Mirror cell doubts

[Sagitta]

Question for the more experienced builders here. It's my first mirrorcell...

I have designed and built my telescope's around a mirror cell and mirror
(and secondary and holder), made by a friend of mine (see picture 1).

Now I'm about to make a totally new mirrorcell with triangular bases, Delrin
support knobs (a bit small at 16mm in diameter but I think this will do)
I read Mike Lockwood did not test differences when using smaller than his
recommended size 3/4" - 1" (~20-25mm).
I intend to make a 180° stainless steel sling support and also whiffletree supports
(to compare the two systems in performance). I can just remove the sling then,
and add the wheels to the whiffletree bases which sit about the same distance from
the mirror as the thickness of the stainless steel wire.
Naturally I will have the whiffletree wheels protrude from the bases by the same amount.
If the cable proves better, the wheelless whiffletree bases will serve for protection in case
my sling would get loose or snap (highly unlikely but .. well you never know).

When thinking about collimation, I figured I could make it easier maybe by having
one of the collimation points/bolts in the primary 'opposite' the one for the secondary
closest to the focuser. This way tilting the diagonal away from or towards the focuser
and tilting the primary and secondary towards each other should make collimation more
straightforward (green line of sight in the top-view of the sketch).
So far I've had no regrets and I want to keep it this way. Also, I don't have the option
to drill new collimation-bolt holes in the fixed part of the mirrorcell since I made holes
for ventilation all over and I don't intend to make this part again (see picture 2).

My current cell and the one I'm about to make moves as a single unit when collimating.
The primary does not move in the cell through collimation.

For the new mirrorcell, I think I have two options now.

1. I use the conventional horizontal triangle-support bars (red lines in the sketch) and
so only one of the collimation bolts will be roughly under one of the triangle-support
bars (remember the bolts do not move the triangle-supports anyway).

2. I make a cell with vertical beams for the triangle-support bars. My only worry here is cog
issues.. would it matter that two of the triangle-support beams are at one side of the mirror
instead of two on top and one at the bottom (it's like this with the current cell but I figured
it might be more of an issue with a sling support).

In the sketch, the collimation points have been designated A, B, C (-1 for the secondary mirrof, -2 for the primary mirror).

The 'conventional' versus 'mine' sketches' viewpoint is from behind the primary mirror, looking through its cell.

 

 

[Chriske]

This cell is very badly designed....
Download PLOP and you'll see why.

[Peter Drew]

I have a 17" Dobsonian, the mirror "cell" consists of a backplate fitted to the full tube, a counterplate that is adjustable for collimating the mirror and the mirror support is solely a disc of heavy duty plastic bubble material. The originator of PLOP would have a fit but it has worked well for years with no problems for visual purposes.   ? 

[Sagitta]

The cell works very good actually, but I want to make a possibly better one. I used plop to design the new cell. Of tremendous help also was

http://www.loptics.com/articles/mirrorsupport/mirrorsupport.html

and

http://www.cruxis.com/scope/mirroredgecalculator.htm

Everything's been taken care of. I'm just having some doubts yet for the orientation of the triangle bases in relation to the positions of the collimation bolts (see 'my two options').

This is the interpretation of Plop's results (I don't do 3d design so everything goes on paper).

 

[Sagitta]

14 minutes ago, Peter Drew said:

I have a 17" Dobsonian, the mirror "cell" consists of a backplate fitted to the full tube, a counterplate that is adjustable for collimating the mirror and the mirror support is solely a disc of heavy duty plastic bubble material. The originator of PLOP would have a fit but it has worked well for years with no problems for visual purposes.   ? 

Where does the mirror rest on at the bottom?

[Peter Drew]

The sequence is mirror-plastic bubble disc-mirror counterplate (10mm aluminium)- tube backplate (10mm aluminium.)   The collimating screws are threaded into the backplate and press against the counterplate, a central screw holding the counterplate to the backplate holds the two together ?