Deep Space Lucky Imaging

– theory

– gear

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

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. 

My first attempt at lucky imaging

In my first attempt at lucky imaging (£700 vs $7,000,000 Astrophotography Shoot Out video) I used 5 second exposures with my little 6inch Newtonian – the ‘Red Devil’. As you will see later it turns out 5 seconds is too long an exposure length for real lucky imaging although it still makes a difference. Even with these overly long exposures the signal was so weak that my shot of M106 was very noisy in all but the core of the galaxy. The noise was further increased by the fact that i threw 50% of my least sharp subs away. The more subs you throw away the more the noise increases but at the same time the more the sharpness of the final image increase. I personally think its worth it as the detail in M106’s  core is GOBSMACKING😉

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 Pixel Scale

Based on the fact that the best seeing we get in the UK tends to be about 1arcsecond and we’re trying to do better than the seeing, I would recommend you pixel scale to be 0.5 arc seconds per pixel or smaller. I appreciate that lots of nerds will want to talk about  over sampling. I personally think that kind of talk is old school. After all you don’t worry about oversampling when you are shooting the planets (or at least not in the same way). And please dear reader bear in mind that planetary photographers are able to shoot details 10times sharper than the Dawe’s limit of their scopes… and us deep space lucky imagers are trying to do the same.

Seeing

This graph from those clever professional nerds N.M. Law, C.D. Mackay, and J.E. Baldwin  also shows us how seeing affects the results of stacking fewer frames. When the seeing is bad reducing the % improves the results greatly but its also worth noting that the better the seeing the bigger the effect of lucky imaging.

This is what N.M.Law says about this graph (good luck!)

Fig. 4. The factors by which FWHM and FWHEF (full width at half enclosed flux) are improved at slow frame rates (12FPS),
at three seeings. Obtained from linear fits to figure 3 and similarly derived results for the FWHEF. Starting at the leftmost point
of each line, datapoints correspond to selecting frames at the 1%, 2%, 5%, 10%, 20%, 30%, 40%, 50%, 75% and 100% levels
respectively

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

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

GSO Newtonian 10″ f4 £599

The stand out budget lucky imaging scope is the GSO Newtonian. GSO are a Taiwanese optics company who clearly make beautiful mirrors. I now recommend GSO mirrors and optics. The GSO Newtonians are rebranded by different companies in different countries. Same scope different stickers. 

The simple Newtonian design is actually sharper that on axis than both the SCTs and Dall Kirkams AND its LOADS cheaper. The quality of the mirror and the smallness of the secondary obstruction are important factors but the diameter of the mirror is single most important consideration. The fatter the mirror the higher your potential resolution. Our tests show a good 8inch is fat Newtonian is sharp enough to lucky image with but unless you have great seeing you really need a 10″aperture to be successful. 

My affiliate link

(You won’t find a better price)

My affiliate link

(You won’t find a better price)

My affiliate link

(You won’t find a better price)

My affiliate link

(You won’t find a better price)

My affiliate link

(You won’t find a better price)

You don’t have to buy a GSO newtonian. OLD Newtonians are also very good. My little 6 inch 50 years young ‘red devil’ by Edmund scientific has broken the 3arc second barrier and is one of the sharpest 6 inch newts in The BAT. TBH though I think 6 inches is really too small to lucky image with. 8 inches is really the minimum. Big Bertha my 40 years young 10inch f6 newt manages an impressive 1.6 arcseconds of blurriness but because I’m shootin 1/2 or 1/4 second exposures I plan on getting an even fatter newt which will catch more photons per second.

Celestron 11″ Edge HD

Best All Rounder

According to optical expert Es Ried Celestron’s HD Edge telescopes are…” virtually apochromatic across the entire field… a very nice system”

The Celestron 11″ HD Edge is in my opinion the best all round scope its very versatile, great for lucky imaging but not cheap.

My affiliate link

(You won’t find a better price)

My affiliate link

(You won’t find a better price)

My affiliate link

(You won’t find a better price)

My affiliate link

(You won’t find a better price)

My affiliate link

(You won’t find a better price)

Orion Optics ODK12 £5,800

Not really suprising that this scope is sharp but at least we c an confirm it is.

Planewave CDK 12.5: £9,180

Not really suprising that this scope is sharp but at least we c an confirm it is.

Best Cameras for Lucky Imaging

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. 

See Also…