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Discovery Cell/Generic Cell Flaw and Fix

Started by Rod Kaufman, 11/11/2003 10:41AM
Posted 11/11/2003 10:41AM Opening Post
I have found my Discovery 12.5" mirror cell doesn't hold collimation because the springs in the mirror cell allow a small but significant amount of movement as evidenced by migration of the laser dot from the center of the doughnut to the top of the doughnut on the primary as the scope is moved from the horizontal to vertical planes. I believe virtually all such cell designs will produce this de-collimation effect when used with full-thickness mirrors. The "fix" is borrowed from an old Telescopics mirror cell-design which incorporated three locking screws with nuts that pass through the rear supporting plate to the plate that supports the mirror and are screwed-down after the wing nuts are adjusted for collimation. I drilled four holes through the back-supporting bars of the Discovery cell and passed the screws to the back of the plate that supports the mirror and tightened them after collimation. This effectively stopped the de-collimation. I believe the de-collimation problem is un-recognized by other amateurs and is probably a universal problem. I also think this explains why Ed Ting could not resolve Delta Cygni with a test 12'5" Discovery scope in the Novenber Sky and Tel review; his scope was de-collimating as he moved it but he didn't know it.
Posted 11/11/2003 11:06AM #1
Another solution is to replace springs with another material that compresses and expands but won't permit the cell plate to laterally shift or rock. I like to use rubber stoppers. I simply drill holes through the center of the stoppers for the collimation screws to pass through. They are usually 1" tall and come in various diameters. I find them at my local Lowe's Hardware.





Attached Image:

P. Lee Stock's attachment for post 73065
Posted 11/11/2003 05:40PM #2
. I believe the de-collimation problem is un-recognized by other amateurs and is probably a universal problem. I also think this explains why Ed Ting could not resolve Delta Cygni with a test 12'5" Discovery scope in the Novenber Sky and Tel review; his scope was de-collimating as he moved it but he didn't know it.
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It is pretty common for mirror cells to have locking screws. All the Asian DOBs I have seen as well as my Orion Space Probe 130ST have locking screws.

That may explan why I was able to split Delta Dygni with my OPT Starhunter 10, a Guan Sheng 10 in F5 scope. But I think seeing was more likely the reason Ed couldn't split it.

The problem with locking screws is that they themselves can affect the collimation. I normally just leave em loose for that reason.

My 12.5 inch Discovery currently has a University Optics cell, it has locking screws. I purchased a Novak cell for it. These are supposed to be the best. No locking screws but it has dual springs at each bolt that are extremely stiff and uses 3/8" diameter bolts for the adjustment. I have to bring a 9/16" wrench along now but I don't think I will needing to use it much. The cell was designed for mirrors at least 2 inches so I am going to have to adapt it for my 1.5 inch Discovery mirror.

jon