IV. Mounting

4.8 Tube Assembly

The tube assembly posed a unique problem. We wished to have a rotating tube assembly. A Newtonian can be pointed in such a way as to make the eyepiece difficult to get to. When the eyepiece is 8 feet in the air it can be inaccessible. Small telescopes sometimes use a rotating tube. My Cave Astrola is an example of a German equatorial mount with a rotating tube, Figure 1a. We wanted to make an open framework like the Serrurier truss. In the traditional Serrurier truss, the truss assembly is attached to a box that is rides in the declination bearings. The optical tube assembly is a modified Serrurier truss. The modification is in the attachment of the truss tubes to the central box assembly. In the case of the 22" tube assembly the truss tubes are welded to a rings rolled from 1.000" square aluminum tubing. This was done to allow the tube assembly to rotate in the tube box, Figure 4.8.1

a  b

Figure 4.8.1. The rotating tube assembly in the tube box can be seen in (a). The Cassegrain secondary holder can be seen in (b). The Newtonian secondary holder can be seen in (a)

 This modification allows the eyepiece to be rotated to a convenient location for visual use.

 The tube box was fabricated form 0.200" aluminum plate. (The 0.200" plate was originally to be used for the fork!), Figure 4.8.2.

Figure 4.8.2. Welding the tube assembly box with the MIG welder. The MIG welding gun and the control box can be seen in this image.

 A 2" aluminum angle was rolled for me by a local machine shop. The resulting rings would act as a track for the telescope tube to roll on. Two of these rings were fabricated for the back and front of the box, Figure 4.8.3. a  b

Figure 4.8.3. The ring before welding 4.8.3a. A plywood disk was made to act as a welding jig, 4.8.3b. Two cabinet clamps were connect with a 4-way pipe fitting. The disk was clamped to the welding table and welded.

 The rolled rings were formed around a plywood disk cut to inner diameter of the ring. The ring was clamped around the plywood disk and welded, Figure 4.8.3b.

Each ring was welded into the front and back of the tube box, Figure 4.8.4. After welding the rings in the box, a gap was left in each corner. A piece of plate was cut and welded into the corners to finish the tube assembly box.

Figure 4.8.4 shows one of the rings welded in the tube box. The second ring is on the floor prior to being welding.

 The finished box assembly is held in place by a 1/2" threaded rod that goes through the center of the declination axis and box with a 3.250" steel plate welded to the end. The plate loads the outer bearing into its bore. Two 8" diameter 1/2" round aluminum plates are attached to the declination axis and these plated are also bolted to the tube box, Figure 4.8.5.

Figure 4.8.5. The tube box mounted in the fork. Note the 1/2 " threaded rod that loads the outer bearing and the three 1/2" bolts that bolt the declination axis flange to the tube box.

 The telescope tube is made in two sections. This was done so that a round ring could be located at the upper end of the tube box. Each section of the tube is made with two 1.000" square tubing rolled into a ring. (Bill made the rolling machine the rings were rolled with. I made the roller dies.) The connecting tube is made from 1.250" emt rigid aluminum conduit. (This is the same conduit that is used for electrical work). The tube assembly is slipped through the box and held in place with rollers. I used more generator bearings for the rollers. Cams were machined for each roller so that the rollers could be adjusted. Figure 4.8.6a shows the upper tube assembly shortly after welding along with some odds and ends. Figure 4.8.6b shows the first mock up of the tube assembly. The entire tube assembly can be rotated 3600 so that the eyepiece can be conveniently positioned. a  b

Figure 4.8.6. The upper tube assembly after welding 4.8.6a along with some other parts. The first mock up of the tube assembly 4.8.6b.
  PREVIOUS   NEXT
22" Telescope Index
Home