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Anyone else notice this on a 8" GSO Dob?

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#1 howard929

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Posted 30 November 2012 - 12:08 PM

On mine, there's 3 primary mirror clips located at 6, 10 and 2 o'clock. The cell is keyed to the OTA. When I loosened the clips to accept a credit card, the mirror would clink back and forth as it teetered on the clip located at 6 o'clock.

Anyone have one of these scopes and notice this?

#2 ThreeD

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Posted 30 November 2012 - 12:41 PM

So are you saying that the edge support for the mirror is actually the clip at 6:00? Usually the clips themselves do nothing when the scope is in normal use and only come into play if the OTA is pointed near or below the horizon. If the scope if viewing at the typical viewing altitudes (and even far below what would be considered normal) the mirror will be resting on the back and on the lower edge support.

Could it be that the clink you are hearing is the mirror sliding from one side to the other and not forward/back? If the edge support is at 6:00 then it would make sense that the mirror moves slightly to one side or the other to find a second edge support.

In my XT8i, which is of course isn't GSO, the back of the mirror is actually stuck to the back of the cell by three small (~3/8" diameter) dobs of silicon. With a small mirror like these 8" scopes have this doesn't cause any problems and should solve the problem I suspect you are experiencing.

#3 tag1260

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Posted 30 November 2012 - 01:26 PM

My 12" is somewhat the same way but the clips are at about 7, 11 and 3 o'clock. I'm now modifying my cell to give better support. (Or at least I think I am) I don't think these cells were the best thought out as it really wouldn't have taken much to rectify some of their flaws.

#4 Gastrol

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Posted 30 November 2012 - 01:36 PM

On my Lightbridge 12, also a GSO, I believe I'm able to change orientation (rotate) between the inner cell and the outer cell (ring) which is keyed to the OTA at the seam. I'll take a closer look later as I have my scope apart at the moment.

#5 simpleisbetter

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Posted 30 November 2012 - 01:56 PM

My old 12" GSO also had the mirror tabs at around 3, 7, and 11 o'clock. Easiest fix is you should be able to just remove the inner cell and rotate 120* wrt the outer cell, until the tabs are out of the way and your mirror doesn't "high center"?

If not, insert a pair of shims at around the 4-5 and 7-8 o'clock positions to keep the mirror in place. Just enough to build it up and keep it from shifting side to side but not enough to make the mirror tight or snug in the cell.

The other alternative if neither of these work is get a dremel tool and file down the high spot.

#6 tag1260

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Posted 30 November 2012 - 03:05 PM

That actually won't work on mine as the cell will just line up the next bracket in the same spot. Don't know if yours is the same way though.

#7 Gastrol

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Posted 30 November 2012 - 03:20 PM

The other alternative if neither of these work is get a dremel tool and file down the high spot.


Good idea, that should work. I assume you mean file down or flatten the seam on the OTA.

#8 simpleisbetter

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Posted 30 November 2012 - 03:21 PM

Okay, sounds like either shims or filing will be needed.

#9 tag1260

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Posted 30 November 2012 - 03:28 PM

Yea, mine has six spokes on the cell and on three are the adjusting screws and the other three have the brackets so turning it does nothing. I'm actually going to drill and tap two holes at 7:30 and 4:30 to mount a couple of nylon screws to support the mirror from the sides.

#10 simpleisbetter

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Posted 30 November 2012 - 03:33 PM

No need to go that far, just some thin cork, felt, or paper shims (like file card material) inserted at those positions would suffice, and should stay in place fine.






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