Adapting telescope (2" focuser) to Fuji X?

[Astigmatism]

I have a Dobsonian Newtonian with a 2" focuser, and want to put my X-T5 at the prime focus, maintaining the open path of 2" parts. That is, I don't want to step down to 1.25" adapters and then back up again. I'm pretty sure my telescope is fast enough that putting 1.25" hardware in there will cause vignetting.

What do I need to do this?

I've investigated 42mm and 48 mm T rings, extension tubes, T tubes and T2 tubes, 1 mm and 0.75 mm thread pitches, and some very nice Wikipedia articles on various tubes and camera thread standards . Also I bought three different kinds of extension tubes (for other purposes) none of which measure 2". I'm not getting it, though. What's the magical secret here?

 

BTW I also still have to investigate whether the prime image is far enough outside the focuser. But I can't really pin this down before I know what component will be first on the camera body.

Thank you!

[jerryy]

The shorter M48 to X-mount adapters are usually the ones that work.

The trick with Newtonians is that the focus spot is almost inside the focuser.

Edit: I am updating this portion, the older stuff that was here is still valid, but 'a picture and a thousand words and all of that'.

If you look at the focuser closely, you see it has the outer part and the inner part sitting inside. Loosen the screws on the outer part, take the inner part out of the focuser and slide in the adapter with the camera attached. Tighten the screws and adjust the focuser as needed.

The overall “trick” is to find the scope specs listing for what is called back focus (for many refractor scopes it will be 55mm, but for reflector scopes it will be very different depending on what kind of scope it is, rc scopes have very different back focus distances than do Newtonians than do …) Okay, Fujifilm X-mount cameras have what is called the flange distance (the distance from the mount spot to the sensor in the body) of 17.7mm. If your scope or accessory has a back focus of 55mm, subtract 17.7 from 55 to get 37.3mm that you need extension tubes to cover.

Most Newtonians have very short back focus distances, so you almost need to have the camera inside the focuser. Other bodies, Canon, Sony, Nikon, etc. with deeper flange distances are more troublesome when trying to use them with standard Newtonian scopes.

Edit: It occurs to me that I could hopefully make this a bit clearer by demonstrating with one of my scopes, so in a bit, I will post that and not rely on looking at other sites for visuals. I will try to do so quickly and come back and edit this posting.

HTH.

Edited December 5, 2024 by jerryy

[jerryy]

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The M48 to X-Mount adapter with lens cap.

The focuser. Loosen the three (screws / bolts) and remove the inner part.

Inner part removed and some M48 extension tubes are added to the adapter. Just use long enough extensions for the focuser to hold onto the combination but not so long that when you put them into the focuser, it hits the secondary mirror. That will cause a lot of bad juju. (The small mirror at the top of your Newtonian is called the secondary).

Adapter combination attached to my trusty X-T10.

Slide the tube into the focuser, tighten the (screws / bolts) and you are good to go.

Edited December 5, 2024 by jerryy

[Astigmatism]

Wow, jerryy, you knocked this right out of the park! Thank you! This would be great as some permanent sticky!

I think I have ordered something that will work. It's M42, not M48 -- I couldn't find both parts in M48 on Amazon or B&H Photo. However I bet this will not cause any vignetting. I've had tools for predicting vignetting, but don't know where working ones are, so I'm gambling a little. What I ordered:

Astromania 2" T-2 Focal Camera Adapter for SLR Cameras

Fotodiox Lens Mount Adapter Compatible with M42 Screw Mount SLR Lens on Fuji X-Mount Cameras

I already have one of the Fotodiox adapters which I built into an adapter for mounting a microscope objective 160 mm from my camera sensor. It can be trusted. However it doesn't minimize the length added to the optical path, and I didn't find any others that were shorter.

The scope is an "Orion SkyQuest XT10 IntelliScope Dobsonian Reflector Telescope" (this name is almost as long as the optical tube). My manual doesn't list the back focus distance, but I am hopeful, because using 2" eyepieces requires a special adapter that extends the focusing tube outward, as if the back focus is surprisingly large.

Thank you for a very thorough useful answer!!

[jerryy]

This one may work for you, it is a little expensive:

https://www.telescopeadapters.com/2-prime-focus-adapters-for-fuji/564-fuji-x-mount-2-ultrawide-prime-focus-telescope-adapter.html

This one will generally work, but you will need to get another step down ring to go with it:

https://agenaastro.com/baader-wide-t-ring-for-fujifilm-x-with-d52i-to-t-2-and-s52-tring-fw-2408331.html

The Astromania company you are working with does sell M48 extension tube sets, such as this one:

https://astromaniaoptics.com/products/astromania-astronomical-2quot-m48-extension-tube-kit-for-cameras-and-eyepieces-length-5mm-8mm-10mm-20mm-30mm-m48x075-on-both-sides?VariantsId=10423

They have others, just click that you want them to ship from the US, they have local resellers.

Newtonian scopes are really nice; since they are reflectors, they have no chromatic aberration but you do need to keep the mirrors aligned (collimated) to avoid coma troubles. The reason the back focus is so short is when Issac Newton invented them, way back when, the design used an eyepiece inserted right at the focuser. Cameras were not around back then. Since then, the design has undergone a few changes, trying to get it more camera ready, (those new designs have joined the ranks of telescopes called astrographs) but overall they are still just about the same as when Mr. Newton used his invention to peer at the heavens.

When you get things all worked out, post some images!

Edited December 5, 2024 by jerryy

[Astigmatism]

The two items I ordered aren't compatible. The threads on the camera side are coarser than the threads on the telescope side. I still haven't found (on my own) a camera part and a telescope part that both specify thread size (including pitch) and match.

So I think I'd like to try the first thing you mention in your last post, jerryy, the "Fuji X Mount 2" UltraWide Prime Focus Telescope Adapter" at Telescopeadapters.com. But I have a question about it. The photos seem to show that the barrel has two slightly different diameters. The barrel is narrower close to the camera and wider further from it. There's definitely a small shoulder visible in the photo. If the wider part fits into the tube, the narrower part will be under the clamping mechanism (three screws bearing on a split brass ring). That seems wrong. If the narrower part fits, the wider part won't go in in the first place.

Can you tell me what's up with that?

 

Thanks!!

[jerryy]

That is a little odd the threads are not matching. Usually the pitch is the same for those. 🤷‍♂️

Regarding that first thing I mentioned… That adapter is a bit of a multi-purpose adapter. I guess More Photos are in order:

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The inside is deeply threaded.

This is typically what you would use it for to get your camera’s sensor into prime focus range. (I did not put my trusty XT cameras on this time so that they would not obscure the view. But ...

the pieces come apart, so that you can use other extension tubes if you want to. (M48 threaded).

Look inside your telescope’s focuser, your scope most likely has that brass ring sitting there. similar to the one pictured here.

The three bolts / screws clamp down the ring onto the piece you put inside.

You will have to trust me that the ring clamps tightly whether you use the wider or the narrower place on the tube. I do not know why the manufacturer made it like that, but it does work. [For what you are doing with your Newtonian, you would slide the tube all of the way in and then clamp it, or use a shorter extension tube.]
 

edit: If I were to guess, I would guess that if the brass ring fits entirely inside of the indented narrower part of the tube, doing so would help keep the tube from slipping in either direction, and that makes sense, but that place is not the only clamp down spot, other telescopes with different back focus spots would not use that location to clamp the tube.

 

Edited December 10, 2024 by jerryy