Sunday , April 20 2014
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20” Telescope

By Mike P. Usher

The 20” home-made telescope with the wheels still attached. Model is Jackie Richards of the Everglades Astronomical Society. PHOTO BY MIKE P. USHER

The 20” home-made telescope with the wheels still attached. Model is Jackie Richards of the Everglades Astronomical Society. PHOTO BY MIKE P. USHER

Why build a twenty-inch telescope? For me, the reason was I could. It greatly helped that I enjoy being the center of attention – and this telescope pretty much guarantees that when we both appear together. I usually claim it’s the largest telescope in Southwest Florida, but I do not actually know this for a fact. There have been rumors reaching me of larger ones hidden away in unspecified locations, but if they exist, the owners must keep the telescopes to themselves. On the other hand, anyone can look through mine if they attend one of the numerous free, or inexpensive, events I will be taking it to throughout the winter.

The term twenty-inch telescope comes from the diameter of the mirror, which is installed in the box on the bottom of the telescope. Quality telescopes are always described by the diameter of the main objective, being a mirror or a lens. Usually an f-ratio is thrown in, which is the ratio of the objective diameter to the overall focal length. If you are a camera buff this is a familiar concept as camera lenses are described in exactly the same way. In this particular case my telescope is an f/5 which means the focal length is 100 inches. For a camera this would be described as “fast;” in a telescope it means it shows a wide field.

A focal length of “only” a hundred inches does have certain advantages; it means a 6’2” man (me) can stand with both feet on the ground when looking at much of the sky and just a short stepladder when the telescope is pointed straight up. Another club member I know has a slightly smaller objective but a longer focal length – he spends much of the night perched on top of a ladder. It is possible to get a mirror with a smaller f-ratio of 4 or even 3.5, but only at a much higher cost; f/5 is a nice compromise.

Nevertheless, this thing is a beast to transport. Weighing in at 197 pounds, it requires careful thought, planning, some muscle power and a large vehicle. The telescope is too large to pass through most doorways, so I get it in and out of the house through a sliding glass door. I use a pickup truck for transport, although an SUV or a minivan with the seats popped out would work as well or better. Creative people could even use a hatchback. Two wheelbarrow handles with matching wheels make the telescope fairly easy to roll up a ramp into the truck. Tubing and the secondary cage on top of the telescope are removed for transport and assembled on site.

Next time: How is this telescope put together?

If your non-profit organization would like to have a telescope viewing event for free by the Everglades Astronomical Society please contact Charlie at cpaul651@earthlink.net.


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