sg2

Telescope Specifications

1.) Dobsonian

2.) Truss-type construction

3.) f/5, 17.5" main mirror, temperature controlled

4.) 3.1" secondary mirror, temperature controlled

5.) 4.5" reflector telescope as finder

6.) Crayford focuser

Design Issues

It wasn't that long ago that I never would have thought of building a telescope. And my first telescope, Stargate-1 , hasn't yet seen first light, and I'm already designing its successor. Why? Already, much to my dismay, everything has finished out over-size and over-weight. For example, the mirror box, with mirror in place, will weigh a whopping 106 pounds! And the combined height of the ground board/rocker/mirror box/secondary is 5-1/2 inches too high to fit stacked together in my Jeep. With that, below are the original five design objectives, plus five new objectives:

1.) Rich-field telescope

2.) Easy to transport and set up by one person

3.) Quick assembly and disassembly

4.) Light, and compact when disassembled

5.) Low-cost when compared to commercially-made telescopes

6.) The entire telescope (excluding ground board) will be made from welded aluminum.

7.) The unassembled telescope (excluding truss tubes) will be able to fit stacked into my Jeep.

8.) The mirror cell will be designed around the mirror cooler (provided that it passes field trials).

9.) The mirror box will be much shallower, as will the rocker.

10.) The combined weight of the rocker and mirror box shall not exceed 80 pounds.

To say the least, I have learned a lot from my first experience. And before I finalize a design, I will learn as much as I can from the Stargate-1 before building the Stargate-2. At least, I will be able to reuse a large part of it, including the secondary cage, truss tubes, side bearings, ground board, primary mirror, mirror cooler, and control and power electronics. Also, I plan to add several new experimental features to this design:

1.) A small fan supported above the main mirror, in the shadow of the secondary mirror.

2.) Adjustable stoppers made of Delrin intended as protection for the mirror in transit.

3.) An iceless cooler. Originally for the mirror cooler, ice-cooled water was to be the coolant. But I have developed a system to keep the coolant cold enough to be useful.

The mirror box was the main focus of the Stargate-1's redesign. A shallower box was a prerequisite in order to meet the 33 inch height requirement of my Jeep's rear hatch. But with the mirror (the bulk of the weight) so close to the center of rotation, balance became a greater concern in the design process. Assuming my calculations are accurate, the need for counterweights should be less than ten pounds. In fact, the reflector telescope that I bought to use as a finderscope does double-duty as a counterweight.

Weight was another design issue. I will build the entire mirror box out of welded square aluminum tubing and plate. Numerous pieces in key places will be welded to make the box rigid. In my newest design, I shortened the box again, adopting a more mininmalist approach- I reduced the weight to nearly 25 pounds (as opposed to 31.5 pounds for the preceeding design). Initially, I decided to leave off covers for light shielding. As shallow as the box is now, I'm not sure how much of a benefit they would provide. But then I realized how open the mirror would be, particularly in transport. The frame was supposed to be the hand-holds for carrying the mirror box. Instead, I had to include a pair of handles.

The biggest difference between this mirror box and its wooden predecessor are the adjustable stops mounted in the corners. These stops will be made of machined Delrin parts. This makes for a more minimal mirror cell. The sling attaches, now, to the inside of the mirror box. On one side, the sling is held to the box with screws. On the other, it is fastened to a bolt, which is in turn, attached to the mirror box. I drilled a pair of very fine holes into the bolt, then carefully tapped them, which worked well. The sling is cinched tight, then the nut on the bolt is tightened.