Hi-Resolution Deep Space Lucky Imaging
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. Happily us nerds can reduce the atmospheric blur by employing a technique known as lucky imaging.
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 and throw away the blurry images your resulting stacked image will be much sharper than the single 60 second exposure. Lucky imaging means us amateurs now have the ability to resolve detail that is less than 1 arc second small. That’s 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.
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.
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.
PROs
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Can resolve incredible amounts of detail
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Might help negate a wobbly mount.
CONS
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You end up with thousands of subs so you need a good computer to process them.
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You throw away about half of the subs so you need to spend twice as long gathering data to catch the same number of photons
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Short exposures don’t catch enough photons to resolve the dim bits (this is especially true if you don’t have a fast telescope)
In practise you would probably want to use lucky imaging to resolve fine detail in bright objects like a galaxy core and then also take longer exposures to catch the fainter parts of your target like the galaxy’s spiral arms.
M106 Shot with my 50 year old red devil
Luminance: 1200x5s, 120x60s
RGB: 40x30s x2binned
Ha: 60x180s
Equipment for lucky imaging:
My 6 inch lucky imaging rig
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Telescope: Edmund Scientific Newtonian (with a 35mm Orion Optics Uk secondary)
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FL 900mm, Diameter 152mm, F6
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Mount: 2006 Skywatcher HEQ5 with Rowan belt upgrade (and a Dark Frame Optics tune up 6 years ago).
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Camera: ZWO ASI178MM with home attached cheap peltier cooler (temp controlled by an LED dimmer)
PROs
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High quality mirror: probably about 1/10th wave accurate
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Small secondary: Old school visual guys say if you can get the diameter of the secondary down to less than 20% of the diameter of the primary then its negative impact is barely noticeable.
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The light only hits two optical surfaces (the primary and secondary mirrors). The more optical surfaces the light has to bounce off or pass through the blurrier the final image. That’s why camera lenses with their 9 element (or more) design never match a simple 4 element apo telescope’s crispness.
CONS
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No matter how good the mirror and how small the secondary physics limits its resolution of this little 6 incher to about 1 arc second. Fatter scopes have a much greater potential.
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At f6 its quite slow. A faster telescope would catch photons more quickly. Faster telescopes can therefore take shorter exposures which potentially makes for a sharper image.
My telescope recommendations
Obviously to make lucky imaging worthwhile you need a very sharp telescope that is fat enough to resolve details that are only 1 arc second big. Apo refractors are the best telescope design in terms of sharpness but they are very expensive when they get fat. If you have a 5inch or fatter apo then lucky imaging is worth doing. Anything smaller and its not.
One of the most popular and best value large refractors: Explore Scientific 127mm triplet:
Newtonian telescopes (and Muskatov and Schmidt Newtonians) are the next best in terms of sharpness. 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. i wouldn't be surprised if a standard 8inch Newtonian trumps the supersonic 6inch mirror and tiny secondary obstruction that I have in my red devil. I also suspect a standard 8inch newt would trump a 5 inch refractor despite being 1/3rd of the price.
Good old Newtonians (designed by a Brit!!!) are the way to go. But which one? My rather slow f6 red devil doesn’t catch photons very quickly. This limits its ability. If I had an 8inch f4 Newt I would be capturing photons twice as fast. I could reduce my exposure time from 5 seconds to 2.5 seconds. That would increase the sharpness of my final image and I'd end up catching twice as many photons… So fast newts are better. The downsides to a fast newt are that you will probably need a coma corrector to reduce the coma that you will see on the edge of frame. And collimation is critical. I plan on doing a detailed video about collimation. Its super important. Until then please check out astrobabies guide.
Newt 12" f4
Newt 10 " F4
Newt 8" f4
I'm recommending newtonians with mirrors made by Taiwanese optics company GSO. Word on the street is that they are better than the already very good Chinese mirrors made by Synta.
In the UK I'm recommending the 8"or10" Skywatcher Quattro
Maksutov Newtonians are a very interesting choice and should by slighly sharper than a regular newtonian with the same aperture
You can also get super fancy newts which, hold collimation better and are more expensive (but hey you only live once!
My camera recommendations
For lucky imaging you really need to go mono for best results. The colour camera’s bayer filter blocks 2/3rds of the photons from ever reaching the sensor and lucky imaging relies on getting as many photons as you can as quickly as possible. And even with a mono camera I personally wouldn’t bother with short exposure lucky imaging techniques when collecting my colour data. You don't need your colour data to be as sharp as your Luminance data anyway.
ZWO ASI2600MM PRO
The ZWO ASI2600MM PRO or the similar QHY268M are probably the best cameras for lucky imaging if you own a scope with a focal length longer than 1000mm
Modern CMOS sensors’ read noise varies from about 1-3 electrons per pixel depending on your gain setting. The lower the read noise the better we will be able to see the faint bits of the target when we stack the final result. For lucky imaging we should therefore use a very high gain setting to reduce the read noise to its lowest value possible. I used a gain setting of 275 zwo asi178MM.
The 183MM PRO is also a great camera if your focal Length is over 1000mm. Its half the price of the ASI2600MM because of its smaller sensor but when your shooting galaxies you don't need a big sensor anyway
Do you need a cooled camera?
The amount of noise produced by dark current every second is halved every 6deg C cooler the sensor gets. Lucky imaging exposures are short though, so short that the dark current generally is dwarfed by the read noise. So when lucky imaging you don't need to worry too much about the cameras temperature. However when shooting 30 or 60 second long exposures then dark current is an issue and bc we need to take longer exposures to pick up the faint bits of what we're imaging then I think having a cooled camera is a good idea. My home made peltier cooling hack which I used in the video is useful but its only good enough to take the temperature down to just above the dew point. Any colder than that and the sensor starts to dew over.
If you have a telescope with an 800-900mm focal length then I think the small 2.4nm pixels of the zwo asi178MM are hard to beat but there are many great CMOS cameras with slightly larger pixels that would work brilliantly too especially with slightly longer focal length telescopes. My dream is to pair big bertha, my 10inch f6.3 F1600 newt with the ZWO2600mm, I think the ZWO ASI183MM PRO looks like a good and slightly cheaper bet too
ZWO ASI178MM
This a great camera for planetary imaging too
My mount recommendations
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.
Essentially the mounts I recommend are here. If you really want to take the hobby seriously then maybe you should invest in an even better mount. Or you could see if Dave Woods from Dark Frame Optics can tune the one you've got.
TBH I don't yet know how important having a good mount is for lucky imaging. I suspect if you are able to get down to 1 second exposures then guiding is unnecessary but even 1 second exposures could benefit from having a finely manufactured mount whose worm gear is microscopically smooth. Dave Woods from Dark Frame Optics has to use special machines to smooth the worm gears on the factory produced cheap amateur mounts from china that he tunes up. Also a good mount will reduce the number of frames you have to throw away. And of course a good mount will mean your longer exposure shots (to get the faint bits) will be sharper. I will find the answer to this conundrum when I start being able to analyse data from the good folks who are taking part in THE BIG AMATEUR TELESCOPE
Fancy Mount: I've got an old version of the Avalon Linear Fast Reverse. It uses belt drives rather than worm gears for extra smoothness
I am attempting to get a group of astrophotography nerds together to share lucky imaging data and thereby compete with giant multimillion dollar telescopes. I want us amateurs to be able to capture unimaginably good images and potentially shoot objects that the professional telescopes struggle to get (like variable nebula). We can only do this if we work together. If you are a lucky imager or if you are a whizz at processing then please come and join us by following this link
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