12 inch Dob collimation?

[doug_parkes]

hi Ice fellow Ice in Space members,

my brother bought a 12 inch Dob (it looks Taiwanese but I don't know the name/model/series information, unfortunately) and has had a lot of trouble collimating it.

I had a look at it (I have some experience with Dobs, having built a 381mm f5 Kriege & Berry Dob some years ago) and can see what he is getting at!

The primary mirror cell has three collimation screws and springs which screw up into threaded bosses within the movable section of the cell. But, 30° removed from each collimation screws are fitted locking screws (three in total).

How do you collimate this model?

The springs are just adequate to support the weight of the mirror but during collimation attempts the spring tension is sufficient to lift the mirror and ruin what collimation one has achieved? The instructions (which I have not seen) apparently advise the operator to back off the three locking screws and then simply work with the spring laden adjustment screws. Fair enough but as soon as one collimation screw is loosened the spring pushes the mirror up.

For collimation tools I use: a standard wire-crosshair Cheshire; a home-made 1.25" laser collimator; a 2" Kendrick laser collimator (with a 45° flat protruding into the tube and it also has a small removable built-in Barlow for the Barlowed-laser method); a star test when observing.

I also fit a small webcam to my computer and position the camera (by clamping it to one of the secondary spider vanes) so that I can see the returned laser spot projected onto the 45° flat on the end of the 2" laser collimator. It works really well. Collimation can be done by one person sitting at the primary mirror end, watching the red spot's progress on a laptop computer screen. I also collimate my Society's 400 mm Dob that way (with its 2511 mm focal length), sitting at the back of the mirror cell watching the laptop screen.

I would appreciate any tips on how to collimated the "six screw" Dob.

Doug http://www.iceinspace.com.au/forum/../vbiis/images/smilies/shrugging.gif

[monoxide]

the 12" gso has weak springs too, i just roughly collimate using the collimation screws then re-collimate with the locking screws.
its a pain but it holds..
failing that you could replace the springs with some heavy duty ones from your local hardware store and do away with the locking screws

[erick]

Yes, replace the springs, one at a time, with a suitable stronger spring from hardware shop. Use the "locking screws" for transportation only. Loosen them off, then collimate with the three big screws. Ignore the "lockers" until it's time to pack up - then nip them up. Bounce the 12" tube for kms in the trailer and back out again next week, loosen off the "lockers", and you'll find collimation of primary has hardly shifted - that is my experience. The previous owner of this scope believed in pulling the primary well down on the springs to put them under significant compression, thereby reducing the possibility of movement.

Adding a stronger spring to the secondary mirror centre bolt is another good move. Collimation of my 12" survives many km of transport and handling with little change.