1000mm russian catadioptric lens converted into telescope (not working).

[Marco Gilardetti]

I have been given a russian 1000mm catadioptric lens which had been converted into a telescope by adding a rack-and-pinion focusing tube, a 45° prism and an eyepiece holder. I would like to use it, but it seems that it can't focus with any of the eyepieces that it came with. Apparently, a much longer tube would be needed, and the eyepiece should sit much more far away (like: half meter) from the barrel of the lens in order to see a focused image.

Does anyone perhaps own a similar gizmo, and can figure out or suggest what may be wrong? Is there any missing lens/part perhaps? I can hardly imagine anyone doing this conversion only to get an unusable bucket of metal and glass, I'm pretty sure that it had to be functional at the beginning.

As a side note, by searching over the internet I have spotted this neat conversion kit which apparently does the same thing. Unfortunately, as it can perhaps be seen in pictures, the conversion on my unit was done pretty roughly, by filing out the original 42mm thread before attaching the rack and pinion tube with screws.

https://www.adrianololli.com/articolo.asp?ID=2861

Any help is welcome. Thank you.

 

[Horatio]

If it was made for a film camera, wouldn't the plane of focus be closer to the mount?

 

[Dali]

Does it focus at infinity?

Like Horatio, I think that the focal plane is much closer to the lens than you think. What if you remove the diagonal prigm and keep only the r & p focusser and the eyepiece?

 

[AgX]

You obviously got the wrong eyepieces.
As stated above, from the photos of this lens with mounted cameras you will see roughly where the focal plane is situated.

 

[Bazza D]

If possible I would take the adapter off and fit the lens to a camera and see how the lens works.

 

[reddesert]

The problem is not the eyepieces, but most likely the (lack of) back focal distance that other people have suggested. If you look at the conversion kit link you provided, the diagonal and eyepiece are much closer to the rear of the lens body.

For a start, remove the diagonal and try putting the eyepiece directly into the focuser. If it isn't exactly the right size tube, just hold it in place and see if you can get it to focus. As usual, you need more extension to focus on a closer object, meaning closer objects focus at a longer back focal distance. Try both very distant and closer objects to see which way you need to go.

 

[Donald Qualls]

The eyepiece has its own focal length; that should coincide with the final focal plane of the lens itself (and therefore should be close to the flange-to-film distance of the mount). I actually suspect the extension tube on the mount is too long since it should probably be in the 40-50 mm range from the mount flange to the focal plane. Try removing the diagonal and mounting the lens directly in the focuser, but even that is probably too long. Assuming the lens has its own focusing helicoid, you could try fabricating a shorter tube to accept the eyepieces and use the lens focusing mechanism. Your image will be inverted, of course, but that's normal for astronomical type telescopes.

 

[Marco Gilardetti]

Thanks everyone for your respective comments. By dealing separatedly with the lens and the eypieces issues, as many of you suggested, I first checked that the lens could actually deliver an image, and later managed the whole assembly to work.

The problem was that at first I thought the helocoid had been deactivated, as its stop screw had been removed and the rack-and-pinion focusing device had been attached. This proven not to be the case: the target can be focused by using both the helicoid and the rack-and-pinion, but if one of the two (or both) are not sitting in the proper range (which happens not to be too wide) the target will never be focused. Once reassured about the functionality of the lens, and once checked by holding an eyepiece by hand that an image could indeed be seen, I fiddled around with the helicoid until, by trial and error, I could focus with the whole rack-and-pinion tube attached. Once the helicoid and the rack-and-pinion are both set in the "sweet spot" range, sharp focus can be achieved by moving any of the two, at the observer's choice.

Evidently, when I first tried the "telescope", the helicoid and/or the rack-and-pinion were completely off. I believe that the eyepieces that I got are partly for astronomy and partly for microscopy, however both seem to work quite all right. The helicoid is very stiff, probably due to hardened grease and shold be re-lubricated. Has anyone ever disassembled one of these lenses and can give me some directions, or does anyone know where a tutorial webpage is located? Thank you!

 

[Dali]

No need to change the grease if you plan to use the lens for astronomy. Use the R&P to focus instead.

Yes, some of your eyepieces are for microscopy. Their barrel diameter should be different from those used for astronomy (31.75mm & 50.8mm mainly).

 

[Donald Qualls]

Microscope eyepieces are usually 23 mm, as I recall, which is even smaller than "consumer" telescope eyepieces (0.965"). If they fit and will hold in the drawtube, they'll work, it'll just be a little trickier to figure out what overall magnification you have (normally you'd divide the objective length by eyepiece length, but microscope eyepieces are marked in diameters magnification; you then multiply by the objective magnification, which you don't have for that lens).

 

[Dali]

Usually, the get a micropscope eyepiece focal length in millimeters, you divide 250 by the eyepiece power. As an example: Your eyepiece marked 12.5x should have 250 / 12.5 = 20mm focal length. With your 1000mm lens, it gives x50 magnification and more or less 2mm exit pupil diameter.