*
Sunday: Cloudy until the afternoon and then only short sunny periods of weak sunshine. With the added weight of the 2" Lacerta, solar prism I decided to check the balance of the OTAs on the mounting. I had the distinct feeling that I was pushing the absolute limits of the four x 5kg counterweights. [44 lbs in olde money.]
By slackening off the Declination worm housing/support plate nuts I reduced friction to what little is left in the main bearings. Which are self aligning, 50mm bore, cast iron, flange type, ball bearings. The pair of OTAs was a little nose heavy so I adjusted the tube balance weights on their rails. Soon the entire revolving mass was in a state of near equilibrium.
I wanted to ensure that the pair of OTAs would stop at random positions. So I then had to reduce the offset weights on the stalks. They were initially heavier than required to balance the entire 6" OTA attached to the side of the 30" long saddle. I had deliberately placed the tube balance weights opposite the 6" refractor on the saddle. So they were helping to manage the offset 6".Eventually I reduced the balance weights until the 6" wanted to be positioned gently downwards. Moving the "stalk" weights out a little more again then neutralized the balance around the Declination shaft.
Spinning the OTAs produced no obvious bias but was very impressive! I could then lock the Declination axis worm into its wormwheel again. Allowing two axis freedom is much too dangerous with such large instruments unless they are already close to perfectly balanced!
The image shows the set-up before I replaced the offset "stalk" weights with much lighter ones. The 6" H-a refractor is surprisingly, easily balanced with these. It was too dark for pictures by the time I had finished balancing. Now I can reduce the lengths of the threaded stalks and increase the barbel weights slightly to compensate and maintain their combined moment. Anything projecting from the sides of the OTAs is a nasty "head magnet" for the unwary climbing up to the observatory floor. So I want to minimize the projection of these extended weight stalks.
Note the "posters" on the obs. wall with large, block text tables of magnifications of both telescopes against eyepiece focal length. With GPC compensated powers alongside for when [if] I ever get my misaligned binoviewer back from T-S. I also have an AWR button pressing menu for the Intelligent Handset IH2 [paddle.] There's nothing worse for somebody who needs reading glasses than fine print in a darkened, red lit dome. I don't want to be finding and then donning spectacles just to see what power I'm using.
Only now could I safely slack off the RA worm while keeping a firm hold on the 7' long 7" OTA. You can't just remove the friction and expect such a large structure to remain still. It has quite a serious moment [total effective mass x distance from the pivot.] It also has a large arc of swing [at both ends!] Which is only just contained within the hemisphere of the dome and its lower, octagonal walls.
Meanwhile the intrepid "mechanic" is standing firmly within the 3/4 of a sphere of potential range of motion. It may not quite have the power to hit me out of the observing slit and on out to the garden boundary. It can still be painful to be struck without warning by the accumulated mass.
The objective of the 7" alone weighs 10lbs. The 6" objective is no lightweight. 20kg of counterweights will do what they will when released. Then there's all the extra OTA balance weights, two telescope tubes, the focusers, PST and solar prism goodies.
Thankfully the release proved that the main counterweights were still [just] in charge. The OTAs wanted to gently nose dive if left to their own devices. More adjustments of sliding weights and the polar motion of the entire mass was very slightly, main counter weights down but predictable.
I really ought to have an adjustable weight to balance around the polar axis. I can't put it on top of the Declination, bearing housing because that would upset the torsional balance. It would need a balance weight on rails on either side of the Dec housing to keep things neutral. I had the idea of using large, eye bolts to support these threaded, weight rods. Though this wouldn't be as "pretty" as brackets and a plain, tubular rail. The main advantage of Dec side weights is overall compactness. Extending the Declination shaft, to improve the leverage, would increase the projection of the main weights into the already cramped work space.
Absolutely worst case scenario when balancing is the OTA's wanting to swing downwards in a hurry when released! This could happen if the main counterweights were not heavy enough to balance the combined OTAs. Not this time. I re-clamped the RA worm nuts and all was safely gathered in. At least for the moment. The drives would no longer have to lift heavy stuff uphill. Nor resist its impending descent. Both problems leading to noisy motor stalls. A sure sign of imbalance.
The gibbous moon was rising when I went in for a cup of tea and rest. More later if it stays clear. It did, but the moon was blindingly bright. With little to see except on the terminator. I went up through the powers to 220x but Plato was so poor in contrast that I could see no minor craters. I even tried the Baader Solar Continuum and the Orion 2x Shorty Barlow but it didn't help. So I switched off, locked up and retired indoors to prepare for dinner.
*
No comments:
Post a Comment