Telescope Adapter (Eyepiece Projection) for 1.25" Eyepieces and Micro Four Thirds Mirrorless Camera

Previously I developed an adapter which connects the camera directly to the telescope focuser, so the telescope is the camera's lens. This is called the Prime Focus method. In my case, the focal length is 910 mm. While this works, the images of planets are very small. They can be scaled up (cropped), but because they do not cover many pixels of the camera sensor, they don't preserve much detail.

Another approach is to take a picture of the image produced by an eyepiece, which magnifies the image. The amount of magnification depends on the focal length of the eyepiece and the distance in front of and behind it. I have not yet determined this for my adaptor, but probably will at some point.

There are various focal length and magnification calculators available, but they seem to vary and will depend on measuring the actual distances resulting from the adapters. (I'm starting with a 15mm Plössl eyepiece, and I also have a 6mm to work with... they are not the same physical size.)

As mentioned in my previous post, it starts with a part connected to the telescope's focusing tube which accepts a standard 1.25" eyepiece or other part. 

Next is a tube which inserts into the 1.25" opening and holds the eyepiece at some distance up from it. So that tube is slightly larger than the eyepiece and contains a shelf so the eyepiece rests at the right height. The outside dimension therefore depends on the specific eyepiece to be used. I'm using an SVBony eyepiece, so I measured that carefully. Maybe in the future I can make it larger to accept others. But since the design is all parametric, other users could resize it if needed.

Next is a tube that slips over the eyepiece holder and can slide up and down. This is important in order to "project" the image onto the camera sensor at the desired size. More on that later. I made some estimates of the minimum and maximum lengths. This part includes threads for a set screw to lock this outer tube onto the inner tube at the desired length, without screwing into the actual eyepiece. I went through a couple of iterations of exterior shape to get one that prints an overhang with no drooping. I got some knurled screws from Home Depot that work fine.

It ends with a part connecting to the camera's bayonet connector. I found the Micro Four Thirds connector as a STL file. (It's pretty complicated and works well so there's no reason to redevelop or change it.) The "standard" way to connect this to telescope adapters is called a T-mount, which is a big ring, 42mm in diameter if I recall correctly. I had some issues with the standard T-mount threads so I'm using a somewhat larger custom thread for now. I may go to a standard thread at some point. This part has male "T-mount" threads, and the outer eyepiece tube has female threads, so it all fits together.

Depending on the distance to the object being imaged, the telescope focus travel may be extreme. To accommodate a wide range, I developed an extension tube that can be inserted between the camera connector and the outer tube. This extension is a 14mm long hollow tube with male "T-mount" threads on one end and female on the other. I think the tube is needed for celestial objects, and not needed for terrestrial objects. More on this later.

The inner and outer tubes need to slide snugly but smoothly over each other, so they need to fit well. Due to shrinkage of the PLA plastic I used, it took me several iterations of adjusting a tolerance variable before these fit properly. Here is how all the parts appear together, in cross section:

And visualized:

And in real life:

Because the M43 bayonet connector has a lot of fine details, I used my smallest nozzle on the previous design. It's 0.15mm. Unfortunately after printing a few parts it became hopelessly clogged. I went down a rabbit hole trying to unclog it with solvents. Since that was taking a long time, I tried printing with a 0.25mm nozzle, and it worked fine too on the M43 as well as the threads. It's maybe a little more rough, but it works. In the meantime I ordered another 0.15mm nozzle and some 0.15mm cleaning wires.

If the tubes are not set to the right length, the image is highly vignetted, i.e. a small round image in the middle of the camera sensor, highly distorted. By extending the tube the image gets larger and less distorted, until it becomes a usable image. I'll need to do some tests to see if the distortion completely straightens out or remains an issue to deal with in post-processing.

This tube will bear the weight of the camera and needs to be robust enough to not allow it to bend or fall. The thicknesses of the inner and outer tubes are parameters so I can thicken them if necessary. It seems to me that on the one night I have tested it, there was one part that I felt needed to be thicker so that a set screw would not distort it. I think it was the insert on the lower tube, that goes into the telescope focuser. I'll have to check on that again.

The good news is that the adapter did work as intended. The bad news is that I had tracking problems with my telescope's computer, so I was not able to get any useful images. I've taken some steps to improve that, and have some more testing and calibration to do. I also need clear skies and decent temperatures to do some further tests. I'll update this article as I get more results.

These parts and the Fusion 360 design file are now published at https://www.thingiverse.com/thing:4834778

Another step will be to figure out the length needed to make a lower tube for my 6mm eyepiece, which is considerably longer than the 15mm. The length of the adapter may increase the leverage that the weight of the camera places on the telescope tube, which may change the power that the telescope drive motors need to exert to move the whole assembly. We'll see how that goes.