we can use to measure the Full Width Half Maximum (aka blurriness!) of the stars. We now have a database of hundreds of scopes (which can be seen here). The blurriness of the stars depends on the seeing conditions as well as the optics of the telescope itself, so this is not a perfect test BUT if the seeing conditions are good you would expect a very sharp scope to produce stars that have 2 arc seconds of blurriness or less.
All the scopes to the below have been tested by The BAT and are able to resolve stars with a FWHM of less than 2 arcseconds
Best Cameras for Lucky Imaging
Sensitivity (mono or colour?)
Lucky imaging relies on taking very short exposures. To gather enough photons in that short amount of time your camera needs to be sensitive (or your telescope needs to be very fast). Colour cameras have a problem here because they collect 1/3rd as many photons per second as a mono camera when the mono camera is shooting luminance frames. That’s why I’m not recommending colour cameras or dslr cameras for lucky imaging. Note this isn’t a hard and fast rule. Please try it if you have one.
Cooled or uncooled?
Dark current can be reduced by cooling you camera. However very short exposures are dominated by read noise not dark current. So cooling your camera is far less important when lucky imaging.
As we’re hoping to use lucky imaging techniques to resolve details around 1arc second wide I’d say as a rule of thumb we want our pixel scale (that is the amount of sky each pixel ‘sees’) to be around 0.5arc seconds per pixel. So my recommended choice of camera relies on matching the size of the camera’s pixels to the focal length of the camera.
Small sensors are actually quite useful for lucky imaging. Each sub doesn’t take up much space on your hard drive – and when you’ve got 10,000 subs that becomes important. I actually only shoot cropped with my qhy268m to save hard drive space. Also we’re generally looking at the fine detail when lucky imaging and small sensors are naturally looking at a smaller area of the sky.
Please note I have tested the read noise from all the below cameras myself using sharpcap sensor tester software which is why my values may differ from those released by the cmanufacturers.
My mount recommendations for lucky imaging
For years astrophotographers have said you must spend more money on your mount than anything else. The reason is that traditional long exposures are extremely susceptible to mount wobble. Lucky Imaging utilises much shorter exposures. Over a short period of time you may find that your mounts wobble is negligible. Or that your mount wobble is sporadic in which case you just throw away exposures during the unlucky moments of mount wobble. Its a bit like double lucky imaging where you only keep the frames where both the atmosphere and the mount aren’t wobbling. The flip side of the coin is that hi resolution imaging is imaging at sub arc second scales and it is therefore critical to minimise all sources of blurriness as much as possible.