Many people have come to love the ease of Collimation that 'Bob's Knobs' can provide. These are thumbscrews that replace the 'tool necessary' screws that ship with Commercial Schmidt-Cassegrains. If you wish to purchase a set of Bob's Knobs for $19 then be my guest; if you want to save some money then read on.
Bob's Knobs are basically 6-32 cap-head screws (newer scopes may be metric) that are cut to length and have a plastic knob pressed onto them. If you have a few basic tools you can make your own with an outlay of around $2. Go to your local fasteners store and buy three stainless-steel 6-32 (or the new metric size) cap screws and ask for knobs for them. Might as well get a couple of extras while you are there. Now for the fabrication...
- Measure the length of the original screw. On my C8 this was .300"
- Holding the new screw in a vice carefully cut it to length with a hack-saw
- Using a file or wheel smooth the end
- Using your vice (or better yet a drill press with an old hose fitting) press the plastic cap onto the screw
Note: If you buy the brass knobs (they're great) you don't need to press anything, just cut, smooth and insert
- One at a time replace the old screws with your 'knobs'
- Rough collimate
- Take a piece of paper and punch a hole in the middle
- Stand back about 6 feet and look at the scope through the paper
- Tweak the screws until all the 'circles' are concentric. Don't forget, if you loosen 1 tighen the other 2 (and vice versa)
- You probably won't need more than 1/2 turn to do everything, use small adjustments
- At the end, make sure all the screws are left slightly snug
- Take a piece of paper and punch a hole in the middle
The plastic knobs (like Bob's Knobs)
The brass knobs; easier to use and about $1 total more
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