Shims in cells

[Ben the Ignorant]

Hi.

As a response to ebeygin in the whole-scopes section, here's how I insert shims in refractor cells. I stick 6 strips of adhesive tape at 60° from each other. If that's not enough, I add a layer of narrower strips. I forgot to photograph the job I did in the cells, so I'm showing it with a cap.

If the layers become too thick I remove those barely visible strips this way: I lift the corner of a strip with the tip of a thin blade, and remove it with tweezers.

Its edge remained visible because this strip is narrower than the previous one. 3 strips at 120° don't hold the lens as well, the large empty space between them still allows the lens to play. Plus, 3 strips have to be thicker, to the point the lens drags against them when you want to insert or remove it. 4 strips at 90° don't work too well either. Because of tolerances/inequalities in the lenses edges and (plastic) cell, 6 strips hold better with less pressure. All this is known by frustration experimentation, so trust it. In other shim jobs, I've noticed that 5x72° works well, but only with small circumferences.

Worries:

- a friend asked if the glue could seep onto the lenses; no, the "glue" on transparent adhesive tape is a solid, like gum, not a liquid, and it doesn't melt at daytime temperatures. I've kept rolls of tranparent tape in a drawer for years, the glue never seeped out. However, don't use electric tape because its glue does flow out with time (also something I know through frustration experience), and it's too thick for accurate centering, anyway.

- too much shimming? The inside focus pattern will seem ok, but outside focus the pattern will be an hexagon with rounded edges, the clue that the 6 strips pinch the lens; another thing I learned through shameful blunder experience.

Tips:

- making the strips narrower has the same effect as removing one or two broad strips, because the friction area is reduced, the lens will enter the cell more smoothly, with less binding.

- blacken the lens edges before you shim the cell, because the layer of ink is thick enough to cause binding (another piece of wisdom got from aggravation experience)

- the two 80mm scopes I worked on required two layers, and then the removal of one or two strips. It makes the contour a little uneven but you don't care because the lenses have to be centered relative to each other, not relative to the cell.

- the first strips have to be about 25mm-30mm long, the next about 10mm shorter

Personally I shim only the rear lens, and add centering screws at the front, but locking the rear lens with shims, and tapping the front makes the process less random. Don't forget the alignment marks on the lens edges.

Yeaaahh! I finally found how to write between pics!!!

[Ben the Ignorant]

Ebeygin tells me he doesn't see the DIY pics on his computer, but I do. Does someone else have the same issue?