7" f/12 iStar folded refractor 30: Final preparations:

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I spent the late afternoon fitting two plastic knobs with projecting M5 screws to the bayonet plate. Slightly larger holes in the matching OTA plate allowed the threaded rods to lock the two plates safely together. Only now could I be confident that the lens would not just "fall off" the front of the OTA when gravity beckoned.

WARNING! Flash photography:

It occurred to me afterwards how to greatly improve this arrangement.  I will obtain more knobs with a larger M6 thread. Then have anchored nuts on the far side of the OTA front plate. Plain holes and sleeved threaded rods will retain the knobs on the bayonet plate. This will not only lock the plates together, to stop them sliding apart, but allow the plates to be pulled tightly together for accurately repeatable collimation.

I also fitted the short dewshield by trapping it against the bayonet plate using the collimation screws. The 'pull' screws are now changed to socket head type like the 'push' screws. The original pull screws had needed to be much shorter than those used on the 'straight tube' iteration. There I had T-nuts fixed to thick plywood rings as the counter-cell fixing to the tube. I didn't like cross-head screws in this situation because of the dangers of pointing a cross-head screwdriver into the murky depths of the blackened dewshield. Hex socket-head screws will happily accept a matching Torx screwdriver with perfect location and a powerful drive.

Then I had to refit the clamping bars on the MkIII mounting's saddle so that they did not collide with each other in their rest positions. That simply meant reversing which side the open slot was arranged on the lower batten. I still need to look at this arrangement because the battens can easily prevent the OTA being placed securely onto the 'rails.' It may simply require some trimming of the clamping batten's width.

Now the counterweights had to be refitted to improve the balance slightly. I am using a sleeve to push the weights out from the declination casting. A longer sleeve, or an added weight, will push the center of mass a further half an inch outwards.

Fitting the objective, along with its bayonet plate and dewshield, onto the mounted OTA was the next [huge] hurdle. Wasn't this supposed to be easy? I tried doing it with the OTA horizontal and pointing west. But the MkIII's axes locks could not manage the huge imbalance! So I made a batten to support the declination axis with the heavy counter-weights but it was not remotely ideal. The entire mounting could easily have been levered up and off the top pipe of the massive, stainless steel, tripod-pier! Anxious moments followed as I very gingerly fitted the objective.

Ideally, I should have set the OTA, at the correct height for balance, pointing at the Pole Star.  The OTA front plate would then have been much higher but at a more comfortable angle for fitting the objective bayonet. This, however, would have raised further problems. The OTA would no longer have been balanced, longitudinally, without the heavy objective being already in place. A clear case of chickens and eggs. The axes locks was certainly not up to the task. Back to the support batten resting on the lawn to support the counterweights and praying.

I finally managed to slide the objective bayonet plate horizontally into place, while working from a stepladder, but it all felt very unsafe! Now I could finally screw down the locking knobs to fix the bayonet securely in place. It was getting dark as I finished the reconstruction and tried moving the OTA around the sky. It was not perfectly balanced and required further fine adjustment of the OTA along the rails and more fiddly re-clamping with the four wing nuts. Fit washers under the butterfly nuts and they drop and jam against the batten sides! No washers, and the friction is too high and makes turning the butterfly nuts painful on the fingers. Not very happy with this clamping arrangement at all. Though it does hold the OTA very securely it is a fiddle to get right while supporting the OTA with one's third, or fourth, hand. This aspect needs thinking about some more.

The answer is not a heavy tube balance weight [like I used before on the straight tube] but a fixed stop for the OTA on its rails. Hanging, hooking or fixing a temporary weight to the bottom of the OTA will solve the balance problem while fitting he objective. I shall then be able to fit the objective, via its bayonet, when the the OTA is pointing safely at the Pole Star. But more of this later: The Moon is still beckoning, clear and sharp, at ~40 degrees altitude, to the south and directly over the house roof. Though it looks much higher than Stellarium suggested. Jupiter was till only a bright blob hiding behind the hedge.

Click on any image for an enlargement.

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