Author: Tom Droege Date: 980529 Key Words: Instrumentation, Construction, Mechanical, Electronics
This TN is "show and tell" for the Mark IV mount and the planned house at the Canterbury woods observatory. Plans will be available for those that want to construct a similar housing.
Figure 1 is a side view of the mount. The telescope is in its parked position pointing south which is to the left. The green components are fixed to earth. There are two 24" wide by 18" tall aluminum plates separated by three 1/2" by 32" aluminum bars. The plates are mounted to whatever is used for a base with 1 1/2" x 1 1/2" aluminum angles. Two of the bars contain the bearings for the RA motion. The third bar contains the motor for the wheel and string driven right ascension (RA) drive. In case one thinks flimsy when the word "string" is used, think instead something that does not cut with diagonal pliers.
The red components are the polar axis for the RA drive. The bar at the bottom is the RA drive wheel section. This is a short section of a wheel (bottom right in red) that is a radius of the RA motion. The third spacer bar mounts a motor and a lead screw. The lead screw contains follower blocks tangent to the wheel section with long threaded sections that drive "string" that wraps around the wheel section. Screw irregularity is reduced by using a rolled thread lead screw, and by engaging many threads to generate an average motion. The details of the string drive are not shown.
The red platform sticking up and to the left mounts the motor which drives the declination axis.
The blue components move in both RA and declination. This motion is shown in the drawing of Figure 2
and the model of Figure 8 with the telescope elevated to point near the equator. The telescope tube is mounted through thin plates attached to a cage which moves in declination. The 5 1/2" dia. 22" long lens tube is fitted with a light shield shown in black. The shield is shown 12" long and 8" in diameter. We may be able to make it a little longer.
The electronics are mounted to the cage top and bottom, balanced on the two telescope sides.
The camera and its mounting bars are shown in magenta. Linear bearings in the cage support the camera head. The head is driven along the lens axis by a linear stepping motor mounted to the cage. Its motion of 0.001" per step allows for fine and repeatable focus. The details of the focus drive are not shown, but the system is designed to allow the CCD plane to be maintained perpendicular to the lens axis as the camera head moves in focus. The design is such that precision alignment can be maintained through pinning if this proves to be necessary.
Figure 3 is a view of a model of the mount. The pieces of the lens tube support cage are omitted because I got tired of cutting up little bits of balsa wood. The mount is shown pointing near the zenith and rotated in RA. The RA motion is limited to about +/- 30 degrees from the meridian.
Figure 4 is the model in close to its park position, pointing south at the horizon but rotated somewhat in RA.
Figure 5 is a model of the enclosure that we have designed for the dual telescope. The white pieces at the bottom are 2"x4" x 8' long construction lumber. This will help to calibrate the size. The housing is about 48" high at the peak. The red part of the house moves on the silver rails made from 1 1/2" x 3" aluminum channel. The right hand roof section covers the dual telescope. The left hand section is a "dark" box for the flat field system. When the roof is in its closed position as shown in Figure 5, the telescopes look directly into opal glass plates that are illuminated from light sources 30" away at the end of the dark box section. The arrangement is such that as the roof section closes, the light shields on the end of the lens tubes mate into rings on the wall of the dark box. This results in a completely dark capped telescope even though light leaks into the main telescope enclosure, which will also be designed to be moderately dark.
Figure 6 shows the roof section just starting to open. It moves due south as it opens. The roof is designed with an overhang on the north end of the enclosure which is the only part not covered as a "box in a box".
Figure 7 has the roof section fully open but with the telescope still at the park position.
Figure 8 shows the telescopes rotated up and pointing near the zenith.
Figure 9 is a drawing similar to the model photograph of Figure 8. North is to the right.
The top cage bars (Figure 1) extend across the space between the two telescopes and thus limit the motion to a little past the pole. This is to insure a rigid structure.
Figure 10 shows the telescope pointing at its limit past the pole.
The design has considered how to keep the housing snow and water tight. It is planned to cover the rails with dryer vent tubing. This will prevent ice and snow from accumulating on the track when the roof is closed. The tubing will then fold up as the roof opens. It is planned to seal the other openings with weather stripping, but it is not expected that a fully tight enclosure will be achieved.