14½" reflector

Design
Features
Tube
Mount
Cell

Design

I designed and built this telescope for a member of the Sacramento Valley Astronomical Society¹ (SVAS). 

She had these requirements:

1. She had to be able to lift and carry each part in and out of her  Pathfinder

2. She wanted as large a mirror as possible
3. She was willing to spend about what a commercial equivalent in size would cost

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.

internet links

1. Sacramento Valley Astronomical Society

Features

1. Weight

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. 

These are the separate components:

1. Tube

2. Mirror and cell
3. Tripod
4. Mount
5. Altitude clutch
6. Counterweights

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:

1. .26-wavefront peak to valley
2. .038 RMS
3. .945 Strehl ratio

Its images are sharp -- the owner got her money's worth.

4. Mechanical performance

The telescope has these properties:

1. Flexure: there is little flexure (although some work was required to bring it under control)
2. Bearing smoothness:  machined Delrin-Teflon composite bearings are used, and -- in my opinion -- are smoother than the ubiquitous Teflon and plastic laminate combination
3. Cooling: because the mirror is completely exposed and the tube is open, cool-down time is minimal and fans are not used
4. Convenience: the entire tube rotates on bearings built into the nine-point cell and mount -- this makes viewing very convenient at all elevations
5. Balance: the telescope is balanced on both altitude and azimuth axes
6. Mount: the telescope uses a German equatorial mount designed for the North Pole -- thus, a "German alt-azimuth"

internet links

2. Galaxy Optics

Tube

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

The top end ring was routed from 4mm Alumalite, manufactured by Laminators Inc.⁶. Typically used in the sign industry, a 4' x 8' sheet weighs only 18 pounds. It is lighter than 1/8" aluminum.

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 for collimation.

I tried to use 4mm Alumalite for the tube's bottom end, but it flexed too much around the bolts. The final bottom end ring is machined from ½" aluminum plate.

4. Spider

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.

Because the Alumalite cannot be tapped with threads, the JMI⁷ NGF-DX2 focuser is bolted to the shelf. The hardware needed for this was supplied with the focuser.

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.

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internet links

3. W. M. Keck Observatory
4. Gemini Observatory
5. POP rivets
6. Laminators Inc. aluminum composite
7. Jims Mobile, Inc. focuser
8. Tasco finder
9. Tele Vue finder
10. Orion finder

Mount

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.

The design of this mount, which looks somewhat like a stunted German equatorial, has minimal machining costs. The design has only four aluminum parts that I couldn't make and had to be contracted. Other designs -- such as a fork or traditional Dobsonian -- would have required more machined parts, and would have cost more.

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.

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internet links

11. Hollywood General Machining counterweight
12. Celestron counterweight

Cell

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.

During setup, one of the bearings is removed so the cell can be lowered through the ring and come to rest on the rotation pads located under the assembly bolts on the underside of the base plate. It is then screwed back in place so the tube won't fall out of the mount.

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

To simplify handling and assembly, I made a carrier for  the mirror cell. The carrier is welded from ½" square bar stock, and has three holes that fit over the cell's assembly bolts.

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.

internet links

13. K. R. Anderson Co., Inc. source for Dow Corning 93076-2 RTV