Checking and Placing a Mirror-Donut, the 3-4-5 way

[Captain Scarlet]

Those who noticed my recent “Deconstructing, Fixing and Upgrading a used OO VX8” thread may recall that one of the problems I identified with the scope, with the mirror itself, was that the centre-spot “donut” was manifestly not in the centre. This makes the scope essentially uncollimatable, or guaranteed to mis-collimate.

The method I used to establish this was to take a DSLR photograph of the mirror, face-on, with a reasonably long focal-length camera lens and the picture centred around the centre of the mirror. In Photoshop I then chose three arbitrary roughly-equally-spaced points around the edge and noted their pixel-coordinates. I used a formula to determine from those coordinates, the pixel-coordinates of the true centre of the mirror.

I’ve since worked out a less involved way of checking this, which can be used both to check the centred-ness of a spot already in place, AND to place a new spot centrally when the time comes.

A common suggestion is to trace a circle on a piece of paper or card, fold it over a couple of times, and where the folds meet, there is your centre. That’s fine, but my new method lends itself better to using graph paper, with no need to try to cut accurate circles or curves.

It turns out that a 3-4-5 triangle has a couple of really nice properties: one, it’s a right-angled triangle so its corners can be trivially marked on right-angled graph paper. And second, the centre of the circle that passes through the three points of the triangle has nice round-number coordinates too.

So a triangle that has the coordinates (0,0), (0,3) & (4,0) will have the centre of the circle at (2.0,1.5). All extremely easy to scale up, no nasty pi-related fractions of millimetres to try to judge, rather easy to mark directly onto graph-paper and cut out, and possible to check and re-attach a centre-ring without removing the mirror from its clips, as the 3-4-5 will fit inside them, as shown.

In case anyone likes this and plans to use it, I’ve added a little table below with the triangle-and-centre coordinates of some common mirror-diameters, plus a picture of my own in action. If you look closely at the hole I made in the centre, you can see just how far out my own spot is!

Cheers, Magnus

Edited April 7, 2022 by Captain Scarlet

[Captain Scarlet]

I plucked up the courage actually to perform the centre-spot placement today. I initially intended to use the template to mark a marker-pen spot, and with tweezers place the centre-spot over it. But I found my hand wasn't perfectly still enough to be certain of getting the initial spot in the right place. So I adopted a different strategy.

I taped the triangular spot to my template in just the right place using a piece of low-tack masking tape. I removed the adhesive backing from the centre-spot. I placed two strips of multiple-layer flocking paper, one on each side of it to give clearance, to ensure that when I laid the whole thing onto the mirror, it wouldn't stick until I was happy with the positioning and pressed it down. It worked first time.

The spot I also made from black flocking paper, in my never-ending quest to remove any source of bright scattered light from my tubes (i.e. from a white spot).

Success! Whereas before the spot was fully 3mm from the true centre of the mirror, almost on the paper of the donut, now I've measured it to be 0.2mm from the centre. I reckon I'll not get it better than that.

Cheers, Magnus. Pics below

 

preparing the press-on template:

 

Laid in place and ready to press down:

 

Finished result (central dot marks the true mirror centre in the last pic):

 

Edited April 10, 2022 by Captain Scarlet
Pics added

[rojay]

That's a clever bit of geometry! Looks like you got a good result with it too.

[Mandy D]

@Captain Scarlet Thank you for linking me to this. It is going to be most useful. How is the rebuild of the OOUK 200mm coming along?

[CraigT82]

Nice work Magnus. The ‘stick It to the template then push it down onto the mirror’ technique is the same as the Catseye template that FLO used to stock but don’t seem to anymore. Works really well for getting it dead centre.