How tight should the retaining collar be on a SW conical mirror?

[tomato]

When we dismantled my SW 16” Dob primary mirror assembly, the threaded retaining collar was quite loose, you could waggle the mirror from side to side. I’m guessing the collar should be tight enough to eliminate any play but no more, as this would put the glass under tension, definitely not a good idea.

[Giles_B]

I don't own a SW dob - I own an Orion Optics one which I have dismantled a few times - I'm struggling to understand what the retaining collar is on your OTA.

This could just be differences in build of the telescopes but it would help if you could you send a picture or link.

If it is a part of the mirror cell, there is a small amount of play in the retaining pieces that hold the mirror in place - they hold the mirror loosely in place, and a bit of side to side movement was possible. I understand this is necessary for thermal expansion of the glass, which would fit your conjecture.

[tomato]

The larger Sky Watcher Dob mirrors have a ribbed, conical back, with a central coarse threaded boss. This is used to secure the mirror to an alloy support frame, this is what the collimation screws work against. Presumably SW came up with this design to save weight and manufacturing costs. I’ll see if I can post some photos.

[tomato]

Here is a photo of the cell arrangement and back of the mirror. The conical shape and ribbed structure are visible. The central threaded collar is also visible, the cover plate with the two small screws is, I think, a safety feature to prevent the collar unscrewing and falling off. If it did, there would be nothing preventing the mirror from dropping into the tube if the scope was placed below the horizontal.

[Tomatobro]

Like this

[bosun21]

From what I have read it should only be tightened until sufficient force is subjected onto the individual collimation springs. It shouldn't be loose. I'll try and find the article in which I read it 

Edited May 14 by bosun21