Seemingly impossible step up ring challenge

[tribal-warrior]

Ive come across a telescope adapter that I want to use on my microscope eyepiece tube. The plan is to screw this adapter on to the end of my Sigma 30mm f2.8 lens (with an adapter ring) for afocal imaging. That lens would be mounted to my Panasonic G7.

However, the screw thread of this telescope adapter is of a very unusual size / diameter. It's indicated that it's 2 inches / 49.9mm. However, when I convert 2 inches to mm on google, I get 50.8mm.

The filter thread of my Sigma 30mm lens is 46mm. So it looks like I would need a step up ring of some kind. When looking for step up rings online, the closest I can find is a 46 - 49mm. So there may be some chance that this one might work. But if it's not an exact match, what could I do instead?

 

[Paul Spinnler]

I am copying 1:1 an AI response for you below:

I understand your dilemma with the telescope adapter. It's definitely a tricky situation with those non-standard thread sizes.


The discrepancy you noticed is important - the adapter is listed as 2 inches/49.9mm, but 2 inches is actually 50.8mm. This suggests the adapter might be using a non-standard threading system.


Here's my analysis of your options:

1. About the 46-49mm step-up ring: This will likely be too small. The difference between 49mm and 49.9mm is significant when it comes to threading precision. Forcing it could damage both the ring and adapter.
2. Possible solutions:
   • Custom machined adapter: A machine shop could create a precise adapter that goes from 46mm (your Sigma lens) to 49.9mm (the telescope adapter).
   • Use a larger step-up ring with modification: Get a 46-52mm step-up ring (which is common) and use shims or tape to reduce the inner diameter.
   • Two-stage stepping: Use a 46-49mm step-up ring first, then add a thin adapter ring or spacer to bridge the final gap.
   • Thread tape/PTFE tape: If the gap is small, telescope/plumbing thread tape might work temporarily, though it's not an ideal long-term solution for optical equipment.
   • 3D printed adapter: If you have access to a 3D printer, you could design a custom adapter, though it may require several iterations to get the fit perfect.

For afocal imaging setups like you're planning, having a secure connection is important to prevent light leaks and ensure stability. If you decide to try the 46-49mm ring, test it carefully before mounting your camera equipment.

 

[tribal-warrior]

Great suggestions there. Someone on another forum suggested using epoxy to glue the step up ring to the microscope adapter. I guess I could get an extra wide step up ring that has a wide flat base - easier for gluing on.

I wonder where I could get some shims that are just the right thickness. I admit Ive never used shims for any project before.

By the way, I used to do afocal imaging with this microscope in the past but I had a very crude way of setting everything up. The tricky thing is I have to arrange the camera so that the lens is perfectly parallel with the eyepiece (and centered) otherwise there's weird uneven vignetting. I used to set the camera up with a tripod and a slide bar and it took about 15 – 20 minutes to get the camera placement just right. Way too long for my liking.

Hopefully, using this telescope adapter will make set up time much quicker.