HyperLightbridge Base

Last week, my Meade Lightbridge (aka Looking Glass) arrived in Helotes, TX. I had ordered it with the plan to rebuild it, and this weekend I had the chance to do so. I had previously practiced building some of the design features on the Resurrection scope – large diameter altitude bearing and a small base. So, on my way home from work on Friday I picked up a 4x8 sheet of birch plywood ¾ of an inch thick with great expectations for construction.

I had drawn up a sketch of my idea and a cut plan for the sheet of plywood:

Sketch

Cut Plan

On Saturday evening, the first thing I did was to cut out the two three-foot-diameter semi-circles with a router and a circle cutting jig. This jig was a piece of Masonite cut and drilled to fit the base of my router with an arm that extended out toward the operator. (I got the idea for this by searching the Internet for “router circle cutting jig”.) I measured from the inside edge of a straight router bit to a spot 18 inches down that arm and drilled a hole that would be the center of my 3 foot half circle altitude bearing. I screwed the jig ¼ inch from the edge of the plywood and 18 inches from one end. I then cut through the plywood about a quarter inch at a time around the arc.

After cutting these two half circles, I cut a strip of bumpy fiber reinforced plastic (FRP) ¾ of an inch wide and as long as the half circumference. That was about 57 inches (pi*18). I coated the back surface of the FRP and the round surface of the altitude bearings with two coats of contact cement. After about 10 minutes of dry time, I began at one end of the strip and pressed the strip on the edge. After getting the whole strip on, I used the floor to press the strip on very securely by rolling the semi-circle against the garage floor. Then it was time for a night’s sleep.

The next morning, I used a plane to trim the width of the FRP to the width of the plywood, which is actually 1/16 of an inch under ¾ of an inch.

Then I set about cutting the box pieces to hold the sides of the light bucket. I unscrewed the mirror cell from the bucket walls and lifted the top of the bucket off the mirror cell. I put the mirror cell in the house, away from saw dust. (I also took this opportunity to adjust the mirror holding brackets so that they do not press too firmly on the edges of the mirror.) I measured the diameter of the bucket several places and decided that it was very close to 19 inches. I added the width of my plywood to that and came up just below 19-3/4 inches. I knew that I wanted four identical pieces of plywood 12x19-3/4. I also needed a base and a ground board.

I very carefully measured and drew cutting lines on the plywood. First, I cut the plywood with a circular saw so that I had a 48x48 inch piece. I then divided that into two 24x48 inch pieces and then two more cuts gave me four 24x24 inch pieces. Two of those pieces I cut to 19-3/4 inches long, and then each of them was divided to one foot widths. I put the two remaining 2x2 foot pieces to the side until I discovered exactly what size the base would need to be.

I routed the long edges of each of the box sides, top and bottom, with a round off router bit. Then I drilled and countersunk three screw holes on one end of each of the box pieces. I assembled the box, making sure to drill a pilot hole for each of the 1-1/4 inch drywall screws.
I removed the aluminum altitude bearings from the light bucket. Then I slipped the box over the bucket and secured it to the bucket with a ¾ inch wood screw through two of the holes which had held the aluminum altitude bearings.

My plan was to have the center of my 3 foot diameter altitude bearing at the center of the upper edge of the box. I knew, from the collected wisdom of many owners that the scope was bottom heavy. I confirmed this on my scope when I attached a radian eyepiece, a Telrad finder, and a finder scope to the secondary housing. When my Looking Glass approached level, it wanted to nose dive. I also knew from the collected wisdom of the group that moving the center of the altitude bearing an inch up would make a world of difference. That was where I attached the box – with its upper edge an inch and a quarter above the center of the factory bearing. I also attached a third screw to the topside (opposite the seam in the bucket) to reduce rotary movement of the scope in the box.

I attached the half-round altitude bearings to the box with home made knobs and tee-nuts. I made the knobs by threading three 1-1/4 inch diameter ¼ inch fender washers over a 2 inch long 1/4x20 hex bolt and tightening them down with a nylon insert nut. I drilled ¼ inch holes through the bearing and the box in locations which spread the load and which gave me room to work between the bucket and the box. Then I drilled the box holes to accept a 1/4x20 Tee-nut and drew that Tee into the wood with a hex bolt and a washer. This makes it easy to remove the two altitude bearings from the light bucket without tools. A minute or two is all it takes to install or remove the large bearings. Of course, the bearings were oriented so that they allow both vertical and horizontal use of the scope.

I measured the width between the outer edges of the altitude bearings to make sure that they were parallel. They were just a little under 22 inches from side to side at three locations – ends and middle. That was very good news. All my careful cutting had paid off. Now I was able to build the base.

The width of the base would be the same as the width of the altitude bearings. I cut both the remaining 2x2 foot boards to 21-7/8 inch squares. I rounded off the corners of the ground board by using a varnish can to trace ¼ circles at each corner.

Next I needed to know how high the base needed to be at the center. I measured how far the corner of the light bucket extended outside the altitude bearing. It was about 2 inches. Allowing room for the center screw on which the telescope would rotate, I decided to make the center 3 inches tall. I marked that height at the 11 inch center of a 22 inch piece I was drawing on the part of the plywood from which I had cut the two three foot circles. I used an altitude bearing to draw a curved upper edge so that the bottom center was 3 inches tall and the sides were even. It ended up being about 6-1/2 inches on each side.

At this point, a bunch of routing was done to round off edges. The bottom edge of the rotating base board was routed. The outside corners of the two supports (with the concave rounded edges) and most of their round surface was rounded. Both top and bottom of the ground board were routed. The full edges inside and out of the altitude bearings including the FRP was rounded – the FRP only slightly.

A 5/16 inch hole was cut in the center of the base and ground boards. (To do this easily, place a straight edge from corner to corner and draw an X in the center. A 5/16 Tee nut was placed in the bottom surface of the ground board after that hole was drilled one size larger.

The side supports were screwed to the base from below with countersunk drywall screws. A brace was cut for both the front and the back of the base, adding strength to the support sides. The front was cut to length from one of the 4-1/2 inch wide pieces left over from the cutting of the box sides. The rear brace needs to be a little shorter to allow the swing of the telescope. 3 or 3-1/2 inches tall works pretty well. These braces are screwed to the sides of the supports and to the base. All holes are countersunk.

Only a little bit of work remained. I used the lazy susan azimuth bearing from the stock base, except that I also placed three felt furniture pads just outside the circumference of that bearing. I cut about half the thickness of the pads off so that the azimuth bearing is smooth, but does not turn in the wind. I attached square furniture glides on the upper rails of the supports. (I left small areas which were not routed so that the full width of the rail would support the furniture glide.) I also attached pieces of 1” aluminum bar, cut 2 inches long and drilled to be attached to the outside edges of the supports.

I cut triangular braces for the box, one for each corner, 4-1/2 inches wide. This is to strengthen the box so that a handle can be attached. The handles make it much easier to carry the light bucket. They also allow two people (I’m not strong enough to do it by myself) to lift the scope from the base after it is assembled.

I put a handle on one of the braces for the base, and it can easily be carried on one hand while carrying the bearings or the secondary housing in the other hand.

I really like the feel of the scope now. It is much easier to move around. The light bucket with mirror attached is still a heavy item. My only recourse if I had to move the scope a long way by myself would be to use a hand truck or to remove the mirror cell from the light bucket. That is not too hard, but it would still be a bit of a pain.