Why dotting the mirror center?
To be able to collimate a Newtonian accurately it is important to make a dot at the optical centre of the primary mirror. The physical centere of the mirror is not necessarily the optical centre of the mirror. Therefore it is important to make the dot easily removable, so using paint or a permanent marker didn't seem a good idea to me.Round, square or triagular?
Of course the dot doesn't need to be round, it can also 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.How do you get it centered?
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