doug_parkes
23-05-2010, 10:40 AM
hi all,
I recently discovered a problem with the optical tube of my C9.25: the tube was moving in relation to the primary mirror cell! Further inspection revealed that two of the four attachment screws were loose, allowing 5 mm movement between the tube and the cell.
In the C9.25 CPC series (and probably in other Celestron Schmidt-Cass models) the optical tube is held to the primary mirror cell by four countersunk-head screws M4 by 9.7 mm and fixed with plain nuts??? To be fair to the designers of the CPC, the loose nuts do provide some degree of locking when they are done up tight.
I found two of the four nuts loose and I determined to replace the screws with fractionally longer ones and fit proper self-locking nuts. Two of the four screw heads are hard to access due to the method of fixing the primary mirror cell to the supporting forks. A degree of disassembly of the OTA is required if all screws are to be replaced/tightened.
I obtained some M4 countersunk-head screws 15.8 mm long and shortened them (basically: screw the screw fully into a M4 die; cut off the excess length; file the cut smooth; file a chamfer on the end; and withdraw the screw from the die) so that only 6 mm protruded from the hole when the screw head was seated in the cell. I then fitted a self-locking nut (1 mm of thread extended beyond the nut). This setup provided a generous clearance between the end of the screw thread and the primary mirror.
The optical tube is held to the cell by four M4 screws/nuts and the corrector plate cell is held to the optical tube by three M4 screws/nuts. I replaced the corrector cell screws/nuts as well.
The optical tube is now securely fitted to the primary cell and the corrector cell.
I have attached a .doc file that will assist when collimating a Celestron Schmidt-Cass.
I found that Bob's Knobs were unsuitable on a C9.25 but they worked well in my C8.
Collimation tip: I found that a screw driver easily slipped out of the Philips head screws during collimation (of the secondary mirror) so I replaced the M3 x 10 mm Philips Head screws with Allen-socket-head screws. Using an Allen key for collimation gives more positive control of screw positioning.
Doug http://www.iceinspace.com.au/forum/../vbiis/images/smilies/happy19.gif
I recently discovered a problem with the optical tube of my C9.25: the tube was moving in relation to the primary mirror cell! Further inspection revealed that two of the four attachment screws were loose, allowing 5 mm movement between the tube and the cell.
In the C9.25 CPC series (and probably in other Celestron Schmidt-Cass models) the optical tube is held to the primary mirror cell by four countersunk-head screws M4 by 9.7 mm and fixed with plain nuts??? To be fair to the designers of the CPC, the loose nuts do provide some degree of locking when they are done up tight.
I found two of the four nuts loose and I determined to replace the screws with fractionally longer ones and fit proper self-locking nuts. Two of the four screw heads are hard to access due to the method of fixing the primary mirror cell to the supporting forks. A degree of disassembly of the OTA is required if all screws are to be replaced/tightened.
I obtained some M4 countersunk-head screws 15.8 mm long and shortened them (basically: screw the screw fully into a M4 die; cut off the excess length; file the cut smooth; file a chamfer on the end; and withdraw the screw from the die) so that only 6 mm protruded from the hole when the screw head was seated in the cell. I then fitted a self-locking nut (1 mm of thread extended beyond the nut). This setup provided a generous clearance between the end of the screw thread and the primary mirror.
The optical tube is held to the cell by four M4 screws/nuts and the corrector plate cell is held to the optical tube by three M4 screws/nuts. I replaced the corrector cell screws/nuts as well.
The optical tube is now securely fitted to the primary cell and the corrector cell.
I have attached a .doc file that will assist when collimating a Celestron Schmidt-Cass.
I found that Bob's Knobs were unsuitable on a C9.25 but they worked well in my C8.
Collimation tip: I found that a screw driver easily slipped out of the Philips head screws during collimation (of the secondary mirror) so I replaced the M3 x 10 mm Philips Head screws with Allen-socket-head screws. Using an Allen key for collimation gives more positive control of screw positioning.
Doug http://www.iceinspace.com.au/forum/../vbiis/images/smilies/happy19.gif