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I suspect all of these scope even the DK have to be focused by moving the 2ndry mirror. I believe that used to be done via a rod along the scope and a belt drive of some sort to the focuser. These days an electric focuser would probably be easier. A linear bearing and some pre ground rod for the same might help some one make something suitable for either method. Electrics through the spider vanes or the belt/string or what ever over them.

My feeling is if this is done properly and the tube is stable enough collimation should be a rather complicated one time job providing the main mirror is adjustable for tilt and centring and can be kept in place. Done that way slight errors in the alignment of the focusing travel alignment can be adjusted at the mirror rather than trying to centre and square up etc the focuser. The 2ndry then just needs the usual tilt adjustment.

I haven't really thought through how to focus but another thought that would get rid of batteries is a lever running above or below a vane controlled via a push pull rod at the eyepiece end driven just like that or via a screw.

The part of this that does concern me is the spider. I don't think there is much chance of using tensioned thin vanes as something hefty enough to resist the strain might prove rather difficult to produce sensibly. I had a dob once that used part of a sort of deep oval that went out just past the 2ndry holder. That sort of thing might be a better way to go. On the dob the oval was used to shield the 2ndry mirror from stray light as there was no tube. I also made a dob with a single 1/4in dia steel bar running across the tube but feel that no spider diffraction would be a better solution.

Linear bearings - probably 2 spaced many bar diameters apart. The more over all length of the bearings the better the angular tolerance it should hold. The adjustment could work between them.

John
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I suspect all of these scope even the DK have to be focused by moving the 2ndry mirror. I believe that used to be done via a rod along the scope and a belt drive of some sort to the focuser. These days an electric focuser would probably be easier. A linear bearing and some pre ground rod for the same might help some one make something suitable for either method. Electrics through the spider vanes or the belt/string or what ever over them.

My feeling is if this is done properly and the tube is stable enough collimation should be a rather complicated one time job providing the main mirror is adjustable for tilt and centring and can be kept in place. Done that way slight errors in the alignment of the focusing travel alignment can be adjusted at the mirror rather than trying to centre and square up etc the focuser. The 2ndry then just needs the usual tilt adjustment.

I haven't really thought through how to focus but another thought that would get rid of batteries is a lever running above or below a vane controlled via a push pull rod at the eyepiece end driven just like that or via a screw.

The part of this that does concern me is the spider. I don't think there is much chance of using tensioned thin vanes as something hefty enough to resist the strain might prove rather difficult to produce sensibly. I had a dob once that used part of a sort of deep oval that went out just past the 2ndry holder. That sort of thing might be a better way to go. On the dob the oval was used to shield the 2ndry mirror from stray light as there was no tube. I also made a dob with a single 1/4in dia steel bar running across the tube but feel that no spider diffraction would be a better solution.

Linear bearings - probably 2 spaced many bar diameters apart. The more over all length of the bearings the better the angular tolerance it should hold. The adjustment could work between them.

John
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I'm just a hack of an ATMer, but once you have the spacing and alignment of the two mirrors correct, FOCUSING of the telescope is no more difficult than using a typical focuser such as found on a refractor or Newt.

And I surely hope a wire spider will work, as I plan on using one...if I ever finish this 300MM F20 DK.

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I'm more worried about a pure Cassegrain Tom and people say no way as they always need tweaking each time they are used. DK's are easier to tweak. If things need to stay put then tension should be ok if it's even in all of the vanes and what ever they connect to on the tube is reasonably rigid. They should settle down eventually anyway.

As to the rest there isn't much room round the 2ndry mirror for adjustments - very little on the one I am looking at with a 50mm dia mirror so to me it makes more sense to tilt and centre the main mirror. The tilt is usually there anyway.

The big telescopes probably focus using the 2ndry because of variations in spacing due to temperature changes but people have pointed out that the back focal length can change rather a lot due to variations in this so it has to be set accurately. I don't see any real problem doing that in a way that also focuses. A simple screw adjustment at the eyepiece end would then allow any other errors in construction / optics to be tuned out and it would only be used once. All much easier to do than making a good crayford and a lot cheaper than buying one.

Somehow I suspect this is area is part of the reason SCT's move a mirror for focusing other than the usual one quoted in mag's and books - closer focusing than could be obtained with a normal focusing arrangement.

John
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