March 1998 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - March 1998 Issue

The Telescope Nut
by Jeff Baldwin

The Telescope Nut

The Telescope Nut will cover topics of telescope making, primarily mirror making. This will include grinding, polishing, figuring, repairing and testing mirrors, as well as using, choosing and knowing about telescopes. Paraboloidal mirrors are the mirrors used in Newtonian and Dobsonian telescopes, and that will be where most of my energy goes. There will be discussion on classical cassegrains, refractors, Schmidt cassegrains, Gregorian cassegrains, and other telescopes, either by myself or by other opticians who can contribute to this forum. Constructing telescopes involves carpentry, cabinetry, woodworking and other skills for which I am not very famous, and at those times I will bring in other contributors. Now that I have set a framework for my writing, I will no doubt evolve into something totally different, just as I did with Deep Skies. Who knows?

Choosing a Project:

What size, design and focal ratio telescope should I make? This question is almost unanswerable. Each person has different parameters which they use to answer that question. Lots of books tell the beginner to start with an 8" f/6 paraboloidal mirror for a Newtonian reflector. Here is my logic on the subject.

How big a mirror can I work? How big a mirror can I afford? How big a mirror will my [Mom, Dad, wife, husband, significant other] allow me to work? How big a telescope can I store, transport, carry, share, and setup? What types of objects will I be observing?

It may be best to make the biggest telescope that these parameters will allow. Not all telescopes have to be big, but generally a larger optic will produce more astronomically favorable images. As the aperture of the optic gets bigger the resolving abilities increase, and the light grasp increases exponentially.

Hold it: aperture? - resolving? The diameter of the optic is referred to as its aperture. Resolution is measured by how close the angular measure is between two points of light that the telescope can separate as two points of light rather than an emergence into a single point. OK, back to what I was saying, the larger a telescope's aperture is, the finer it can resolve, and the light it can pass through its optical path increases exponentially. In comparing a 5" telescope with a 10" telescope, if all other conditions are neutral, then the 10" telescope will distinguish two points separated by half the angular distance that the 5" telescope can split, and the 10" telescope will scoop up not 2 times as much light, but rather 4 times as much light as the 5" telescope.

Types of objects that astronomers look at are sort of categorized. Solar/Lunar/Planetary is a type of observing, and Deep Sky is another type of observing. Generally speaking, but not an absolute rule, long skinny telescopes have done better on the planets and short fatter telescopes have done better on the deep sky. All rules can be broken, though. The best view of Saturn I have seen in the last 13 years was through David Schamber's 22" f/5.

A good bet for figuring out which telescope to get is to hang out at star parties with people who have been doing it for a long time (such as club members) and use their equipment with them, and/or check out a variety of club telescopes and see which direction you end up being interested in. Some folks have Dobsonians, some have Schmidt cassegrains, some have refractors, and no two telescopes are the same in every way. After viewing through many telescopes you will settle on what you like to look through, and that will probably be a better way of choosing than to jump into a project with no previous knowledge.

Next time we will start digging in. Be sure to visit the A.T.M. link on our club web-site.

Clear Glass...Jeff Baldwin
For more information on Telescope Making jump to the ATM page.