Secondary Mirror Holders question

[barx1963 (Malcolm)]

One thing that has always puzzled me as far as big dobs are concerned, is that on a secondary holder that is pretty much the standard, such as the Astrosystems ones that are common, the only way to rotate the secondary is by loosening rotating and then tightening with a spanner, then checking, redoing it and hoping you get it about right.
Surely it cannot be too hard to devise a simple geared mechanism to allow the rotation to be done accurately and then locked down? All other parts of the alignment/collimation process are covered, why not this?
Being an inveterate fiddler with my gear, I have to redo my secondary position fairly regularly (at least 4 times this year!) and this part of the process is the most time consuming and frustrating.

Malcolm

[ZeroID (Brent)]

I would have thought that the need to rotate a secondary would be minimal. I also suspect that the mechanism needed to accurately and minutely rotate a mirror would be rather difficult to design and build. Simple angular movements with adjusting screws, guitar wires etc are pretty easy to setup. A accurate rotating system on the other hand ... hmmmm

[Roger Davis]

There is a simple method which is to use a Morse taper. This is used on lathes for mounting various tools. The tapered shape gives grip (believe me when you're drilling out a 25mm hole in aluminium rod the amount of torque that it takes to spin a Morse taper free is quite high) and would hold a secondary mirror no problems. By inserting a threaded rod into the narrow end and using a knurled knob it would be an easy process to align the secondary and tighten the knob to bring it into the locked position.