Friday, November 17, 2017


I am rebuilding my 3" f/4.5 reflector, mostly to make it something to be proud of.   I am not changing the mirror, just building a proper mirror cell (which holds and collimates the mirror) and the spider (which holds the diagonal mirror).  These are usually held in place by three or four legs (hence the term "spider." 

The mirror cell is complete, except it needs to be painted flat black.

The spider is a surprisingly complicated assembly because the mirror has to be adjustable (3 points of positioning) and ideally has some ability to make fine adjustments it up and down the tube and rotate within the tube for precise alignment.  A really fancy spider would hold the diagonal mirror (which actually a 45 degree elliptical) by the edges only, but that was more work to design than makes sense here.  Instead, I will double-sided sticky foam the mirror to a 45 degree piece of aluminum.  The collimation screws allow for minor errors on angle or variations in foam tape thickness to be adjusted after the fact, but I wanted the holder to be pretty close to perfect.  I cut it on a chop saw to approximate 45 degrees, then put it in a chuck on a tilting table to get a perfect 45 degree angle.

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