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Spider vanes tightening and focuser issues

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#1 precaud

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Posted 19 January 2013 - 11:59 AM

My AD10 still hasn't seen first light due to focuser alignment issues. I got a replacement focuser and will do that today. While examining the mounting area prior to removing the old one, I noticed quite a difference in the gaps between the focuser base and the OTA. Big gap (about 1/8") on the front side and hardly any gap on the back side. The tube is being flattened by the front ring mounting screw and especially the spider vane tightening screw. This, in turn, is throwing off the focuser axis alignment. Here's the pic from the front.

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#2 precaud

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Posted 19 January 2013 - 12:01 PM

And here's the gap (or lack of gap) on the back side.

When I got the scope, the first thing I did was go around and snug up all screws/nuts. And, after reading in here that the spider vanes should be "as tight as possible", and "if you have to ask if they're tight enough, then they're not", I hand-tightened all four of them as much as I could.

I did the same to my XT6 when I got it and saw no tube deformation. The smaller tube diameter can obviously take more surface tension.

Interestingly, tube is not deformed as much on the opposite side, which suggests that the focuser hole weakens the tube's ability to hold its shape there.

So the question is - did I go too far? What's the "right" way to deal with this?

I thought of a large washer, but it would have to be big and conform to the tube curvature to be effective.

Any input would be appreciated.

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#3 dpwoos

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Posted 19 January 2013 - 01:10 PM

When I got the scope, the first thing I did was go around and snug up all screws/nuts. And, after reading in here that the spider vanes should be "as tight as possible", and "if you have to ask if they're tight enough, then they're not", I hand-tightened all four of them as much as I could.


Not all of the advice you read here is good. Don't tighten anything on your scope to the point that it causes deformation, including the spider.

#4 precaud

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Posted 19 January 2013 - 01:43 PM

Apparently so. Noone challenged the point in that thread, so I assumed everyone agreed.

I just replaced the focuser and it solved the problems. Incidentally, the front gap with the new focuser is half or less. So even the base castings have variability.

Since I had two of them here, I also compared the 2"-to-1.25" adapters, watching the 2ndary image while rotating the adapter with a sight tube in place. The replacement one was definitely out of round. The old one was right on. Guess I'll keep the old one... :)

So there is definitely manufacturing tolerances involved in these units, and if the errors all "bunch up" in the same direction, it can throw things out of whack and make the secondary alignment impossible.

Maybe I'll finally be able to get some light into this thing tonight!






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