I have several shelves loaded with old FD, Minolta MD, and Nikon F lenses and I love to experiment with them, but none were expensive enough to make me cry if they quit and I have the tools to repair at least the simpler ones. I just drew the line on the 800 FD as I think the owner wanted about $1500 at the time. I have a friend with an 8" Celestron Schmitt and the EF 800L seems to outperform it by a pretty good margin, and yes, he is always fussing with collimation. The Schmitts can be very good if everything is tuned up and better with correction lenses (e..g Celestron Edge line), but if is a big word in this context. I have several older Mirror lenses (Canon FD 500mm, Minolta MD 500mm, Sigma 600mm, Nikon 500mm, and Nikon 1000mm). They are a blast to experiment with, but the 800L will smoke them all looking through samples, I would say the RF 800 f/11 does also. The Nikon 1000mm comes the closest, but it still needs a lot of sharpening to get close to the refractors and then you have the challenge of actually taking the shot. The Mirrors need a stable platform (most tripods are marginal) and then focus (particularly on long distance) is VERY touchy and hard to optimize, whereas with the 800 f/11, the same shot can be handheld and taken in a couple of seconds. I think the biggest challenge with mirrors is accurate focus. It is very hard to determine (even with focus peaking) just where the optimum point is. I believe that is because the secondary mirror effectively blocks the center (and therefore the peak) of the airy disc, so when focusing, you are actually looking at bunch of tiny doughnuts rather than something that looks like a gaussian curve. This means the focus adjustment has a flat spot on top rather than a peak. It also means that the depth of field is very shallow because you don't have the advantage of that peak to broaden the DOF. Here are a few comparison shots. Note that this pole is about a mile away and much of the time is heavily distorted by thermals. These shots were taken at different times and even different years, but the seeing conditions were pretty good in each case. The time-of-day differences make direct comparison more difficult, but you can get the idea. The R5 likes the big Nikon Mirror better than any other body I have tried it on but it is still a bear to focus and this shot was with a 10 second delay to let the tripod settle down and probably the sharpest shot I have ever taken with that lens. The 800L was on a tripod with the stabilizer on and shot with an SL-2 (200D) by simply hitting the shutter button. The 800 f/11 shot was with an R7 handheld (leaning against a post) and the easiest to set up by a mile. All the shots are quite usable with a little post work, but you can see that the RF 800 does not lose the contest by any means, particularly considering it was shot with the high res R7, but that is offset by the 1.4 TC as opposed to 2x (I don't have an RF 2x converter). All shots are 100% crops.
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Nikon 1000mm Mirror with 2x TC (f/22) and R5.
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EF 800 f/5.6L with 2x TC (at f/14) and SL2
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RF 800 f/11 with 1.4x TC (f/16) and R7