10.01.2020 Mounting and balancing the folded refractor OTA.

 *

Friday 43-40F: Another cool, wet and grey day and becoming cooler. Another excuse to work on the folded refractor and its mounting system.

Work completed on the upper crossbar and channel. I made a half round hole in the edge of the plywood crossbar. Then was rewarded with a satisfying clunk as the channel fixing bolt dropped safely into the locating slot. The image shows the layout from beneath. With the saucepan lid protection on the stumpy dewshield.

I pressed tightly fitting birch plywood pads, for the crossbars to rest on, into the upper channel of the massive I-beam saddle. The arrow shows the single fixing bolt for the location channel and crossbar notch.

Lifting the OTA high enough, with the pulley system, proved to be a problem. I should have attached the cord to the saddle side of the framework so that the pulleys didn't close up together. This meant that there was no longer any flexibility to steer the OTA easily into place. I still managed it but with quite a struggle working from a stepladder. Which wasn't the point at all. It was supposed to be an easy loading system working entirely from the observatory floor.

The rest of the morning was taken up in trying to balance the folded refractor. Too heavy for two 5kg weights and too light for three. I tried all my 50mm Ø spacers both inside and between the weights but could not find a happy medium.

I need to add smaller weights where they do some good on the outside of the folded OTA. Bolting weights onto the saddle was not far enough from the axis to do much good. An adjustable weight on a rod would be ideal but it can't pass right through the folded OTA without blocking the light beam.

Yet again I had to release the RA worm from contact with its wormwheel to get sufficient freedom to properly test the balance. Slacking off the clutch screws and pads is never enough for accuracy. I clamped two, 4" Record G-cramps to the outer framework and achieved balance. Now I know what weight I need to add, I could use sheet lead for compactness.

*