Mounting: Worm adjustment and Goto drives.

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It needed careful and repeated checking, using a straight edge above and below the wormwheel and feeler gauges. Eventually I was able to confirm a 1mm error in RA worm height relative to its wormwheel. Unfortunately the worm is set too high and this cannot be reduced.

The worm housing bracket sits on top of a solid plate and square section tube housing the drive, stepper motor. I used 10mm, solid aluminium plate everywhere to reduce flexure to a minimum. Only milling away the plate would reduce the worm's height. Needless to say that isn't going to happen.

Normally, correcting this height error would mean removing the OTA and dismantling most of the mounting to fit a thin, ring shim [large washer] under the wormwheel boss.

To avoid dismantling and heavy lifting, a split shim would obviously be best. 1mm is pretty thin. So most sheet materials could easily be flexed enough to allow a simple slit and circular cut-out to pass easily over the shaft.[50mm Ø] It would just require a sufficient gap be opened below the wormwheel boss to allow the shim to be flexed enough to be slid into place.

It might have been an interesting exercise to work out the desired friction levels between rotating surfaces on the shafts. Early on I tried PTFE [Teflon] between contact surfaces to ease manual pointing.

In practice both shafts rotate uniformly from end to end entirely under the Goto drives and motor driven slews. No manual slewing is allowed. Or the lock on the sky is completely lost until it is re-set with a Sync on a known object or position. Even here it is assumed that the mounting is perfectly set up on the True Pole and the Declination shaft is perfectly at right angles to the RA.

An observatory, with a supposedly immovable pier, should offer the opportunity for better mounting alignment. Since its alignment is not repeatedly undone by tearing everything down at the end of an observing session. The telescope itself should also be parallel to the Declination shaft. Or yet again, Gotos will be relying on false information.

The AWR-ASCOM Goto system relies entirely on counting motor turns. Any slippage of [purely protective] clutches would not be sensed. Leading again to false telescope positioning on the sky. The drives are essentially, completely blind. So must carefully count the exact number of "footsteps" between landings on known objects.

There is no GPS or shaft encoding involved. Only a friendly mouse click on the computer screen, or handset button, to confirm arrival on the chosen target. The IH2 handset offers four different speeds of drive to adjust discrepancies in centering before a Sync is confirmed.

The clutches are supposed to protect the motors if they should stall under an unbalanced load or physical blockage. All too often the long OTA would slouch downwards due to imbalance. Leaving me, yet again, with lost coordinates.

You can't ever physically nudge the OTA safely onto the target. The handset [control paddle] must be used to slew to the target. This takes some getting used to after decades of manual shoving telescopes around the sky. The AWR handset is far more responsive and useful than the Fullerscope's synchronous motor control paddle.

I soon fitted three radial screws and plastic clutch plugs to both wormwheel bosses instead of the original one. Even when these are very firmly tightened the OTA can still move independently of the drives rather too easily. Adding the binoviewer or a 2" star diagonal would cause a major slippage simply due to a sudden change in the OTA's balance.

I have been steadily improving the balance and reducing the mounting's sensitivity. Hence the change to the folded refractor instead of the long, straight tube. Moment is mass x distance from the fulcrum [or shaft in this case.]  Sliding weights became essential to combat these sudden shifts in moment whenever a perfectly normal accessory was added.

A routine in setting dynamic balance weight positions must be established to cope with these perfectly normal changes. Otherwise the mounting will count the paces to Go To the butchers. When it thought it had safely reached the bakers.

It only thought it knew where it started but was really facing the wrong way. Sadly it can gather no clues along the journey to remind itself exactly where it is between landing points. Only a fresh Sync to confirm safe arrival at the next target will help.

None of this is easy with a telescope of this scale and weight and often so far above the floor of the observatory in the dark! It is a steep learning curve even when I thought I knew what was involved.

The masses and the torque to overcome them, or to resist unwanted movement, must be carefully balanced in all planes. I still feel I have hardly started on the journey to event-free observing. More practice with a solar telescope in daylight will help to iron out the problems and build valuable experience in the mounting's foibles.

As yet I have no real observing experience with the big home made mounting. Just getting the drives to work with a planetarium software has denied me useful practice with pointing the telescope. Perhaps if it was too easy I wouldn't learn how to cope with a problem. How grateful I should feel about this forced learning curve is another matter.

Monday: I bought some smooth, white, Hammerite paint for the counterweights today. I did not paint the turned edges after smoothing the casting excesses [flashing] in the lathe.

They were cheap, 2" bore, 5kg, "Olympic Standard" weightlifting disks in rather rough cast iron. Their size was carefully chosen to allow them to just fit in my old lathe. Even so I had to use a reversed boring bar to reach around the sides from the tool post. I have watched the edges slowly rust ever since without getting around to painting them. I chose white paint for maximum visibility in low light conditions.

Click on any image for an enlargement.

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