Choosing a Telescope Part 1

Although it is possible to have many hours of pleasure observing the night sky using the naked eye or binoculars, at some point in every amateur astronomer's career the decision is made to buy a telescope. The process of buying a telescope is a difficult one and mistakes can be expensive. Before you reach for your credit card, you really need to carefully weigh up some considerations.

Portability

Portability is a major issue. Differences in size and optical design create vast differences in telescope portability, and any telescope that you take out and use will be far better than one that sits in the cupboard because it is too heavy or too cumbersome.

Remember, the best telescope is the one that gets used most often. A small portable telescope with a fast set-up time will show you far more than an enormous light bucket which takes ages to set up and therefore is rarely used. Remember also, that if you live in a city, you may want to travel to a less light polluted site with your scope to take advantage of the dark skies. Obviously, the question that then needs to be asked is whether the telescope can fit in your car. Finally, remember that the telescope is only half the system, the mount must also be easy to set up if the system is to be easy to use. Alt-azimuth mounts, those which move up-and-down and side-to-side are probably best for portability but are useful for visual observing at low powers only since stars move in an arc across the sky, necessitating movements both horizontally and vertically to track celestial objects. If you want high magnifications, or the option of doing some astrophotography, an equatorial is needed which can track the movement of stars with one motion. Unfortunately, the equatorial mount is considerably less portable and has a longer set up time than the Alt-azimuth mount.

Aperture

The most important thing in determining the optical performance of a telescope is the aperture, the diameter of the main lens or mirror within the telescope. The more light you can gather, the fainter the things you can see. In addition, larger telescopes have higher resolution, that is they can be used to observe finer detail on the planets or split tighter double stars. There are important qualifiers.

The LXD75 - a typical small 6" telescope.

If a telescope is built badly the telescope will perform poorly. However, most manufacturers routinely turn out units that are reasonably good. Bad ones turn up, but major manufacturers will often fix or replace a poor telescope if you recognise that you have one.

Different optical designs perform differently. Schmidt-Cassegrains, Newtonian reflectors, and refractors all have good and bad points. Variations however, are relatively minor. It is usually adequate to assume that all well built telescopes of a given clear aperture and given quality of optical craftsmanship have a similar optical performance: Real differences will correspond to changes in aperture of usually no more than 10% to 20%.

Atmospheric turbulence, known as seeing, limits the ability of a telescope to show detail, and sky brightness limits its ability to show faint objects. For resolution, i.e. seeing fine detail on the moon or planets or splitting close double stars, atmospheric turbulence usually means that an aperture of no more than 10 inches (250 mm) is useful under the typical Irish skies where I live. Your location may offer better skies and the possibility of regularly using a larger telescope. Poor seeing usually hits large telescopes harder than small ones. When seeing is poor, there may be no reason to take out and set up a big telescope. If you always observe from such conditions, you may have no reason to buy a big telescope. Yet, even in a bright sky, a large-aperture telescope will show fainter objects than a small one. Also, many of us have found dark-sky stable-seeing sites within a reasonable drive of home.

Notwithstanding these caveats, aperture wins big. If you buy the finest 4-inch (100 mm) apochromatic fluorite refractor in the world it will be beaten by an 8 inch Newtonian costing a quarter of the price of the refractor!!

Finally, it is worth noting that the quest for aperture unfortunately directly contradicts the desire for portability.

Performance

The most optical performance per unit of clear aperture comes from modern, high-quality apochromatic refractors but they are outrageously expensive compared to other designs of the same aperture. Also, in sizes much above four-inches in aperture, the tubes are generally long enough to make the whole instrument cumbersome and heavy. However, these instruments excel at lunar and planetary work and apochromats, because of their wide flat fields, are often used for astrophotography.

The most optical performance per unit of portability comes from Catadioptric Schmidt-Cassegrain and Maksutov designs -- but they are still quite expensive. There's a qualifier here: What makes them portable are short, stubby tubes, but for apertures of four inches or less portability of all telescopes is dominated by the clumsiness of the mount, so the portability advantage of Schmidt-Cassegrains and Maksutovs diminishes. Catadioptric telescopes are a great all-round telescope and are probably the most popular world-wide at present.

The most optical performance per unit of cost comes from Newtonians on a Dobsonian mount. Compared to other telescopes of the same aperture, they are clumsier than Schmidt-Cassegrains and Maksutovs, but not nearly as awkward as equivalent achromatic refractors.

Price

It's fair to say that a decent telescope will cost of the order of a few hundred Euro (€) or Dollars, minimum, for something like a 6 inch Dobsonian reflector.

A telescope such as this will show you many thousands of celestial objects and could keep you occupied for a lifetime. You may want Digital Setting Circles (DSC), a computer fitted to your mount which guides you to objects in the sky or even GOTO capability - just dial in the name of the object and the telescope will turn to the correct spot in the sky. If so, you will pay more. Prices start at about €900 for a Dobsonian telescope with DSCs and €1800 for a decent, 5 inch plus, GOTO scope. (U.S. prices for equivalent equipment will be lower, but that's another story). This may seem like a lot of money but it's important to appreciate that a telescope is a lifetime investment.

A good telescope will be useful in 100 years time. In addition, telescopes hold their value well; compare that to, say, a computer which will be both worthless and useless in about 5 years time!! Telescope prices are at an historical low at present as a proportion of wages. If you do decide that few hundred Euro or Dollars is too much, please do not buy a cheaper telescope. You will inevitably be disappointed and will have wasted your money. Worse still, you will probably be put off observational astronomy for life, cutting yourself off from a hobby which can give you enormous pleasure. Make sure you avoid mass-marketed junk refractors 2.4 - 4-inch refractors, advertised as high-power and sold in department stores and camera shops.

Part 2 >