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Hi Dave
I suspect usability will be the main issue. That is, most telescopes come with a focuser, a mount for adding finder scope (a small, wide field, low power scope to locate objects in before centering in the main scope), means for attaching to a mount, etc.
I have photographically tested a Canon 400mm F5.6 wide open and this leads me to believe that optically, it would make a fine visual instrument.
However, I would need to purchase an eyepiece adapter so I could insert eyepieces to use it as a ‘scope. I’m not too sure if a simple adapter will do; that is, would the eyepiece need to be very close to the lens in order to achieve focus. Would the eyepiece reach focus? Most astronomers have several eyepieces and you would need to check that each eyepiece came to focus on the lens.
Mounting rings for telescopes come in standard sizes and you may require some modification to mount a tele lens?
Also, would you want a nice piece of L glass to be dripping in dew, or outside in –ve temperatures? Notwithstanding their tough design and build quality.
I have focused the 400mm F5.6 using Remote Live View on a 40D and it worked extremely well, although it would have been nice to have a motorised focuser which can be fitted on most telescopes to assist in focusing. The 400mm F5.6 won’t accept an off the shelf motorised focuser, although Bert (Avondonk) has designed a very nice belt drive system for focusing his lenses via a motor drive.
Cheers
Dennis
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