Friday, September 04, 2015

repaired CAO Dob

Decided to have a look at the Walker Dobsonian. I had in mind to loosen the turntable and have a look at the sticky focuser. Of course, it became a bigger job.

The rocker box turntable bolt-nut was so tight that without the optical tube assembly, the top would not turn freely. With 17mm wrenches, I loosened the pivot fastener one full turn. It is much better azimuth. But with the weight of the OTA, it stiffened again. I wondered if the slide-y bits need to be checked.

I moved on. As I checked the altitude, I noticed the side elements on the OTA moving. Oops. I found the two bolts very loose. When I looked down the tube, I saw the nut on one bolt was at the end of the threads! Crikey. That would have been bad when it fell off... I put a 10mm spanner on the nuts and a large Philips on the bolts and tightened them. Good. The other side of the OTA was fine. Much better altitude motion now. Still, there's a lot of play in the handle area. I wonder if a shim has gone missing.

Finally, the focuser. Sticking and jumping when at the out position. And not because the lock was engaged. I dismounted it recalling memories of fixing the Centre's other Dob focuser last winter. Gum-like grease again! Ooh. This focuser did have the sled! I had wondered about this last December! Black plastic (Teflon?) about 4 or 5 cm long, with divots for the two small hex grubs. Cleaned everything with paint thinner. Apply white lithium grease to the rack-and-pinion. Reassembled. Much better!

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