 |
by Don Urban
Telescopes and the performance they deliver differ widely, just as do automobiles. They come in many sizes, in various types, at prices that range from $150.00 to $5,000.00 and higher. Just as cars are suited for specific purposes -- from small, inexpensive ones for commuting, to family oriented station wagons and vans, to 130 MPH sport cars -- telescopes are designed and built for specific purposes, some to show fine detail on bright objects and others to make invisible objects visible. All of them can be used to observe the thousands upon thousands of celestial objects just as any vehicle with four wheels and an engine will take you from coast to coast. It's how they do it that is most important.
WARNING!!! If you have ever been inside of a large department store, you've probably seen bright red telescopes in the camera department with big signs proclaiming they are capable of showing you the moon and planets at 450 power. I can't say this is a lie, because if they came with a tripod and mount as sturdy as the foundation of a house and a motorized drive to keep the object centered in the field of view of the eyepiece, you certainly would see something at 450 power. Whether it would be recognizable, however, is another story. At their lowest power they are well suited for observing the Moon, Jupiter, Saturn and Venus and very little else. I wish I had a dollar for every one of them that is sitting in a closet gathering dust because the person who received it as a gift could not find and see anything with it.
If you have less than $400.00 to spend on a telescope, your money would be much better spent purchasing a quality pair of 7x50 or 10x50 binoculars, a sturdy tripod to support them, and one or two books on introductory astronomy. Again, the key word is "quality." Be prepared to spend at least $125 -- $150 for a name brand. If, after twelve months, you have been able to find the Andromedae Galaxy, the Sagittarius Star Cloud, the Beehive Cluster, the Lagoon Nebula, and the Hercules Globular Cluster, you most likely are ready for that first telescope.
The three popular types of telescopes are refractors, reflectors, and Schmidt Cassegrain. Each will be described in detail as to how they work and what they are best suited for. Just remember, the best telescope is the one that gets used most often, and doesn't end up gathering dust in the attic because it is too cumbersome or complicated to carry outside and use.
You will hear or see telescopes referred to as a "4 inch" telescope. What this means is that the aperture (diameter) of the front lens of a refractor or the primary mirror in a reflector is 4 inches in width. A general rule with most telescopes is that you should not expect more power than fifty times the aperture. For example, a five inch telescope has a limit of 250X and a two inch, 100X -- under the best observing conditions (which rarely occur). This serves to reinforce my opinion that those little red beauties you see in KMart are not capable of standing up to their advertised claims! In fact, most of my observing with an 8 inch telescope is done at powers ranging from 50 to 200. The word "seeing" is a term used by astronomers to describe the steadiness of the Earth's atmosphere. When you can see stars twinkling on a cold winter night, seeing is considered poor and any bright object you look at with a telescope will appear to dance around and be impossible to focus. The best "seeing" occurs during the summer on warm, humid nights when atmospheric turbulence is at a minimum.
Contrary to what most newcomers think, a telescope's primary function is to gather and concentrate light to a focus. This is why objects that are invisible to the naked eye appear bright and plainly visible through a telescope. Consider that each of your eyes is a telescope that has a maximum aperture of about 7 mm and will allow you to see stars as dim as 7th magnitude under extremely dark skies. A 102mm (4 inch) telescope has an aperture almost 15 times wider than your eye, but in total area has the ability to gather 212 times more light. The 7x50 binoculars mentioned before will transmit 51 times more light to your eyes.
The rule of thumb is that, as you double the aperture, you quadruple the brightness. Conversely, as you double the power, brightness and contrast decrease by a factor of 4.
Sadly, Americans living within 100 miles of sprawling, light polluted cities, rarely have the opportunity to witness dark skies and the splendor of the Milky Way except when they are vacationing in the country, far from city lights and smog.
Quality refractors range in aperture from 3 to 8 inches, with 5 inches the largest that should be considered by a newcomer, and will deliver the sharpest images of the Moon, planets, double stars and star clusters. Inch for inch of aperture, they are the most expensive type of telescope, since they employ precision ground lenses and no mirrors. Be prepared to spend between $800.00 and $2,500.00 for one in the 3 to 5 inch size. Above this size they become prohibitively expensive and more difficult to handle. They are also the most durable -- assuming that any telescope you own will be handled with care -- and lightest in weight, rarely if ever requiring adjustment. They have no open ends into which dust and dirt can collect. They take up the least space when stored in their protective cases and are easily transported.
Reflectors, properly named Newtonians after their inventor Isaac Newton, offer the largest aperture for the least money. This is because they use highly reflective mirrors that are considerably less costly than lenses of the same size, at least in sizes up to 16 inches,. A 4 1/2 inch reflector can be bought for under $500.00. The most popular entry level Newtonians range in size from 6 to 10 inches (bigger and they become heavier and longer) and are the best telescopes for observing deep sky galaxies and gaseous nebula because of their greater light gathering ability. Among its disadvantages it can frequently require adjustment of the optics, and the mirror never fails to gather dust, even in normal use. In sizes greater than 4 1/2 inches, their tube assemblies increase proportionately in length and weight. They must be handled very carefully and not be subject to bumps or being stored in an upside down position! Also, mirrors usually require periodic resurfacing since airborne pollutants combined with moisture are corrosive to reflective coatings.
Before going any further, the subject of tripods and mounts must be addressed. A fine telescope can be rendered useless if it is supported by a cheap, flimsy tripod that will transmit movement to the telescope. Light breezes and footsteps, not to mention turning the focusing knob can will cause the image to shake so much that observing any object will impossible. By the same token, even the department store telescope will deliver acceptable images if it is mounted on a firm, steady tripod. This having been said, be prepared to spend between 25 to 50 percent of the cost of the telescope for a quality tripod.
Schmidt Cassegrain telescopes were first mass produced in the early 1970's and they have become the largest selling type in the 8 to 11 inch sizes. Because they employ features from both refractors and reflectors, they can be built with extremely short tubes. This serves to make them compact and portable, allowing for larger apertures with less weight. They are sealed from the elements by a glass corrector plate at the front of the tube and like a refractor, rarely if ever require adjustment. They are extremely well suited for astrophotography and some manufactured today come with built in computers that can automatically find any object in the sky. They are also made in sizes as small as 4 to 5 inches, but unlike refractors, the price does not drop very much for smaller sizes. Basic setups can be had for just over $1,000.00 but again, like cars, options can increase the final price to twice that amount.
My final recommendation to anyone contemplating the purchase of a telescope is to purchase a copy of "Sky & Telescope" or "Astronomy" magazine and study the advertisements. When you are ready to take the plunge, purchase from a reliable telescope dealer who will provide "after sales" support. Joining a local astronomy club and attending its observing nights will allow you to become familiar with the kinds of telescopes referred to in this article. This is one sure way to "try before you buy."
