Arie's Dobsonian Telescopes

Mirror cells

What I found tedious with the previous open mirror cells was to exactly determine and mark their center. And this is necessary for obtaining good Argo Navis pointing accuracy. The laser light has to hit the center of the mirror and upon removal of the mirror, this light must hit the center of the pivot bolt (see pictures in the "Installation" section above). However, to align the center of the mirror exactly with the center of the mirror cell required exact measuring and this was always a bit off. And for the binoscope this is not good enough. For instance, good pointing accuracy requires that the center of the Argo Navis pivot bolt is located precisely between the two mirror centers. And these must in turn also be precisely positioned in relation to each other to achieve good alignment of the two independent optical systems. The following way of construction greatly simplified this all.

Instead of making an open mirror cell based on aluminium tubes, I started with a solid, 8 mm thick aluminium plate. I drew the mirror cell on the aluminium plate around a small drilled hole, which served as center (picture below).

On the plate I drew
1) the exact location of the center,
2) the six points on which the mirrors lies,
3) the two lateral points against which the side of the mirror rests and
4) the points through which the collimation bolts go (see picture below).

This approach has multiple advantages. For instance, I also started with a solid multiplex plate as bottom of the mirror box. On this plate I drew the same mirror cells. Now it was easy to mark exactly the two centers of the mirrors/ mirror cells on a straight line at precisely the same distance from the "real center", this being above the prospective center of the Argo Navis pivot bolt. Then I drilled a 6 mm hole through the centers on the aluminium mirror cells and the corresponding centers on the multiplex bottom plate, and fixed them with bolt and nut to each other. Finally I drilled the holes for the collimation bolts through both mirror cells and the wood (see picture below). Each mirror cell will have three collimation bolts (see black arrows), but note the different forms of the two mirror cells. The right mirror cell will have two collimation bolts at the lower site, and the left mirror cell will have these two collimation bolts placed at the upper site. The why of this difference will become apparent in the section on collimation and mirror merging.

I next extended this way of construction by fixing the bottom plate of the rocker box to the combination of the bottom of the mirror box and mirror cells. Now I could again precisely mark the "optical" centers of the mirrors as well as the "real" center for the Argo Navis pivot bolt. In a later phase I could accurately place the sides of the rocker box, in relation to these marked centers.

Finally I did the same with the secondary cages. These are constructed as tunable tops (see for detail pictures the next section). This involves a multiplex bottom plate on which both secondary cages can rotate. I fixed this multiplex plate to the now triple combination of rocker box bottom plate/ mirror box bottom plate/ mirror cells. Again I drilled holes in the tunable top plate, coinciding with the centers of the mirror cells that later served to quide the position of the pivot bolts of the secondary mirrors.

So I made a 'four-component sandwhich' consisting of the bottom plates of the rocker box and mirror box, the mirror cells and the bottom plate of the secondary cages. All fixed to each other and now precisely aligned for:
- the center of entire binoscope,
- the centers of the mirror cells,
- the centers of the mirrors
- and the centers of the secondary mirrors.

Trivial as it may seem, this succession of constructing steps was much more precise than what I did ever before. For instance, after fixing the collimation bolts as before (see the section Mirror cell), it turned out that a) collimation went more smoothly and b) without any slop in the system. This resulted in very stable collimation, which is crucial for stable merging of the images, as will be described below. The preciseness of the subsequent steps also led to a pretty accurate first alignment of the optical systems. This showed by the rather closely merged images at very first light, which was a pleasant surprise!

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