Mirror centre spotting and collimation screws questions.

[cantab]

I had the mirror out of my scope for inspection and centre spotting. It looked like a dog's breakfast at the bottom of the tube but up close all looks fine.

First question: I've blown off the loose dust, it's still a bit grubby but I'm minded not to touch it further. However is it OK to centre spot it with a ring reinforcer without cleaning it first?

Second question: There are three pairs of screws at the back of the scope. In each pair, one goes through a smooth hole in the mirror cell and into a tapped hole in a bracket on the inside of the tube - these screws had to be undone to release the mirror. The other goes through a tapped hole in the mirror cell and bears against a flat part of the bracket. Now which is the collimation screw, and which is the locking screw?

[Moonshane]

I'd probably just add the circle without cleaning. but if cleaning just do the whole mirror.

I think the latter bolts are the locking ones and the former the collimation bolts (I'd take out the locking ones to be honest). some pics would help.

[AstroTux]

Not quite sure what you have there as you only have three pairs of screws. On the Celestron AstroMaster 130 which may be similar, there are three sets of three screws....

1 of each set keeps the mirror cell in the back of the OTA ( sounds like the ones you unscrewed to remove the cell ). These are small black cross-head screws.

1 of each set is the collimation srew. These are large silver screws with a knurled top so they can be turned by hand, and may also be turned using an Allen Key.

The last screw of each set is a mid-sized silver screw with a cross-head top. These are the locking bolts.

I don't know if your setup is the same, but I hope this helps........

[Moonshane]

some cells have one collimation screw that pulls and one that pushes (and then one that locks). I am not keep on these but they may possibly hold collimation better once done.

[cantab]

Here's a snap of the mirror in its cell

The longer screw on the left is just sitting loosely in a hole here, and screws into the bracket on the inside of the tube. The shorter screw screws into the mirror cell. Both have the same cross heads.

I think Moonshane is right that collimation must be affected with the same longer screws that hold the mirror in, while the shorter ones serve to stop it shifting forwards if the scope's tilted down.

Also you can get some idea of the cleanliness of the mirror, though I'd say it looks worse in the snap than in real life.