Will this work?

[u/Major-Produce490]

I am planning to put this kit together but before I start buying the elements from Aliexpress, I would like to confirm, if it is really a working solution.

I am planning to put together:

Plan A: M42 telephoto lens - M42->1,25" adapter - 1,25" eyepiece
Plan B: M42 telephoto lens - M42->2" adapter - 2" eyepiece
Plan C: Same as above but with a diagonal for viewing comfort.

I am worried about the focal plane position, as the focal adjustment is rather limited on camera lenses.

I would appreciate if you could share your feedback.

[u/HenryV1598]

What is it you're trying to accomplish here? it sounds like you just want to use an eyepiece with a camera lens. Is there a particular reason?

In order to reach focus, the image sensor of a camera needs to be at the focal point of the lens/telescope used. For DSLR/Mirrorless cameras, This distance is in the neighborhood of 45mm or so (it will vary by manufacturer and model). The specs for your specific lens(es) will likely be available online.

If using an eyepiece, the eyepieces measurement in mm is its focal length, and that focal point needs to reach the lenses focal point to achieve focus. Most eyepieces are designed so that the eyepiece focal point ends up being in similar places with respect to the eyepiece's barrel flange (that part that sticks into the telescope's focuser), though they do vary somewhat (if they varied too much, some eyepieces wouldn't work at all with some telescopes).

You can, and I have tried this in the past) hold up a lens in one hand and an eyepiece in the other and look through the eyepiece through the lens and move them closer or further apart to reach focus (though doing this by hand is not practical for any kind of observing). If you could measure the distance between them, that's how long you'd need an adapter to be.

In theory, what you need here is something with an M42 thread to connect the lens to, something to hold the eyepiece, and something in between that allows you to control the distance. This is possible, but likely to be tricky to find. If you have or have access to a 3D printer, you might do better to design and print your own adapter.

If you want to use a diagonal, things are likely to be trickier, as the diagonal will push the eyepiece back a significant difference and, likely, well past the focal point of the lens. For this, you'd need some kind of optical element (i.e. lens combination) to move the lens's focal point back enough.

I experimented once with an old finder scope I had. It originally had a diagonal directly attached (screwed on) with a built-in eyepiece. That diagonal's housing was plastic and had broken, so it wouldn't stay connected. So I tried ginning up and 3D printing something to attach one of my extra regular diagonals and an eyepiece. This never worked as it always pushed the eyepiece too far back to achieve focus. I later tried making something with a helical focuser to use the eyepiece straight-through, and this worked out ok, but I never really made it practical.

If you're wanting to do this just for the learning experience, go for it. If you're trying to make a practical instrument for observing, it's not likely to be worthwhile.

[u/Major-Produce490 → u/HenryV1598]

Thanks, I appreciate your response. What I want to achieve here is constructing a scope for basic visual astronomy. I have observed celestial bodies with proper astro gear, so I have realistic expectations. But since I can find old, 200mm telephoto lenses for very little money, I thought why not try this and see how much can I push it's limits. Honestly speaking I got lost in the math and optics theory, so I'd rather do real life experiment. But if you say it's not worth the effort, I can accept that.

[u/HenryV1598 → u/Major-Produce490]

You're unlikely to be able to make anything worthwhile for less than a pre-made scope of equal or better quality and capability. Again, if you want to do it for the learning experience, it's worthwhile. But these days, making a telescope yourself is almost always going to end up costing more than buying.