Tips assist choice of telescopes

October 5, 1985

Light-gathering lens sets quality;
learning before buying recommended

COMET HALLEY IS IN THE SKY NOW, AND DURING THE next four months those of us in Manitoba will have our best opportunity to see it. It never will be easy to observe, and if you are going to participate in this once-in-a-lifetime event, now is the time to begin to plan.

There is no doubt about it, the best way to have a good view of Comet Halley from these latitudes is with a telescope. In addition to taking courses, and reading up on the subject, many people are considering the purchase of a telescope. I think this is wonderful, and my only caution is that you learn about telescopes before you buy, and that you plan to use the telescope to study all aspects of the cosmos, in addition to the famous comet.

Telescopes do two somewhat different things. Everyone knows they make big ones out of little ones. Magnification is only half of the story. Many of the things in the sky are not particularly small, but they are faint. Some telescopes are very good at making bright ones out of faint ones. Much of the business of choosing a telescope is deciding which of these features is more important to you, and how much of one or the other you can afford. Most people try to compromise between cost, convenience and performance.

A telescope is basically a simple device. First, there is a lens to collect light and focus it. This lens actually forms an image of the object that it is pointed at. The quality of that prime-focus image determines the overall optical quality of the instrument. Second, there is a magnifier, called an eyepiece, to allow you to look at the prime-focus image. It is this eyepiece that determines the power of the telescope. Do not be fooled by someone telling you about high powers — if you select the right eyepiece any telescope can be any power. Finally there is a mount to support the telescope looking at the right part of the sky.

The nature and quality of the telescope is determined by the big lens that collects the light. When people compare telescopes, the first measurement they will tell you is the diameter of the lens. The rule here is to buy the telescope with the biggest collecting lens that you can afford, or that you are willing to lug around.

A few years ago, some friends of mine conducted an interesting experiment that demonstrates that bigger is better. They compared a seven-inch (remember, that is the diameter of the collecting lens) Questar with a 14-inch Celestron. The Questar is an exquisite little telescope made to such high standards that the manufacturer claims no amount of money could improve the product. The Celestron was much bigger; both cost (at the time) about the same. It was not a comparison of equal telescopes. It was a comparison of value for equal money spent.

Magnification is expensive

A 14-inch telescope is very large by amateur standards, and the big lens easily out performed the Questar in light gathering ability. Remote cities of stars called galaxies, and glowing patches of interstellar gas called nebulae, were much brighter and easier to see with the Celestron.

The place where the Questar is at its best is demonstrating high resolution of very small objects. Here is where a high quality is required to see fine detail on the surface of the planet Jupiter, to glimpse the dark gaps is the rings of Saturn, and to see that a close pair of stars is really two separate spots of light.

There is a limit to how well a telescope can resolve small objects, and, in addition to general quality, the main factor is the size of the collecting-lens. Although the Questar can out-perform almost all telescopes its own size, and even those a bit larger, the sheer size of the Celestron made it the winner in resolution too. ‘

So, why does anyone buy the smaller, very expensive, telescope? Because its perfection makes it beautiful, and its small size makes it portable and convenient to transport and use. The Celestron-l4 is almost a two-person job to move and operate.

The next thing to decide is what you want the telescope to do. Most people who are new to astronomy want a telescope that can do everything. Magnification is expensive; it requires costly precision optics, and a very sturdy support for the telescope. Often the support costs as much as the tube with glass in it.

The main problem in viewing the comet is its faintness. A telescope that solves the problem by simply collecting lots of light and funneling it into your eye is called a rich-field telescope. It is characterized by its short length for the diameter of the collecting-lens, and the fact that it works best with low powers. The low power means that the optics need not be very expensive, and, in the case of the small ones, a heavy mount is not required. Because of the portability and low cost, these telescopes are popular, and well used.

So what ts the bottom line? Rich-field telescopes include binoculars. A good 7×50 pair will cost $100-$250. (The collecting-lens is two inches in diameter.) A small rich-field, with a 5-1/2 inch diameter lens is about $500. You will need a good camera tripod for it. A compromise telescope (one that does everything: good light grasp, capable of high magnification, and is easily portable), with an eight-inch lens and a heavy mount runs about $1,700. I don’t know many people who can afford a Questar-7, or a Celestron-14, but they cost $11,000 (U.S.), and $8,500, respectively. The best time to observe the comet is around the dark of the new moon. The next new moon is Oct. 13. It’s time to stop dreaming and start observing.

One of the ways to learn all about the comet is to take a course. This fall the Manitoba Planetarium, with the support of the Scientific Research Society, Sigma Xi, is presenting an illustrated lecture series focusing on the return of Comet Halley. Here is the schedule. Oct. 10, Comets and the History of Astronomy, presented by Wm. Peters, Planetarium Producer. Oct. 29, Observing Comet Halley, presented by R.J. Ballantyne, Plantetarmm Director. Nov. 14, Comets, Meteoroids and the Formation of the Planets and Comets, presented, by G. Clark, department of earth studies, University of Manitoba. Nov. 21, The Modern Astronomy of Stars Planets and Comets, presented by R. Bochonko, professor, department of mathematics and astronomy. Dec. 5, Solar Systems Galaxies & the Universe, presented by M. Clutton-Brock, cosmologist, department of mathematics and astronomy. The sessions will be held in the Planetarium Auditorium, 190 Rupert Avenue, from 7:30 to 9:45 p.m. The cost is $6 a lecture, or $25 for the series. There is a family maximum of $60. To register, call the Planetarium.

In 1985 Robert Ballantyne was director of the Manitoba Planetarium. His weekly column in the Winnipeg Free Press, called Sky Watch, focused on the return of Comet Halley.

© 1985 Robert Ballantyne and The Winnipeg Free Press

Sky Watch - Robert Ballantyne | Winnipeg Free Press


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