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New Dob's Mirror Cell Springs Are Too Short?

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#1 Ty Williams

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Posted 11 December 2012 - 12:25 AM

As a Christmas present, my wife got a new Astro-Tech AT10D 10" dob. This is the scope that's sold in a variety of places under the Astro-Tech, Zhumell, and Aperatura brand names. All the same scope.

Anyway, I tried to collimate it for her tonight and I'm having some problems with the primary mirror cell. If you're not familiar with it, the cell has the typical 3 knobs to adjust the tilt of the primary, but the 3 "lock" knobs are half-way between the primary collimation knobs. So they don't really function like lock knobs, they function like an additional 3 collimation knobs. Googling, I found that a lot of people don't like this because it's very difficult to try to get 6 knobs balanced and into collimation without introducing warping stress into the cell. Removing the 3 "lock" knobs seemed to be the most common resolution. However, I ran into trouble trying to implement that.

I removed the 3 "lock" knobs and then tightened the 3 collimation knobs to fully compress the cell's springs. I then worked 2 of the knobs in a loosening direction to try to obtain collimation. At first, turning a knob resulted in changing the collimation. Then it suddenly stopped working. Turns out that the springs had reached full extension and the cell was no longer under the authority of the knob, it just sat there not moving. I assumed I had done something wrong, so I re-tightened and started from scratch again. Same result. Turns out that the springs have only 4-5mm of expansion from fully compressed to fully extended and one of the collimation knobs needs to move about 8mm while the other needs to move about 6mm.

Is this indicative of some other problem? Do I just need to get longer springs? If so, where do high quality longer springs come from? I assume if the spring rate is too low, the primary will sag into them as the altitude is changed.

How do I resolve this?


Thanks,
Ty

#2 kfiscus

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Posted 11 December 2012 - 01:00 AM

A hardware store should have a selection of springs to check out. Or you can go to the Zhumell mega-mod thread here in the Reflectors Forum if you dare... besides discussions on springs, places like ScopeStuff having them, you'll see all sorts of upgrades to think about or avoid.

#3 dave brock

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Posted 11 December 2012 - 01:13 AM

Changing the springs is the obvious solution but failing that you could add some washers to the springs that need the extra length.
Having to move 8mm and 6mm compared to the third seems an awful lot. Is the cell fitted correctly (I'm not familiar with the scope model btw)?

Dave

#4 ThreeD

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Posted 11 December 2012 - 02:35 AM

Having to move 8mm and 6mm compared to the third seems an awful lot. Is the cell fitted correctly (I'm not familiar with the scope model btw)?

Dave

8mm may seem like a lot but it is only tilting that size of a mirror/cell less than 2° (and it seems it would be a lot more).

#5 D_talley

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Posted 11 December 2012 - 03:39 AM

I had the same problem with the AT12IN. Springs were too short. I posted a fix in this forum and it seems to work.

Sorry but a long post, check at the bottom of first page:

Spring Update

#6 Damo636

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Posted 11 December 2012 - 09:05 AM

Pick up a set of these.....

http://agenaastro.co...obs-mlbspr.html

The supplied springs on the GSO scopes are hopelessly light. I have removed the locking screws from my 12" altogether since fitting these.

#7 Jon Isaacs

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Posted 11 December 2012 - 04:40 PM

I removed the 3 "lock" knobs and then tightened the 3 collimation knobs to fully compress the cell's springs. I then worked 2 of the knobs in a loosening direction to try to obtain collimation. At first, turning a knob resulted in changing the collimation.



You do want to use only two of the screws to adjust the collimation, that way it prevents you from slowly walking the mirror down the scope. Only two are necessary.

But you should not start with them fully tightened, you should start with them about in the middle of their travel, that way adjustment is possible in all directions, with one spring fully compressed, you can only go one way with the others, they can never be shorter than the fixed spring.

Jon

#8 Ty Williams

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Posted 11 December 2012 - 04:54 PM

Jon, there's no what that what you're suggesting is possible with this scope. There's not enough travel in the springs even with one of them coil bound, let alone with that one starting in the middle of its travel.

#9 Jon Isaacs

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Posted 12 December 2012 - 04:54 AM

Jon, there's no what that what you're suggesting is possible with this scope. There's not enough travel in the springs even with one of them coil bound, let alone with that one starting in the middle of its travel.


How much travel is there between coil bound and full extension? How are you collimating your scope?

Jon

#10 Ty Williams

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Posted 12 December 2012 - 05:01 AM

Jon, there's no what that what you're suggesting is possible with this scope. There's not enough travel in the springs even with one of them coil bound, let alone with that one starting in the middle of its travel.


How much travel is there between coil bound and full extension? How are you collimating your scope?

Jon

There's only 4-5mm of travel between coil bind and the springs rattling loose. 1-2mm before they become loose, they begin to allow the mirror cell to float into a different position depending on altitude.

Collimating using Jim Fly's stuff.

#11 Starman1

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Posted 12 December 2012 - 12:02 PM

It's common in these scopes because they think that collimation will be done as much with the locking screws as with the main knobs.
You definitely need to replace the springs with those from FarPoint, Bob's Knobs, ScopeStuff, etc. You can dispense completely with the locking screws if the springs are strong enough. I used 250# springs on my 12.5", and the held the mirror cell quite nicely.
Depending on how long your collimation knob bolts are, you may be confined to springs of 1/2" length or so. A local hardware store can provide some decent springs for that purpose.
When space is at a premium but you need strong springs, go with Belleville springs. You can get a hundred + pounds of support in less than 1/4". You can find threads about them here on CN.
I used springs of about 1/2" length when fully compressed, 1" when not compressed. The coil wire appeared to be about 2mm or a little more in diameter, spring O.D. was about 1/2". Stiff springs!






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