The stock Celestron focusing knob is about 3/4 of an inch across and turns on a 1/2 inch brass 'post' that is part of the focusing mechanism. Now, I like to have a bit more 'feel' in my focuser than a 3/4" knob provides so here's what I (with a LOT of help) did to fix things.
The finished item
Steps in creation
- Turn the knurls off of the stock knob for 1/2" from the bottom to a diameter of .725"
- To do this and keep things on centre, turn or find a .500" diameter piece of
stock and chuck this into the lathe. Place the knob on the stock and tighten the set-screw down
as tight as you can. Now machine the knob. This will keep things running on centre.
- Cut a 3.4" circle of 1/2" aluminum stock in a band-saw.
- Place the circle in a lathe and bore a .727" hole in the centre.
- Turn the end of a piece of round stock to .726" (enough to just fit in the hole you bored) for about .300" from one end.
- Drill/tap the end of the round stock for a 1/4"-20 screw.
- Screw the bored aluminum disk onto the round stock with suitable washers and tighten. Now you can machine the
disk more simply.
- Turn the disk down to 3.300" outside diameter.
- Start relieving metal on from .500" from center to the edge to bring the thickness of the disk
down to .260". You'll want to leave the centre inch offset on the
disk enough to put a set-screw in so only remove metal from one side.
- When you've got your rough disk to dimension. Start relieving metal on both sides from the center inch
around your bored hole to .250 from the edge. Take it down .075" or as much as you want on both sides to relieve weight.
- Machine a .150" slot around the outside of the disk to a depth of .050". This is to take a rubber drive belt (find one that
fits) to give a non-slip edge.
- Drill 6 holes equally spaced around the disk in the relieved portion. Three of these should be big enough
for your finger (to spin the disk quickly) and the other three are just weight reduction. Make them as
big or small as you like.
- Drill and tap the centre inch somewhere for a #10-32 setscrew.
- Slide the disk onto your original focusing knob and tighten the setscrew. You're done =).
When you're done the disk will ride with enough clearance from the installed visual back to keep things in order and will run just about even with the outside of the rear mirror cell in case you want to add a motor to things in the future; or, if you don't want to motorize then stretch a rubber belt around the outside for a non-slip edge.
Drawings (not to scale)
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