Telescope eyepiece adapter for Micro Four Thirds camera

I have a Meade reflector telescope which I haven't used in years. Back when I did use it, I had a Nikon DLSR which was really too big and heavy to even try to use on the scope. Even if the weight did not overwhelm the star tracking motors, the scope has enough "play" in the tripod and mechanisms that I figured it would be a pain to try to align. I never really looked into it back then.

A few months ago I got an Olympus OM-D E-M5 Mark II mirrorless camera. It's like a DSLR, but much smaller and lighter. I learned that some of its advanced features make it suitable for astrophotography, so I figured I'd check into it. One way to mount it is called "prime focus", and place the camera where the eyepiece (final lens) would go. The image forms directly on the camera sensor. The telescope essentially becomes a huge lens for the camera. So an adapter is needed that will fit into the lens mount on the camera, and slide into the focusing unit on the scope.

This camera is built around a specification named "Micro Four Thirds", and other makers such as Nikon, Canon, etc. all make them. I figured someone would have designed a prime focus telescope adapter for MFT, and sure enough I found one to download on Thingiverse, contributed by Nikolaus Spence. Cool! It printed easily. Then... I found that my entry-level telescope has a smaller eyepiece adapter than most. Mine needs a 1" diameter, and the download was a 1 1/4" and not parametric (changeable). Bummer! It looked like I'd have to design my own.

The problem is, the "bayonet" connectors for camera lenses are *very* specific and would be rather difficult to recreate. They look like three equally-spaced lobes around a circle, but they are not equal, so a lens can only go on one way. The tolerances are pretty tight. I reached out to Olympus to get the spec, but they don't give it out for free - it's only available to participating manufacturers.

I found a set of reverse-engineered CAD files on a site called Salvaged Circuitry, contributed by Anthony Kouttron. It included drawings, but even better, 3D shape files I could actually import into Fusion 360.






The shape was for both the bayonet mount that goes in the camera, and a flat flange with screw holes that would attach to a lens or whatever. I did not need the flange, so I used a construction plane and sliced it off, leaving just the bayonet mount.

Then I made a cylinder of the size needed to go into the telescope focuser, and joined it to the bayonet mount with cylinders. From the Thingiverse part I got the idea to make fine threads on the inside of the cylinder, to try to absorb any stray light rather than reflecting it.

Print 1:

The cylinder was too thin, and the attachment to the camera base too weak. It broke right away.






Print 2:

I thickened the cylinder and added a fillet at the joint. This one was more solid, fit into the telescope, and fit into the camera mount after a little cleanup. But it didn't latch into the camera when turned to the right point... it kind of jams in place after turning further. There must be something not quite right in the design or the print. But it works!






I tried it on my telescope one night with a clear sky and a nice moon. I found that the focuser-adapter-camera setup did not travel quite far enough inward to achieve focus. I had read about this issue. The way to solve it is to add a Barlow lens, a short tube with a lens that adds magnification and changes the focal length. My scope came with a 3x Barlow, and it worked well. I was afraid the extra length and the weight of the camera would make the setup bend or slop around, but it was solid. I was then able to focus the image, though it was quite tricky. Here's one of the best shots from that night.





















Later I noticed that the first layer(s) had not printed perfectly; one of the bayonet mount lobes was kind of lifted up and not filled well. I also thought that maybe the thickness of the lobes was not quite right and might be the reason the camera doesn't latch it. So...










Print 3:
Some time ago I had bought a set of different sized nozzle extruders for my 3D printer. The standard size for this kind of printing is 0.4mm. I had had limited success printing with another project with a 0.25mm nozzle, but I think part of that problem was the filament. This KVP PLA extrudes really smoothly, so I figured I'd go all the way down the the 0.15mm nozzle. The problem with tiny nozzles is they take longer to print, but this is a small part. At 0.4 it took just over an hour, and at 0.15 about 2:20, which is certainly reasonable. It printed very nicely - no clogs, but a *lot* of tiny hairs inside to clean up. I don't want plastic bits falling down into the telescope.

Print 4:

While #3 was printing, I had an Aha! moment. Maybe the reason I could not reach focus with the adapter was the fillet I added at the bottom of the cylinder. Maybe without that extra 0.1 inch, it would go down far enough to reach focus at infinity. I let #3 finish, then I hid that fillet, exported it and printed it again. Each part only costs 9 cents to print, so why not! Since I had thickened the cylinder at the same time as I had added the fillet I figured it would be strong enough. It is.








Now I need to wait for the moon to come back on a clear night. Maybe before then I'll try it during the daytime on terrestrial targets. After I test it out I'll upload it to Thingiverse and send thank-you's to Nikolaus and Anthony, and tell the folks on the Micro Four Thirds forums.

Update 2020-07-03: I finally got to test this out. The base is too thin, but with some paper shims I got it to work for now. Here is tonight's full moon:

Update 2020-07-18: I figured out how much thickness I needed to add to the base but found that the 3D file for the connector plate I had started with was not changeable. It was imported into Fusion 360 in a mode which precluded changing the thickness parametrically. I ended up redesigning the whole thing from scratch, using the dimensions from the diagram. After a couple of cycles of test-and-adjust I got a version which fits pretty snugly in the camera. The camera socket does not latch on to it, so it depends on friction. It would be better if I could make a version that securely locks in.

I tested this version and photographed Jupiter and Saturn over a couple of nights. (Not great photo quality due to a lot of atmospheric turbulence. I'll get there.) I think I hit the focus limit once, but that could be resolved by backing it out of the eyepiece tube a bit.

Now that I have a working version I shared the files on Thingiverse at https://www.thingiverse.com/thing:4546246