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Building a 10" Reflector

Installing the spider and secondary mirror

My favorite spider is a two-legged affair that I started making about 15 years of years ago and of which I'm still a great advocate. (See article here.) It requires a bit of work and skill in working with metal but everything you need can be had from the hardware store and the local hobby shop. Of course, you can install a conventional commercial spider as well. Installation of both will be discussed here.

Installing a conventional spider

To install a conventional four (or three) legged spider, begin by reinforcing the walls of the tube in the places where the spider legs attach. Specifically, where the bolts pass through tube. Cut eight pieces of 1/4 inch plywood 1.5 inches square and drill a hole through the exact center large enough for the mounting bolt to pass through. Next, drill the four mounting holes for the vanes in their proper positions in the tube. This should be done before the tube is painted but after the resin has been applied. The pieces of plywood are painted on the sides facing the tube with resin and positioned, one inside and one outside of the tube, so that the holes in the plywood pieces match the mounting hole, and are then held in position with a C clamp. You will have to get a C clamp that is a bit deeper than normal so it can reach the position where the plywood squares are located. It is important that the plywood pieces are gently squeezed into the tube so that the resin is uniformly compressed and applied between the surface of the cardboard tube and the surface of the plywood. This operation needs to be done because Sonotube, or any cardboard tube, it is not made of a particularly robust material and will deform or fail when significant pressure is applied to small areas. You have now created four areas that can adequately support the inward pressure that will be required to hold the spider vanes securely in position. You can now mount the commercial four-vaned spider as you normally would in any telescope. Because the top of the tube rotates you'll have to take special care to make sure that the secondary mirror is precisely located in the center of the tube so that collimation will be maintained upon rotating the top. (See below for this technique.)

Making and installing a two-legged spider

As regards the two-legged spider, the basic parts consist of a 3/8 inch connecting or long nut, commonly used to connect threaded rods together, and two pieces of strip brass that can be purchased at the hobby shop. The strip brass that I have used is 3/4 inches wide and 0.64" thick. You need to cut two pieces of strip brass that will be long enough to reach from the side of the tube to the center where it will be soft soldered to the long nut. The two legs of the spider will be spaced to form an angle of approximately 30 degrees. The brass strip spider legs will be mounted to wooden strip supports similar to those recommended above for the conventional type of spider.

As can be seen, two strips have been attached to the wooden pieces with 6-32 screws. The ends of the brass strip have been bent over 90 degrees in a vice and hammered into shape. The vanes have been screwed in place and then bent so as to have their inner ends approximately where the connecting bolt will be. The very inner ends are also bent slightly so as to lie flat against the sides of the long nut. The strips used for the legs as shown above are painted black because they were taken from a spider I used previously on another telescope. At this time the inner ends of the vanes should be cleaned and fluxed for soldering.

 

The nut is now positioned in the center of the tube by eye and held in position with a C clamp. Clean and flux the nut before inserting it between the ends of the vanes.

 

A stick is cut so that it will just fit inside the tube and the exact center is marked. This stick is used to determine the center position for the nut. In the above picture the C clamp is not seen. Sometimes, inward pressure from the vanes is sufficient to hold the nut in place, otherwise the C clamp may be loosened sufficiently to allow movement. Later, just prior  to soldering, it is firmly clamped in position.

 

The same stick is used to checked position in the opposite direction.

 

A threaded rod having a length so as to nearly reach the other end of the tube is screwed into the nut and held in place with another nut so it will not move. The same centering stick is used to find the exact center at the opposite end of the rod. When the rod is centered at both ends the nut may be soldered to the ends of the vanes. When soldering is to be done, the C clamp is firmly tightened and soldering accomplished. Apply most of the heat to the nut since it is massier and will take longer to reach soldering temperature. Avoid excessively heating the brass strip. Allow the solder to draw in naturally between the surfaces. Cool by wiping with a wet bunched up piece of paper towel soaked in water. Take care not to burn your fingers.

 

After removing the C clamp and centering rod the spider will look like this. The spider is then removed and the spider as well as the interior of the tube given a final coat of paint. I use a brush to apply the flat black to the interior of the tube but I suspend the spider from a rod screwed in the nut and sprayed on the flat black.

 

After re-assembly, the diagonal itself is now screwed into position. Note the piece of square brass at the outer and of the left vane. Pieces of brass like this can be used as shims for final adjustments.

 

Concluding comments

In The Best of Telescope Making Journal, a two volume publication of Willman-Bell, an article by Richard Suiter and William Zmek addresses various kinds of spider vanes and their impact on diffraction as seen at the final wavefront. Anyone interested in serious high-resolution observing should read this chapter. A recent reading of this chapter and a subsequent discussion with Bill Zmek suggests that the ultimate form of diagonal support is a 180 degree, or semi-circular, single vane type. This diagonal support appears to produce a diffraction pattern that offers minimal disruption to the wavefront and no recognizable spikes. This essentially single vane support is structurally sound and can be made as simply as the two vane support shown above. As in the two-legged support discussed above I would make it out of 0.64" hobby brass.