Of course the dot can be made square or triangular, without any problem. But I thought it would be easier to centre a circular dot in my Cheshire. The field field stop in the Cheshire is 16 millimeter. So I wanted the dot to be somewhere around 14-15 millimeter in diameter. The diameter of a support ring is 14 mm, so exactly the right size.

To be able to put it exactly at the (physical) centre of the mirror I drew a circle of the same diameter as my primary and, exactly at its center, a 14 mm circle. I used a simple CAD program, but a compass can do the job as well. After cutting out the circles, I taped the large one to the mirror, as shown in the picture below. It's easy now to place the center dot exactly where it belongs: at the center of the mirror.

The picture below shows a close-up the mirror with the paper jig and the center dot in place. The last photograph shows the end result: the mirror with the center dot.

Another way of getting the job done is here and, for a third way to do it, look here.

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Email: Jan van Gastel