III. Optics

3.2 Mirror Cell

A 9 point flotation mirror cell was fabricated to support the 22" mirror. The cell is integrated with an end cap that bolts on to the end of the tube. The end cap is made of 3 sections of 0.250" aluminum plate left over from fabricating the fork. The three sections are bolted to three 2" pieces of aluminum channel that are attached to the Cassegrain focus mounting plate, and form the round end cap. The three triangle plates that make up the 9 point supports are indirectly mounted to the end cap with valve springs from a Chrysler Hemi engine with three 3/4" stainless studs through the end cap, Figure 3.2.1a.  A 3/4" Stainless steel nut provides for adjusting the mirror. The Hemi valve springs provide the push to the bolts pull.

a  b

Figure 3.2.1. The nine point flotation cell and end cap 3.2.1a. A u-joint is used to allow the triangles of the flotation system to "float" 3.2.1b.

 The three triangles that make up the nine point cell are set on three u-joints, Figure 3.2.1b. The u-joints provide a pivot so that each of the triangles can "float" or self align. The u-joints are placed in a bore above each of the Hemi valve springs. A 1/2" bore is in the back of each triangle that accept the u-joint. The triangles are secured to the u-joints by a set screw. The mirror rests on 9 teflon pads that are glued to the triangles. The teflon pads are the white circles on each of the triangles. Yes, it is possible to glue teflon. The pads were made form a special teflon strip with a glueable backing.

The mirror, Figure 3.1.2a is a 21 11/16" clear aperture and is 2 3/4" thick. The mirror blank was made by Hayward Scientific Glass Company. The mirror's glass type is C3, hence the designation Hayward C3. The coefficient of expansion is about twice that of Pyrex. The mirror weighs 98 lbs.

a b

Figure 3.1.2. Figure 3.1.2a is the 22" mirror out of the cell. The cell rests on the nine point flotation cell and the mirror retainer secures the mirror in its cell, 3.1.2b. The coating is an aluminum coating with no over coating. The coating is some 20 years old. Even uncoated aluminum lasts a long time.

 The mirror is placed on the teflon pads on the nine point flotation cell Figure 3.1.2b. After the mirror is centered on the cell the retaining ring is slipped over the mirror and bolted in place. Since the telescope is transported in a trailer the mirror cell retaining ring is bolted on with 6 pairs of bolts, Figure 3.1.2a. The entire assembly, mirror, mirror cell, and end cap, is bolted on the tube end ring of the tube assembly with nine 1/4" bolts, Figure 3.1.2b. a  b

Figure 3.1.2. The mirror is secured in place by the mirror retainer 3.1.2a. The assembled mirror cell assembly is then bolted to the end ring of the telescope tube, 3.2.1b.

 An 1/8" thick circular plate was cut to fit over the mirror for protection during transport. The mirror maintains its alignment pretty well even after repeated transportation.
 
 
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