Curved Vane Spider and Secondary Mirror
Curved Vane Spider and Secondary Mirror. 

Secondary mirror curved vane spider:

I used 7475 T761 aluminum sheet, 37 thou. (.037) thick to create the curved vanes.  I used a router compass to cut two disks from 1" thick melamine sourced from a display table I got for free from a store that went out of business, and screwed them together to create a round mold 2" thick.  I had read that 7000 series aluminum is easy to bend and hold a shape at a low temperature, and that it can be done in the oven.  Not even.  It is extremely tough and resilient material.  I cut the material on the band saw with a fence, and drilled a hole in both ends.  I screwed one end to the edge of the mold, and then jammed it against a wall and bent the strip around the mold and screwed it in place at the other end of the strip.  I'm here to tell you it took FULL force to get it to bend.  Thin and light though it seems to be, it is VERY tough material.  I then heated the strip by waving a propane flame over it until quite hot, then quenched it with a spray bottle.  I repeated the process three times before the material would more or less hold the shape.  I made a couple extra vanes, and picked three that were closest in shape.

 

Hans of Destiny Spiders provided the secondary holder and the hexagonal piece of aluminum that the holder mounts to and the spider screws to.  I had originally made some vanes out of carbon fiber, but that did not work, and Hans wanted to support that experiment.  Carbonfiber is a great material for a variety of applications, but in this case, aluminum was better.  I decided to reverse the mirror mounting screw that goes through the center of the hexagonal part the vanes screw to.  Hans had it with the head inside the wood mirror holder, with the thread end stick through the spring and out the top of the hexagonal part.  I wanted to mount the mirror in a box for transportation, so I made a mirror mount from a big chunk of balsa wood, cutting the 45 degree angle with the Makita sliding compound miter saw.  In the middle I drilled a hole large enough for a pair of dew control resistors, and mounted a T-nut inside.  The mounting screw has a large knob I made by using a hole saw to cut a disk, tapping the center of it and threading it on the mounting screw all the way up to the head of the screw and using 5 min. epoxy to hold it in place.  This knob is used to attach the mirror in the transportation box as well as attaching the mirror in the telescope.

 

The secondary mirror is rather heavy though, and the curved vanes, while strong enough to hold the mirror along the telescope tube's axis, they are not strong enough to hold it centered when the tube is tilted over on it's side, so the mirror sags.  To solve this I used the thinnest available piano wire (I might mic. it if I remember, but it's probably about .010, basically the thinnest they had at the hobby store) stretched from the center to the tube's edge at three places.  Each curved vane mounts at the optical tube with two 'bolts'.  One  is a 1/4 20 bolt and the other is a piece of 1/4 20 all thread rod with a hole drilled through one end, through which I passed the end of a length of piano wire and wrapped it around itself once or twice to hold it in place.  The other end of the piano wire I soldered to an electrical ring end connector with the plastic taken off.  The ring connector is screwed to one of the two screws that attach each end of the curved vane to the central hexagonal aluminum piece.  The piece of all thread sticking through the optical tube has a nut, and that is tightened to adjust the mirror until it's supported in the center of the optical tube (well actually offset a little bit of course to accommodate for the shape of the truncated angle of the cone of reflected light).  The three piano wires hold the mirror quite rigidly in place in the center of the light path.

 

Why all this effort?  I'm sure you know, and you can read more detail at Hans' web site, but in short, anything placed in the optical path creates diffraction effects.  A straight piece of metal will create diffraction spikes around bright objects like car headlights in movies and stars in the telescope.  By bending that metal in an arc, the diffraction spikes are still there, but thrown in a fan around the object instead of being four actual bright spikes.  The effect is the spikes are not visible to the eye.  Now I had to compromise that a bit with the piano wire, but they are thread thin, and my expectation is any visual artifacts will be slight.  FOLLOW UP: I can't see any diffraction from the super thin wires.

 

 

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