Lighten up!

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  • Member since
    March, 2008
Lighten up!
Posted by Antitax on Saturday, January 04, 2014 9:43 AM

   My 12-inch GSO dob came out very good, however the smoothness of motion could be improved. I expected I'd also have to deburr and polish several metal parts in the focuser, but when I took it apart I was surprised - for the very first time - that this piece of gear had no misshaped or rough metal at all, accurate cuts and mirror polish on mobile surfaces everywhere.

Surprise

   The only part not perfectly shaped was the little plastic bushing that pushes against the Crayford focuser's rod. I wrapped a piece of fine sandpaper around a screwdriver that matches the focuser rod diameter, rubbed it a couple minutes against the offending plastic part and voilà, it mates the focuser rod perfectly, which is felt as a buttery, more silent and more accurate focusing motion.

   That was nothing compared to the chore of rectifying metal in some other scopes. Now the up-and-down (elevation) axles don't need lube in most dobs but this one has a plain steel tube and the mirror is thick, thus the combined weight is bound to cause a little stiction. From day one I had lubed the left-right (azimut) motion with candle wax between the three teflon pads and the baseplate. After a few dozen turns to spread the lube and wear things in, it seemed smooth enough. However the central pivot still had a rough steel washer.

   With practice I understood the motion was not smooth enough; I replaced the steel washer with two CD's lubed with pasty silicon grease, and released the central bolt a bit: it must be tightened by finger strength only, with no tools, until it just no no longer plays. Tightening more induces stickyness. That was another bit of improvement, nice! The two large elevation trunions took a layer of same silicon grease (only a very thin film of it), and also slid better. However the elevation was now slicker that the azimut, room for improvement there had been reavealed.

Hmm

   Going against an established advice of not using liquid lubes in dobsonians, I dipped a fingertip in ordinary 15W-40 engine oil, and spread a very thin film of it on the azimut CD's and the elevation trunions (after removing the silicon lube with solvent).

   Now it is finally corrected enough that stiction has all but disappeared, and the scope feels like it's much lighter in weight, the sure sign of effecient lubrication. Backlash and resistance to motion are nearly nil, the scope starts to move with light fingertip touch, and centering is trouble-free at 214x on both axes.

Yes

   No need for special lubes or expensive aftermarket materials here, unlike what I read, but they might be necessary for heavier dobs. By the way I had evened out the trunions' surface and the baseplate by means a few passes with a slightly wet rough sponge. This is the equal of polishing metal bearings, and as the rest of the tune-up, costs nearly nothing. I love it when a broke guy's concept of a telescope can be improved for cheap, things remain in the right spirit.

Tags: I'm cheap

TS 8x40 Wildlife, 10x50 Marine/Fujinon 16x70/TS 80mm triplet, 6x30 finder, EQ-3 mount, TS 2" 99% diagonal/Celestron C5+ and 6x30 finder, DIY tripod/5" Bahtinov/12" GSO dob, 8x50 finder/Meade 2" 24mm 82°/Hyperion 24,13,10mm 68°/TS Expanse 17mm 70°/SW 7mm Panorama 82°/Ultima 2x barlow/Astronomik UHC-E filter/Baader O-III/Astro Solar 5" & 80mm filters/Sky Atlas 2000/Rükl's Moon Atlas/Canon 400D/5mW green laser

  • Member since
    July, 2003
  • From: Eastern SD.
Posted by johnjohnson on Saturday, January 04, 2014 12:45 PM

Well done Benjamin

Nothing like a well tuned Dob. Smoothness = quality feel. Stiction is worse than stiff movement in my book. I have always refrained from using lubricants on my Dob. Reason is I am usually in a fairly dusty / sandy enviornment and feared contaminats collecting to the lubricant and then becoming embedded into the teflon pads. I clean the pads and contact surfaces with window cleaner and have found that to be sufficient.

Now that you have tuned up the Dob, time to take it out and put a few hundred million light years on the new ride!

JJ

20" F5 Obsession, OMI mirror .987 Strehl. 10" F4.7 reflector. 6" F5 ST reflector. 120mm F7.5 EON. 80mm F11.3 guide scope. SkyWatcher EQ-6 Hyper Tuned.   Flicker Astro Site   More Astro Images

  • Member since
    March, 2008
Posted by Antitax on Saturday, January 04, 2014 1:42 PM

   Gosh, someone blew my cover, I no longer have my secret identity! Pirate Yeah, first I didn't want liquid lubes either but the three pads in the base still have their candlewax lube, only the trunions and the central pivot plus CD's in the base have an oil film. Almost nothing can reach between the CD's and the wood plates cause they're too far in.

   Between the trunions and their teflon pads, dust will collect anyway and stick there, and screech against both surfaces. No prob because rubbing the cylinders with a rough kitchen sponge from time to time will make them slick again. And when teflon pads become too worn out (everytime a pope dies), they're easy and cheap to replace. The oil film is barely visible, only a different shine on the black plastic betrays it's there. It's too thin to collect much of anything.

   Besides, I suspect oil will reduce abrasion by dust.

TS 8x40 Wildlife, 10x50 Marine/Fujinon 16x70/TS 80mm triplet, 6x30 finder, EQ-3 mount, TS 2" 99% diagonal/Celestron C5+ and 6x30 finder, DIY tripod/5" Bahtinov/12" GSO dob, 8x50 finder/Meade 2" 24mm 82°/Hyperion 24,13,10mm 68°/TS Expanse 17mm 70°/SW 7mm Panorama 82°/Ultima 2x barlow/Astronomik UHC-E filter/Baader O-III/Astro Solar 5" & 80mm filters/Sky Atlas 2000/Rükl's Moon Atlas/Canon 400D/5mW green laser

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