Collimation bolt positioning

[John]

My Orion Optics 10" F/4.8 Europa has OO's standard 3-point adjustment mirror cell. It's rather basic but does get the job done and holds collimation pretty well all things considered.

With the design of this mirror cell, the collimation bolts are positioned close to the centre of the cell / mirror - around 50mm out from the centre in fact.

When I've looked at the design of the Skywatcher 10" mirror cell I notice that the collimation bolts there are right out on the perimeter of the cell, at the edge of the mirror, 125mm out from the centre.

Like most things with astro scopes I'm assuming that the positioning of the collimation bolts along the radius of the mirror has some significance but I can't figure out what it is - I'm wondering if collimation bolts positioned closer to the centre of the mirror allow finer adjustment for each turn of the screw wheras those positioned at the mirror rim produce slightly coarser adjustment for the same amount of screw turn. Or maybe the other way around ?. Or am I "barking up the wrong tree" with this ?.

[astromerlin]

It really depends on the pitch of the threads used. Given the same pitch screws in each case the SW adjustment would be finer.

[barkis]

The mirror sits in a cell which is it's actual support, and it is the cell that is moved during collimation.

Large mirrors are usually supported at nine points, distributed so as to support the mirror equally over it's area.

Smaller, up to 200mm, will probably have three support points, equispaced about 70% out from the centre.

Ron.

[John]

The mirror sits in a cell which is it's actual support, and it is the cell that is moved during collimation.

Large mirrors are usually supported at nine points, distributed so as to support the mirror equally over it's area.

Smaller, up to 200mm, will probably have three support points, equispaced about 70% out from the centre.

Ron.

Thanks Ron

I was just wondering about the significance (if there is any) of where the collimation bolts are positioned in relation to the mirror / cell. Those bolts are either connected to the cell or push against the cell depending on the design.

From astromerlin's post I gather, assuming that the thread pitch is the same on all bolts concerned, perimeter positioning gives slightly finer adjustment.

I guess what I'm getting at is that I've noticed that during the final stages of collimation, that is where you move from well collimated to spot on collimation, you need to apply only the tinest adjustments to the tilt of the primary mirror and at that stage the ability to perform really fine adjustment becomes important I feel. Some mirror cell designs might be better suited to achieving this.

[Bizibilder]

Most cell bolts will have a metric thread pitch of either 1mm or 1.25mm ie one turn advances the bolt by that amount.

So, in the case if 1mm pitch, turning the bolt 1/4 turn means 0.25mm and 1/8 turn (easy enough to estimate) moves the bolt 0.125mm (or, for us old folk, very roughly 10 thou and 5 thou respectivly - a piece of ordinary A4 photocopier paper (80gsm) is around 3.5 thou for reference).

All in all a pretty fine adjustment!!