Michael A. Covington    Michael A. Covington, Ph.D.
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AVX declination axis freezes

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2017
November
8-13

Celestron AVX declination axis freezes
when installing saddle or at other times

The other day I installed an ADM dual saddle on my Celestron Advanced VX (AVX) equatorial mount and encountered a strange symptom: When I fully tightened the 4 screws under the saddle, the declination axis became extremely stiff or completely unmovable. Normally it is supposed to be about 2 to 3 times as stiff as that of a CGEM because it doesn't have the same type of bearings. But not frozen stiff!

First I checked the plastic ring that serves as a bearing at the top of the declination axis (to keep the saddle, which moves, from contacting the outer casting, which doesn't). It was there, doing its job. See Don Halter's posting on Cloudy Nights; he had the same problem and attributed it to excessive thickness of the plastic ring. In his case, removing the plastic ring cured it. Not in mine.

But then I thought about it a while. What if the essence of the problem is that the whole declination axis is sitting a bit too low in the outer casting? By "low" I mean "away from the saddle." Then tightening down the saddle would compress the plastic ring and keep it from working properly as a bearing.

I flipped the mount head upside down, removed the counterweight bar, and had a look. There, a retaining ring (tensioner) holds the declination axis in place. The retaining ring is secured to the declination axis by two set screws so that you can tighten it just enough and then lock it in place.

Well, it was too tight. The set screws had come loose, allowing the retaining ring to rotate freely. Then, whenever I tightened the counterweight bar lock, I was also tightening the retaining ring. Miraculously, I didn't tighten it enough to make the mount freeze — until I replaced the saddle and took up all the slack at the other end of the axis.

Cure?

Step 1: Make sure both set screws are loose (mine already were). There are two, opposite each other. Use a 1.5-mm hex wrench.

Step 2: Loosen the retaining ring, then re-tighten it moderately tight, not super-tight.

Step 3: Re-tighten the set screws, good and tight. They have a job to do.

Step 4: Add a washer and a bit of grease so the counterweight bar lock can no longer directly apply torque to the retaining ring.

The washer I used was this one, from Home Depot:

I had to use sandpaper to enlarge the central hole a tiny bit; maybe it had a burr, or maybe a good 26 mm of diameter was actually needed. Someone who knows the market for industrial hardware better than I do can probably find a better one, maybe even Teflon. They are called thrust washers or spacer bushings. This Amazon search turns up several.

2017
November
1-7

Please pardon my absence...

Nothing is wrong, but I'm busy writing the second edition of Digital SLR Astrophotography, and doing other important work, rather than writing Notebook entries.



An intelligence test

Here is something I posted on Facebook several days before the Texas massacre, all the more relevant in the discussions that are now necessary.

Why is it so impossible for some people to rationally consider the following two things at once?

(1) Guns don't kill people by themselves; people kill people.

(2) Guns, especially certain types of guns, make it appreciably easier for people to kill people.

Plenty of people, both those who want to decrease gun control and those who want to increase it, do understand both points. From there it's a matter of gathering facts about where the main hazards lie and how to mitigate them. I can have worthwhile conversations with everyone in that set.

But I have been running into people who find it just impossible to consider items (1) and (2) at the same time. It's as if I were asking them to calculate a 4-dimensional matrix rotation in their heads. They get angry and spout illogical nonsense.

Or else they feel they are forbidden to admit that one of them is true (usually the second one), that if they do, they're surrendering to the opposing side, as if logic were a war or a football game, where if there are two propositions, they cannot both be true; one has to win and the other has to lose.

A related point: Annoyingly, some people wade into any discussion of gun control and start railing loudly against some unreasonable position that no one there has actually advocated. Why? To save themselves the trouble of listening and reasoning?


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