Telescopes

What astronomy resource is complete without some information about telescopes? Well.....none...so here is some info. It's not actually history, but we shall deal with that at a later date.

This page is intended for those searching for some basic information about telescopes. Maybe you are interested in learning for learning's sake, or maybe you are actually about to spend vast quantities of money on a first telescope to enrich your astronomical interests. Either way, here is some good information for you. Also assumed is that you have already used binoculars on the sky for some time before reading this. Often, a good pair of binoculars is the very best way to start learning observational astronomy.

Learn all you can:

Step one: Always learn all you can about something before spending a lot of money! Some telescopes cost well into the thousands of dollars, which is a little more than many people can afford. Read through old issues of Astronomy Magazine and Sky & Telescope for hardware reviews and prices. Seeing as you have internet access, read through the SCI.ASTRO.AMATEUR news group for people commenting on their equipment (complaints and raves are always being made about one manufacturer or another). Sometimes the manufacturer will have a representative out on the net answering questions, too. You can also call manufacturers for informations packets, although this too can cost you. One company actually sends out a video detailing your choices along with a packet of advertisements: all for $15.00(US).

Telescope types:

There are two basic types of telescope: Reflector and Refractor. Another type combines the two and is called a Catadioptric system.

Reflectors typically consist of a main light-gathering mirror, called the primary mirror, and one or more smaller secondary mirrors that capture the primary's light and aims it towards an eyepiece. The most common reflectors are Newtonians, consisting of a primary mirror, one secondary mirror and an eyepiece:

A Newtonian Reflector

Newtonians have the benefit of being more affordable at larger apertures (primary mirror diameter) than refractors of the same diameter.

Refractors are simply lens systems that have a large light gathering lens, called the objective, and an eyepiece. They are typically more expensive than reflectors because of the higher cost associated with lens making.

$Picture: Refractor$

A Refractor Telescope

Last is the catadioptric telescope which has been made increasingly popular by companies like Celestron International and Meade Instruments Corporation. These systems consist of both lenses and mirrors. First, light passes through a "corrector plate", which is really a large lens that corrects for certain optical characteristics of the primary mirror. Light passing through the corrector plate then bounces off of the primary mirror and onto a secondary mirror attached to the corrector plate. From there it can be reflected to the eyepiece. In the case of cassegrain systems, the light is passed through the center of the primary mirror then to an eyepiece.

A Catadioptric Telescope (Schmidt-Cassegrain Variety)

Which one to choose?

There have been many suggestions concerning this topic! The best way to go about this is to try as many as possible before buying. Newtonian reflectors are in general more affordable than either refractors and catadioptrics, although they require alignment and maintenance more often. Some prefer refractors for their low maintenance and the lack of a secondary mirror's light obstruction. The modern schmidt-cassegrains are popular because of their portability and the fact that they are more sealed than newtonians.

Listen to well-seasoned amateur astronomers. People who have spent many cold nights under the stars will have the wisdom to know good systems from poorly made ones. When discussing brand names, however, be aware that no one brand is usually better or worse than the other; rather it is the quality control of their manufacturing facilities that should be questioned. I have seen both superb and very poor optics come from the same manufacturer in a month's time. This would lead me to say: Try the actual telescope before buying it! Also ask about customer support, warranties, service, etc...

Consider portability: Are you going to travel with the scope? Will it be permanently mounted? Does it come with a transporting case? Is the mounting portable? Does it require more than one person to assemble for a night's observing? Will the system and accessories fit into your car? I have found that a lot of good equipment goes unused because of the difficulty involved in setup and use.

What will you be observing? Many prefer long focal length systems for planetary study and short focal lengths for wide-field, deep-sky observing. That typically leaves refractors for planetary work and reflectors for deep-sky, but catadioptrics can do both fairly well. Try them all at a star party if you can, just to get a sense of the different systems available.

You will also want to consider the mounting....

Mounts: what type?

There are two basic types of mounts: Equatorial and Altitude-Azimuth (Altaz). Let us first define the two. Altaz mounts are like old camera tripod mounts. The scope can rotate around two axes: altitude (up/down) and azimuth (left-right). Equatorial mounts are also a two-axis design: right ascension and declination. Motion around the right ascension axis is about a line pointing to the north celestial pole. This allows the scope to follow a star by moving only one axis, since all stars revolve around the celestial poles. The declination motion adjusts the scopes position north or south of the celestial equator, an imaginary extension of the Earth's own equator.

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