Six Inch Diagonal Mirror and Mount Testing Updates

We are approaching star testing time with steady progress on the secondary (9.5-inch convex hyperboloid), so it was time to purchase the six-inch diagonal tertiary mirror. Discovery Optics delivered three weeks early (love it!). It just arrived today, so we haven’t had a chance to test it, but here’s a look.


Diagonal mirror looking across the minor axis

Six inch diagonal mirror

Meanwhile, the mount testing is a start-stop thing. With the encoders all in place, we have the computer software installed for the SiTech controller and all the wiring in place for the first time. A few weeks ago we spent an evening setting the number of ticks per revolution on the altitude and azimuth axes. This turns out to be a simple, four step process on each axis: set a starting point; command a move to a known angle distance away that you can measure; when it doesn’t move there, move it with the handpad and click the next button to capture that encoder count; command a move back to start; adjust by handpad to get it right back to the perfect start and click the button to capture that encoder count. I think that was five steps.

The simple part was clicking the buttons, so that was my job.  The complicated part was getting the mount to repeat the cycle after a number of iterations so that we don’t have to make any handpad adjustments.  We got kinda close on the altitude axis and then decided that was good enough for that night. The mount was consistently wrong.  The azimuth was a pleasant surprise – we only did three iterations before we found it not needing more than a tiny nudge to register it.

The moment of truth had arrived!  We pointed the scope at a star and synced it in Sky6. Then we began a move to the second star. Altitude was doing just fine, but azimuth? It stopped. Zero motion in either direction. We could hear the motor spinning, but no go. Sigh. This was remarkably like the torque problem we saw a year ago, except up until the failure, it was pushing the scope around with no problem.

We yanked the az drive from the mount and benched it the next day.  In the process we ripped off the jack for the control cable.

Damaged RJ45 Jack

Once the replacement jack was ordered and repaired, and the cables repaired, the motor still would not turn the gear train. We removed the Bartels gear assembly and once open, it was clear what was wrong. The spur gear on the end of the servo was sitting, precariously, on the very end of the shaft, spinning, making no contact whatsoever with the main gear. No teeth, no torque.

It must have been loose, right? After a search at the local hardware store, found the 0.050 allen wrench and gave it a good crank. Reassembled it all and with everything working, finished assembling new cable arrangements so that everything just unplugs at the bottom and the top of the telescope base. That way when we have to service the azimuth drive again, we won’t have to (ahem) cut the wire to get it out.

Test 2. Larry and Mark made adjustments to the mount to make it run smoother while the repair work was in progress. Got the drive reinstalled with the new cables – yes, Mark has incriminating photos of a badly soldered connector that had to be fixed on the spot. Then we learned a couple of things about the altitude drive.  The first thing was that if we don’t keep the timing belt tight, the mount gets to a certain point where you hear an audible thud and you see a hesitation. Investigating, we found the timing belt slipping a cog. Crank the tighten-down screw a couple of revolutions and we’re good to go.  Second thing was that no matter how we tried, we were getting deviations at start and stop of tick determination. On a hunch, we checked balance and it turns out that for an alt-azimuth scope, the altitude has two balance adjustments to make – north of 45° elevation and south of 45° elevation. Should have guessed it – after all, we’d placed that weighty 2″ refractor on the back of the 300 pound mount – what were we thinking? Nothing that a couple of well placed scrap boards secured with C-clamps couldn’t balance out to get near-perfect tick counts at top and bottom. And the azimuth was near-perfect again. Rolled it out, started the moves to sync it to some stars and bam – the az drive stopped again. We pulled it, tried to install it so there was more shaft engaged by the spur gear, but it just jammed (set screw encountering gear – not good). Bench time again.

Consulting with the altaz group, decided to use a longer set screw and drill a small detent in the flat of the shaft. Seems to fit snugly in place. Should be just the ticket. Stay tuned – next test run is a few days from now!


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