After it came to me how inefficient focusing can be with with faster telescopes and ordinary focusers, I figured why not to check it out. It appeared to be too bad, and my assumption that the least controlable movement is not much better than 1/100 of a full turn of the knob was just an out-of-head guess.
I cut out an 8" circle, had it attched to the knob of an ordinary 2" R&P, ordinary Crayford and an 1.25" helical.
I was in for a surprise. The smallest move I could repeat a few times measured ~1/4 mm for the R&P and ~1/3 mm for the other two. But it required near-zero backlash/slippage and near perfectly constant finger pressure (even small change in finger pressure would make it up to several times worse). A reasonable smallest controlable move (one that can be achieved most of the time) is probably somewhere around 1/2 mm on the edge of an 8" knob, and that comes down to less than 1/1000 of a full turn - 10 times better than I thought. It can be somewhat better than that but, also, if the focuser is too stiff, or too loose, or mechanically inferior, it can be considerable less precise.
This focusing precision allows approx. 1/6 wave smallest focusing increment at f/5, and four times less at f/10 for the R&P and Cryford. Fine for f/10, but needs better for fine focusing at f/5. Small helical is over half a dozen times more precise, because one full turn makes for les than 3mm travel (as opposed to 20+mm for the R&P and 20-mm Cryford).
These results match fairly well those that I came out for the SCT (it was easy with micrometric focuser), with the smallest controlable knob move also somewhat better than 1/1000 of a full turn. It translates into a less than 1/20 wave smallest focusing increment, which is adequate.
Even with the results as approximative as they are, the likely conclusion is that an R&P or Crayford w/o fine focusing still are not adequate for fast instruments, although not nearly as much as I thought. But they might be still good enough for apertures having ~1/3 wave, or more, of seeing error in 1 arcesec seeing (~12" and larger).
I cut out an 8" circle, had it attched to the knob of an ordinary 2" R&P, ordinary Crayford and an 1.25" helical.
I was in for a surprise. The smallest move I could repeat a few times measured ~1/4 mm for the R&P and ~1/3 mm for the other two. But it required near-zero backlash/slippage and near perfectly constant finger pressure (even small change in finger pressure would make it up to several times worse). A reasonable smallest controlable move (one that can be achieved most of the time) is probably somewhere around 1/2 mm on the edge of an 8" knob, and that comes down to less than 1/1000 of a full turn - 10 times better than I thought. It can be somewhat better than that but, also, if the focuser is too stiff, or too loose, or mechanically inferior, it can be considerable less precise.
This focusing precision allows approx. 1/6 wave smallest focusing increment at f/5, and four times less at f/10 for the R&P and Cryford. Fine for f/10, but needs better for fine focusing at f/5. Small helical is over half a dozen times more precise, because one full turn makes for les than 3mm travel (as opposed to 20+mm for the R&P and 20-mm Cryford).
These results match fairly well those that I came out for the SCT (it was easy with micrometric focuser), with the smallest controlable knob move also somewhat better than 1/1000 of a full turn. It translates into a less than 1/20 wave smallest focusing increment, which is adequate.
Even with the results as approximative as they are, the likely conclusion is that an R&P or Crayford w/o fine focusing still are not adequate for fast instruments, although not nearly as much as I thought. But they might be still good enough for apertures having ~1/3 wave, or more, of seeing error in 1 arcesec seeing (~12" and larger).
