PST etalon motor drive Pt.3 Mock-up with foolishly long drive belts.

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I was cheated! Early sunshine found me in the dome setting up for imaging the sun. Before long it was completely overcast! Uniformly grey. Grr!

New version of SharpCap safely downloaded onto the laptop.  v3.2.6137.

I used some solar down time to compare camera and eyepiece focus points on 500 yard distant treetops. A thread on SGL, by a novice, solar scope owner, raised some useful points I had never thought to check for myself before now.

The ZWO cameras seem to focus at the same point relative to the 1.25" nosepiece regardless of sensor position or body size. Going on a sample of two. Possibly three. 120, 174 & 178.

This seems logical to avoid searching for focus after a mid session camera swap. A 2x WO Barlow on the camera nosepiece pushed the camera position 25mm outwards. Eyepiece focus was 22mm further out with the camera and Barlow removed. 

Worth remembering. Or adding to my observatory notice board. Memory like a sieve? Print the vital stuff and hang it on the wall to save searching back through my blog.

The post van doesn't usually arrive until after lunch. The timing belts are confirmed out for delivery today. I don't have a belt long enough to test the motor on the etalon band. No proof of pudding where reality is untested. Talk of the devil! The post van has brought my timing belts at 12:00.

The etalon drive requires some considerable tension! I fixed the motor bracket in a vice and held the PST etalon stack in my hand. It is 43F in the dome. The etalon adjustment feels stiffer than ever! Needs more friction between timing belt and etalon band. As I suspected the etalon band is too slippery for the rubber timing belt to get enough grip without lots of tension.

It seems counter-intuitive but I reversed the band and had more grip on the smooth side. Should I try a lighter grease to reduce friction of the underlying 0-rings? Removing the O-rings removed all friction but things became very sloppy. The etalon inner drive ring is effortless to move within its range of arc. A radial screw joins the two to provide the drive between the inner and outer rings.

So the friction in the O-rings is the only real difficulty. The outer ring is far too floppy without them. I'm going to try silicone car door seal grease. Instead I found some pure silicone oil for O-ring plumbing lubrication. Applied sparingly but thoroughly to the rubbing surfaces. Sadly this did not have the desired effect. The friction hardly dropped at all! I haven't used enough to leak into the etalon optical system. I had a good clean up afterwards, with clean tissue, to avoid migration.

What about PTFE/Teflon plumbing tape? Remove the O-rings and replace them with a couple of turns of tape. It's not as if there is much relative movement and no real loads to speak of. Concentricity between the rubbing surfaces with minimum slop is all that is required.

A rough wind of tape later to test the theory: Put in the drive screw. Effortless rotation! The O-rings really are where all the friction lies. The O-rings provide smooth stability between the rings and deter involuntary tuning. They are just a little too tight for comfort.This  explains the subjective sensation that gripping the band between finger and thumb tightens the movement. While wrapping a finger around the etalon band feels smoother.

Both belts proved to be too long. More shorter belts ordered. My search of a DIY outlet provided only stainless steel hose clips. Even then, they only had 60mm and I needed slightly greater diameter.

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