Saturday, May 25, 2013


The structure that holds the diagonal mirror in a Newtonian telescope is called a spider, because it typically has several legs.

Last night, I was finally ready to do collimation of the optical path, and I discovered that no matter what, I could not get the diagonal mirror centered in the tube!  Why?  Was it this far off in the old tube?

No.  I had to cut down the mirror cell bottom plate because the inner diameter of the tube is 19.875" instead of 20.25"; I forgot that the spider required similar surgery.  It was easy to forget, because the legs of the spider are made of .030" aluminum (in the interests of minimizing weight and diffraction of light), and they are flexible enough that they just bent to fit.  But they did not bend symmetrically.

My first concern was that I was going to have to start from scratch on this, but it turns out that there was enough spare room where the legs attached to the center point that I was able to redrill the holes in the legs 3/16" over and solve the problem.  The picture below was taken with my little HP PhotoSmart camera, and it isn't terribly sharp, but you can see where I moved the holes:

I may buy a ring roller at Harbor Freight today to make tube rings.  One of the reviews indicated that one of the plastic parts broke almost immediately, which doesn't surprise me.  I will be rolling 1/8" or 3/16" thick aluminum, so I doubt that I will be stressing it much, and it sounds like other than that one part (which I can probably machine a replacement for out of aluminum), it is an adequate tool.

UPDATE: After spending a bit of time, I discovered that because the spider legs are held in tension (rather than compression, as it more typical of spiders), it is very dependent on the order of tightening to keep the body centered.  After a little experimentation, it seems to be well-centered, and collimation went very well.  But I am a bit concerned that it might not hold collimation as it moves across the sky.  Perhaps a more conventional (that is to say, commercial) spider would be a better choice for this.

Frustration: I bought the ring roller at Harbor Freight -- but it appears that some previous purchaser had tapped the taper pin that holds the handle to the roller before putting the handle in place.  There seems to be no way to remove the taper pin now without drilling it out.  So tomorrow I will go back and ask for one that has the taper pin not yet inserted.

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