143mm f/2.8 Refractor (aka Canon 400/2.8) for visual

[Captain Scarlet]

As the title suggests, I have a Canon EF 400mm f/2.8 lens. Those raw numbers suggest its effective aperture is 143mm. I would love to use this lens for visual from time to time.

The problem is, the distance from the lens’ rear flange to prime focus is only 44mm, ie the depth of a Canon EOS DSLR. I assume that is not enough to accommodate the light-path of a 1.25” diagonal. I suppose I could use it Japanese-style straight through, but I’d prefer not.

So, my question is, does there exist some sort of focal extender or other arrangement to create more back-focus to allow such a diagonal?

Cheers, Magnus

[vlaiv]

You could try barlow lens.

F/2.8 is going to be extremely difficult on any eyepiece. I guess that only central part will be sharp and rest quite astigmatic.

Barlow lens should clean up things. Maybe get x2.5 or x3 one.

Don't get telecentric - I don't think that they extend focus position as much as regular barlow.

Get barlow with longer focal length (if you can - look at the specs and avoid shorty barlows - get regular one).

Even in that combination it is going to be very hard to get diagonal in place. Your best bet is prism diagonal as it has shortest light path - yet these don't do well on fast beams like F/5 - F/6 as you'll have with barlow.

https://www.teleskop-express.de/shop/product_info.php/info/p9870_TS-Optics-T2-90--Star-Diagonal-Prism-with-28-mm-free-aperture-for-Observation-and-Photography.html

That has only 40mm of optical path - but that is between T2 threads. 1.25" adapter is certainly going to add another 25-30mm so you'll have 70mm total not counting the barlow lens.

You could try all of this - but I'm not sure it is even going to work, so if you like experimenting then ...

[Arneb]

I once explored doing this with my 500mm F4 for Nikon F. Nikon used to make a dedicated adapter comprising a prism and a fixed 10mm eyepiece but it is discontinued and it may not have worked with my 'G' or 'E' lenses which don't have a manual aperture ring (unless I could have somehow prevented the lens from from stopping down to the smallest aperture). I'm not sure if Canon made something similar. There is a third party Kenko adapter though:

Kenko LENS2SCOPE Adapter for Canon EF/EF-S Mount, Angled View K-LS10-CEAB (adorama.com)

There are also adapters for converting M42 lenses into scopes on ebay. These comprise a diagonal and, I believe, a fixed eyepiece. You might be able to use one in conjunction with an EF to M42 adapter. The quality of the components are anyone's guess though. Those who've tried turning their telephotos  into scopes (based on the limited discussion I managed to find buried in photography forums) seem to have created their own adapter along these lines:

Lens to eyepiece adapters - Turn your Lens into a Telescope - Bob Atkins Photography

I didn't pursue it any further as I wasn't convinced that I would get acceptable results with a home made adapter and didn't fancy sinking the $$$ to buy one of the Nikon adapters given the possible compatibility issues. I had only considered it for a safari where the primary focus was nature photography rather than astronomy and I had to travel light. In the end I just used a pair of binoculars for some lovely low power viewing.

Anyway, good luck with the project!

Edited April 21, 2021 by Arneb
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[Louis D]

I did this years ago with a much slower telephoto lens for the OM mount.  I put a short macro extension tube on the rear and added PVC plumbing parts and thumbscrews to hold an eyepiece at prime focus.  It worked well enough that I bought a telescope the next year.  I suppose a T-mount would be better because then you'd have a threaded mount to attach to, but I was unaware of them back then.