Sunday, May 9, 2010

6" F/5 Dobsonian: Drilling a focuser hole

I was really nervous about making the focuser hole--drilling in a cardboard tube can be messy.  On my Coulter 8" when I replaced the focuser, I had to enlarge the focuser hole, and it was a quite big mess.  I thought of trying to use a rotary tool, but I couldn't figure out which bit was appropriate, and had no scrap to try it out.

But I decided that the glue-impregnated cardboard just might take a hole saw, and risked drilling with a hole saw.  Success!  There was just one little bit of tearout on the inside, but I glued that down while impregnating the hole with water-diluted Titebond II.

The focuser is going to be a Crayford.  It's almost finished--it just needs some 5/16" dowel glued on the underside for mounting on a curved tube, and I'll buy that tomorrow after dropping the kids off at school.  I'll post on it when it's finished.  This morning I got to do some trigonometry to try to figure out where to place the dowels so the center of the focuser board would be tangent to the telescope tube, and the dowels would be tangent to the focuser board and the tube.  Somehow the calculation didn't work out.  Then I found a simpler method with Pythagoras' Theorem, which did work out.  Geometry is fun.  I don't know how some people manage to do serious woodworking without using trigonometry.  I guess they might be really good at scale drawings, or just able to eyeball things in ways I can't.

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