Skywatcher 16" Dobsonian mirror cell design

[tomato]

Does anybody have any inside knowledge on SkyWatcher's mirror cell design on their large Dobsonians? After reading a bit on constructing a mirror cell for large Dobsonians, SW seem to buck the accepted thinking by only having a single central support point, presumably due to their mirror having a weight saving ribbed conical design. 

I am planning to transplant this cell into a light weight compact mirror box design but it will certainly be a big deviation from the multi point Wiffletree approach. Having said that, there are a lot of large aperture SW Dobs out there and I haven't found any references to owners  bemoaning the mirror cell and embarking on modifications, I have certainly been satisfied with the cell's performance with regard to holding collimation etc.

[Mr H in Yorkshire]

It looks like a very minimalist design, surely a lot of engineering cleverness must have gone into it. As they say, if it ain't broke...

Edited December 22, 2023 by Mr H in Yorkshire

[tomato]

1 hour ago, Mr H in Yorkshire said:

It looks like a very minimalist design, surely a lot of engineering cleverness must have gone into it. As they say, if it ain't broke...

Fair comment. The mirror does not have a flat underside so a conventional multi point cell would not work anyway. I think we will just transplant it as is. I am intrigued to see what lies beneath that central cover plate though.

[Tomatobro]

Skywatchers manual dated 24/04/2012 shows a slightly different cell configuration to the one in the picture. There are three extra support tabs that come in from the sides.

[tomato]

Ok, that photo is not of my scope, I need to check what is there.

[Mr H in Yorkshire]

3 hours ago, tomato said:

I am intrigued to see what lies beneath that central cover plate though.

My bet: the mirror appears to be attached only at the centre - so some sort of expanding sleeve or other device to to hold the mirror securely, collimation is obviously by the three peripheral push/pull units that move the whole cell. That's an interesting mirror casting, I have an ancient Russian Mak MK65 with a shaped mirror (thinned towards the periphery) but I've never seen cast ribs like that, any idea what the material is? I only ask because reading around most attempts at ribbing result in 'print through' effects when grinding the mirror. I was just thinking, imagine if that was a traditional full thickness mirror, trying to make a lightweight version would be a lost cause, in fact you'd probably get a hernia using it!

[tomato]

There is a comment in the description of the mirrors on the larger SW stargate dobsonians that the mirrors have fused ribs rather than glued, I’m not sure if this innovation is age or size related.

I can see why a full thickness mirror of this size or bigger would need a 27 point cell, it would appear that these mirrors can successfully use a single central support and hence it must have been quite a breakthrough in mirror manufacture. I’m surprised I haven’t read more about it.

[Tomatobro]

This is from the SW manual

[Peter Drew]

Schmidt-Cassegrain mirrors are single point central mounted with no apparent distortion problems.  My very thin 30" mirror of conventional profile is centrally mounted via a 3" core.  It rests on a 30" disc of heavy duty bubble pack having 1/2" cylindrical bubbles that cannot be "popped" by hand providing in the order of 750 suspension points. The backplate stub that located the mirror was undersize and a peripheral strap was adjusted to lift the mirror clear of the stub.  Initial astigmatism was cured by wrapping insulation tape round the stub to make a good fit on the core diameter and dispensing with the strap.   I made a few 10" Newtonians with conical profiles ground on, cored and centrally mounted like a SCT with no issues.  Or retaining clips.        🙂