Buying a Telescope Part One -- Refractors


The refractor is generally the least expensive instrument to buy and also is the easiest to maintain. This telescope is based on the same concept as a telephoto camera lens or binoculars. A big lens at the front of a long tube (called the objective lens) refracts (or bends) light rays to a small point (focus) at the back of the tube. A second lens (commonly known as the eyepiece) magnifies the visual size of the object you're looking at and adjusts the precise focus for your eyes.




The most economical of these type telescopes come from companies like Mead, Swift, Bushnell or Tasco. They have a rudimentary up and down, side to side motion (generally called "alt-azimuth") a small spotting scope and a right angle prism to make looking straight up into the sky less head-wrenching. They usually come with two lenses, offering 50 and 100 x (times or power of magnification). These are excellent for viewing the moon and brighter planets. They cost around $100 to $200.

A slightly better telescope (for some people, at least) has what is called an equatorial mount. This mount, when adjusted properly, allows you to follow the motion of the moon, stars and planets with a turn of a single control or may even allow you to put a motorized motion control device (called a clock drive) that automatically moves the telescope to keep the celestial object centered at all times. This clock drive allows you to take time exposure photographs of deep space objects with great accuracy if you have a special mount that lets you put a camera over the lens or inside the tube at the point of prime focus. Some of these equatorial mount telescopes also come with computer controls that will actually position the telescope for known objects (or may let you add on such a device at a later time -- upgrading a telescope is an important point to remember when buying a more expensive instrument). Instruments such as these are generally available from companies like Meade, Unitron, Bushnell and Swift. There are also many makes of quality custom telescopes (see listing at the bottom for suppliers).

Telescopes with equatorial mounts usually are priced from $300. The clock drive is an extra $150 to $200. The computerized star finder -- if available for a given telescope -- is often more than the price of the telescope, itself. Another $300 to $500. Making the total for an entry level semi-professional telescope around $500 to $850.

You actually don't get much more light gathering power or magnification with this extra expensive type of telescope. You just get a more professional mount to facilitate tracking of deep sky objects.

The amount of light a given telescope gathers and the maximum magnification available is determined by the size of the objective (or front) lens. Rule of thumb is 50x (power or magnification) per inch of usable surface area. These small refracting telescopes generally only have a 2.4 or 3" lens, hence you are limited to between 100 and 150x of magnification. You can go a bit higher, but the objects start getting fuzzy.

Good eyepieces to have, include: 6mm, 12.5mm and 20mm. A 4mm can also be good for maximum magnification, although when you get into a small, powerful eyepiece like a 4mm you need a good, high quality brand of orthoscopic design. Usually denoted by the letter "O" before or after the 4mm. The Huygens (denoted by the letter "H") or Kellner (the letter "K") design is adequate for the 6mm to 20mm eyepieces. The standard, professional physical size for an eyepiece is 1 1/4" while some lower cost telescopes use a smaller diameter size that is only .965" -- which means you may have problems finding a good selection of eyepieces and accessories such as a barlow lens (a device that doubles the magnification of a given eyepiece, but it also darkens the object and lowers the clarity).

You also want to make sure the objective lens of the refractor is airspaced not cemented, for longer life and cleaner viewing. Cemented lenses tend to darken and frost after a few years, the cement can also become molten in very hot weather, such as direct sunlight on a hot summer day. Multi-coated or fluorite coated lenses are a plus!


* * *

Some sources of retail telescopes include:

California
Skyoptics
Star Safaris

Colorado
The Science Company

Illinois
My Telescopes

Minnesota
Opticmall

New Hampshire
The Telescope Store

Oklahoma
Astronomica

British Columbia, Canada
A and A Astronomical

Ontario, Canada
Khan Scope Centre

Telescope Makers include:

Meade
Stellarvue
Tasco Sales
Tele Vue Optics
Unitron Telescopes


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