Well, struggling with headcold (groan), I put a day into modding my dob on Saturday.
Major work - installation of EbonyStar and new teflon sliders.
For all the cautions I had read, the EbonyStar was easy to work with. I started off carefully jigsawing it held tightly between 2 pieces of threeply, but by the end of the day I was cutting my altitude bearings with tinsnips!! Yes, I guess if you are finishing off someone's kitchen benchtop, it is critical how the edge is cut since it will flake slightly, or worse, but not a problem for this job.
Started by cutting a 48cm dia circle (jigsaw between wood with finest metal cutting blade), a hole in middle for pivot bolt (just a hole cutter on drill), then holes so I still had access to the four screws that connect the baseboard to the sidewalls. While I had the screws out and the baseboard disconnected from the walls, I took the opportunity to coat all exposed particle board surfaces with estapol to try and retard moisture ingress.
Then glued the EbonyStar into place using contact cement. I was concerned about getting the laminate accurately located before the surfaces touched, but it was no problem (with an extra pair of hands). Hammered it down - mallet onto block of wood - then stood the base down, on the EbonyStar for an hour to let it set under pressure.
The new teflon pads were obtained from Astrosystems:-
http://www.astrosystems.biz/pivot.htm
I fitted the three large ones to the ground board after levering off the smaller, stapled pads provided. I used a knife to bevel off the leading/trailing edges of the teflon pads before I fitted them - about a one mm bevel.
Then I fitted the new pivot bolt (also Astrosystems - see above) which comes with a large teflon pad to provide centre support. The instructions discuss shimming this center teflon pad to reduce the load on the three main sliders, but with even one milkbottle washer, the baseboard rocked on the centre teflon, so no shimming.
With that all done and the bolt pulled up relatively loose (in my case, it needed a couple of washers at the top - my base and ground boards must be thinner that it was designed for - I should have bought the 2" bolt option), the base spun nicely on the ground board. I could notice an improvement, but it wasn't friction-less. However the test will come on a cold night - but I'm optimistic there will be less stiction.
Now, the altitude bearings. I had been wondering what to do here. I started by replacing the teflon pads with the new ones supplied. The originals were already set at about a 90 degree subtended angle, so I stuck with those positions. Again I beveled the leading/trailing edges. I decided to have a go with wrapping the plastic bearings with EbonyStar. I knew I had to thin the backing so I could wrap around the small radius, so out came the trusty electric planner - dull and uneven blades - and I had a shot at planning off half a mm or so of the backing. Well, amazingly, after a bit of experimentation on offcuts, it worked, and I had a satisfactorily thinned piece of EbonyStar for two bearings. Here is where the tinsnips came in. It was now thin enough that I could carefully cut out the bearings covers - 21mm x 350mm. I carefully sanded the sharp edges down and fitted the bearing covers. I designed it so it would wrap around and overlap, then I put one screw through the two ends and into the plastic bearing. A bit of careful work to ensure the EbonyStar cover was taut over the bearing and careful placement of the screw so it did not foul on the base over the travel of the OTA. No glue, I'd decided I wanted to be able to take them off, if unsuccessful.
Now, I had guessed as much, but the radius of the bearing had increased, and the teflon pads were thicker, so the OTA now rests not flat on the teflon, but on the top edge of the teflon. OK, I trimmed a bit more teflon off to allow a greater contact area, but you should see how smooth the altitude movement is now! I may have to reinstate the friction springs, but let's see how I go. I am already used to adjusting my speaker magnet counterweights frequently as I change eyepieces and altitude, so I may get it to work OK. But I may have to rely on the big rubber bumpers I previously placed at the ends of the OTA travel, if I turn away and it starts to tip!
Minor work - rotate focuser and fit support for counterweight under top of OTA. Finish "Eric's Knobs" on the secondary mirror adjustment.
I wanted the focusser at about 45 degrees to the axis of the OTA. Thanks to advice on the Forum, that was easily done - undo two grub screws and the focusser comes off the base. Relocate and tighten the screws. Interesting, a lot of stuff has to be held firm by those two grubscrews when the focusser is loaded up.
I find that, as I approach the zenith, I need to move a counterweight from the top side to underneath the OTA - of course, it slides down. (If I have it on the top side, I just place it above the finderscope holder to stop it sliding.) So, I mounted a small stop under the top of the OTA to stop the speaker magnet sliding down. Used a piece of hardwood, painted Bintel grey, suitably rasped to the curvature and with a slight angle to catch the magnet. This is the first time I took a drill to the OTA itself, and probably not the last. Was careful to catch all the drill swarf. It turned out that I had to remove the spider and secondary to screw the S/S self-tappers into the wood. No problem, but I wondered how well I'd be able to realign the secondary. It did seem to go back OK, though the mirror had rotated, I seemed to be able to return it to position easily and recollimate.
Lastly, finished off my knobs for the secondary mirror tilt. I'd grown sick of the Philips head screws, so found some long M4 bolts (cabinet handle fittings are M4), found some of the wooden plugs that fit into screw holes on furniture, drilled through these, added a few nuts and washers and I had Eric's Knobs. Only trouble, they very slightly protruded into the light path, so I filed flutes into the wood to significantly reduce this, with the added advantage of increasing the grip, gave them a paint and reinstalled and rechecked and reset collimation.
OK, I need a careful re-collimation when I have time - particularly to set the secondary mirror correctly under the focuser. I want to put in the white cardboard blocking the primary mirror reflection to do this properly.
Then roadtest at the first opportunity. Saturday night would have been brilliant, but I collapsed feeling dreadful and have struggled through today. I can feel a day's sick leave coming on for tomorrow - the cough is developing now
Next big job, change from left-handed to right-handed by relocating the focusser to the other side. Sometime in future, hopefully with a 10:1 focusser.
Congratulations if you have read this far. Thanks for persisting. Headcold permitting, I had fun.
