I offered to build one for her. I wanted to use some lighter materials: Alumalite6, an aluminum-plastic composite for tube ends and small aluminum tubing for the tube.
The weight of the mirror constrained the size of the telescope. We agreed on a 14½", 2" thick Galaxy2 mirror. Anything larger would have been too heavy when combined with a cell.
The finished telescope weighs very little:
Because there are so few parts, setup is simple and quick.
These photos show the components before and after assembly. I have put it
together in less than five minutes.
3. Optical performance
The Galaxy Optics mirror is a "premium," or best of a lot of ten. An interferogram was supplied with the mirror and indicated:
Its images are sharp -- the owner got her money's worth.
4. Mechanical performance
The telescope has these properties:
1. Cage tube
I was inspired by the geodesic-style truss tubes used for giant telescopes like the Keck3 10-meter and the Gemini4 8-meter, and settled on a fixed cage design for the tube. The tube just fits in the owner's Pathfinder.
The fixed cage permits smaller tubing than is used with truss-tubes telescopes bolted together at setup. I chose 3/8" O. D. 6061-T6 aluminum tubing with an .035" wall thickness.
The joints were made with all-aluminum POP5 rivets, or blind rivets. They are easy to use. I crimped the tubing in a vise in order to get a thin, flat cross-section at each joint. There are 16 axial and two lateral struts. I cut the struts to length, crimped and bent them so they would spiral around the tube from end to end, and then riveted them. The tube took only a couple of days to make.
The struts are attached to each tube end ring with small aluminum brackets made from ½" aluminum angle. The brackets are screwed to the rings.
2. Top end
Alumalite is easy to cut and lends itself to woodworking tools. Two .015" aluminum faces sandwich a corrugated polyallomer plastic core. It comes painted in a variety of colors and does not corrode. To improve the aesthetics, I filled the edges with aluminum putty to hide the corrugated core.
3. Bottom end
The tube's bottom end attaches to three mirror cell assembly bolts. These bolts
also provide collimation -- the tube is adjusted relative to the mirror and cell -- not
vice versa. This simplifies cell design since its attachment to the mount need not provide
The spider is like a Novak spider. The diagonal is positioned axially on a
central bolt; it uses three screws for tilt adjustment. Each vane end has a post
with a screw hole; blocks attached to the ring hold the screw.
5. Focuser shelf
The focuser shelf -- also made of Alumalite -- attaches to the inside of the top
end ring with a strip of 1/16" aluminum angle. The angle is riveted to the shelf and
bolted to the ring. The shelf is stabilized at the rear through an attachment to a tube
that, in turn, is riveted to two of the tube struts.
6. Reflex sight
I used a high-quality Tasco8 PROpoint PDP5CMP precision gun sight for a zero magnification reflex sight. A gun sight maker invented the electronic red dot gun sight and over a half-dozen firms make them. Most non-Telrad ATM reflex sights are made by gun sight manufacturers and sold under brand names like Tele Vue9 and Orion10.
The PROpoint has a knob with four detented dot sizes, from 4 to 16 minutes of arc. The 4 minute setting -- 4" area at 100 yards -- is smaller than any dot I have seen in other sights. The sight has an eleven-position detented rheostat for brightness settings, and has built in windage (azimuth) and elevation (altitude) adjustments. The latter adjustments cover a wide angular range and simplify alignment with the telescope's optical axis.
The best feature of the sight is its huge 45mm aperture. You can get right behind it and clearly see large areas of sky. With its coated optics, there is no need to use both eyes with this sight. It's a class act and is great for star-hopping.
1. Machining cost constraints
Next to the mirror, machine shop charges are the costliest element of an
aluminum telescope. Thousands of dollars can be saved by eliminating as many contract
parts as possible.
2. Mount Design
The mount consists of two major parts: the mount and the tripod. The mount has a large ring which holds the mirror cell; this ring is welded to a triangular side piece which, in turn, is welded to the altitude axle. The axle is cut from a 4" O. D. aluminum pipe.
To minimize flexure at the welded joint between the mount ring and the side piece, additional ½" rod bracing was welded in place.
A counterweight rod extends from the other end of the altitude axle; the counterweights offset the weight of the mirror, cell, tube and the mount ring. The counterweights were purchased from Hollywood General Machining11 (or Losmandy): one is 21 pounds, the other is 11. These weights are manufactured for use in Losmandy and Celestron12 equatorial mounts.
The altitude axle rests in a fork mounted over the tripod. The fork rotates in the tripod, providing azimuth motion. The altitude axle has Delrin-Teflon rings that rest in the fork; these provide the altitude motion. A clutch is attached over the axle to secure the mount in position, reduce flexure and provide added friction.
The tripod, also cut from 4" O. D. aluminum pipe, has three legs cut from ½" bar stock and welded to the pipe. The fork is attached to a round rod which drops through the tripod and turns in a Delrin-Teflon ring bolted to the bottom of the pipe.
1. Cell design
The cell uses a nine-point whiffletree. The base plate of the cell holds three rod end ball joints; attached to each ball joint is a pivoting triangle. Each triangle point has a weld T-nut bolted to it upside-down to provide a pad for the silicone adhesive.
There is no play in the ball joints. This ensures that the lateral forces -- transmitted through the cell's pads -- will be uniformly distributed over the nine cell points. Without such uniformity, mirror flexure and astigmatism are possible.
Dow Corning 93-076-2 RTV silicone adhesive (purchased from K. R. Anderson Co.13, Inc.) was used to glue the mirror to the pads. Temporary wooden blocks were positioned next to each pad to raise the mirror while the adhesive cured. This permitted a 2mm-thick adhesive layer.
2. Tube rotation design
The end points of the cell's base plate have three Delrin-Teflon roller bearings
bolted to them. The inner part of the mount ring forms a bearing surface for the rollers.
The rollers are flanged to hold the cell in the mount ring.
These roller bearings are finger-tightened -- just enough so that the telescope's tube will rotate easily, but not so much that the tube is loose and flexes in the mount ring.
3. Mirror carrier
4. Mirror cover and cleaning
An ABS plastic mirror cover fits snugly over the mirror. It is placed over the mirror or removed from it after assembly or before disassembly.
Because the mirror is all-metal and won't rust, it is easy to clean. After removing the plastic cover, water from a hose is poured over the mirror's surface, rinsed with distilled water and then the cell tipped up so most of the water runs off. The remaining small drops of water are dabbed off with a folded and pointed paper towel.