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Telescope Basics
How to choose a telescope

There are many factors to consider when choosing a telescope. Making the right decision can be the difference between it ending in the closet for years vs spending many nights in the backyard or observing site. There are a lot of different types of telescopes and each has a specialty. This web site contains several pages that will help you make the decision that is tright for you. Before you head out to make your purchase I urge you to at least read this first page.

While cheap department store telescopes are often sold by marketing their "power" or magnification, the truly important factors are aperature and configuration. Any telescope can give a highly magnified image but if that image is blurry or washed out who cares.

The terms defined below will help you understand what is really important when trying to determine the quality of a telescope. You should understand these terms before choosing a telescope.

Important Terms
Read through all these terms because they are all related to each other. For example, 'aperture' determines 'resolution' which determines 'maximum magnification'. They are all important when selecting a telescope.


How fat is the telescope tube? Aperture is the measure of the diameter of the main mirror or objective lens of a telescope. In general, the larger the aperture, the better the resolution and the fainter the objects you can see.

The most important factor for gathering light is aperture. When you look the tube of a telescope, a "fatter" tube has a larger aperture. In other words the bigger the circle at the end of the telescope closest to the stars, the bigger the aperture.

Telescope manufacturers usually advertise telescope aperture measured in millimeters or inches. You may see an add in a magazine for a 8" LX200, or a 12.5" inch Newtonian. On the other hand you may see a 80mm refractor, or a 125mm mak-newt. These numbers weather in millimeters or inches represent the diameter of the primary mirror in a reflector or the primary lens in a refractor.

When it comes to aperture a reflector gives the most for your dollar. This is because it is easier and thus less expensive to create a large mirror than a large lens. This is why the very large telescopes you see at observatories are mirror based. On the other hand a good quality refractor while expensive will often give better definition or contrast.

Focal Length (determines the power of magnification)

The focal length is the distance from the primary mirror (or objective lens, for refractors) to the focal point (which is also called the prime focus).  The focal point is where the light rays converge.

The longer the focal length, the more magnification you will get for a given eyepiece.

Focal Ratio
  Simply the focal length divided by the aperture. You often see this describes as the F value of a telescope i.e. F4 or F5. You need to be sure you are using the same units when performing the division.
  Angular resolution is the ability of a telescope to show fine detail. The greater the aperture of a telescope, the more detail it will reveal. The theoretical angular resolution of a telescope is equal to 4.56 divided by the telescope aperture in inches. The lower this number is the better the resolution. This number is referred to as "Dawes Limit."
  Definition is a telescope's ability to reveal the contrast between two areas having nearly the same brightness in an extended object image. This is especially important in planetary observing. Dust, bad coatings, thermals, poor atmosphere ic conditions (seeing), or optical aberrations can upset a telescope's contrast factor, thus diminishing its ability to reveal low-contrast detail. Of these, poor seeing is by far the most frequently encountered.

Telescope Components

While there are many parts to a telescope, a telescope consists of three basic components. The job of first part of the telescope is to gather light from planets, stars and galaxies. The second part, the eyepiece is used to magnify the image that is gathered by the first part. The third component is the mount.

Main Optical Tube
The purpose of the main optical tube is to gather light. The diameter and composition of the primary surface determines how much light is gathered.
  Type of Telescopes
    Refractor Light is gathered and refracted by lenses.
    Reflector Light is gathered and reflected by a set of mirrors.
    Compound Light is gathered and focused by a combination of lenses and mirrors.
    The materials that make up the telescopes optics determine haw accurately the light is gathered and focused at the eyepiece. Type of glass, coatings, and smoothness are all important factors.

Type of Mount
  Equatorial - Easily follows the celestial equator.
  Alt azimuth - Altitude (up and down) and Azimuth (Side to side)
  Fork - can be configured and equatorial or alt azimuth.
  Dobsonian - A popular type of mount for large hand operated telescopes.
  Manually Controlled or Go-To (computer controlled)

There are many types of eyepieces each with their own benefits. It is important to understand the following basic terms when selecting an eyepiece.
Eyepiece Focal Length
(Magnification = telescope focal length ÷ eyepiece focal length)
  The magnification is determined by dividing the focal length of the telescope by the focal length of the eyepiece. So if your telescope has a 1000mm focal length and the eyepiece has a 10mm focal length you will have 100x magnification. So as you can see if you have a set of eyepieces, say 32mm, 26mm, 9mm, and 6mm, you have four different magnifications or "powers" for your telescope.
Apparent Field of View
  The apparent field of view (AFOV) and is a fixed property for each eyepiece design. For example, Plössl eyepieces have an AFOV of 50º, Radians have 60º, Panoptics have 68º, and Naglers have 82º.
True Field of View
  Amount of sky you will see. Eyepieces also determine the true field you see in the sky. To calculate the true field of view that you will see (in degrees), divide the eyepiece field stop diameter by the telescope's focal length and multiply the result by 57.3: 
Exit Pupil
  The exit pupil is the image of the objective that is formed by the eyepiece. It's where you place your eye to see the full field of view. For reflector telescopes, it's best to avoid exit pupils larger than 7mm or smaller than 0.5mm. Refracting telescopes have no upper limits on exit pupil sizes.
  The configuration determines the values for all of the above mentioned attributes. There are many configurations of eyepieces. Each type is suited for viewing different types of objects in various telescopes. Some are suited for planetary observation, some for deep space objects.

  The diameter of the focuser, 1.25", 2" which determines the eyepieces you can use. Not all 2" focusers are equal. The quality of the manufacturing and the gearing determine how accurately you can focus on an object. Some of the better focusers have a course and fine adjustment.

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Try them out!

The most important thing I can stress when thinking about the purchase of a telescope is to visit a local amateur astronomy club. You will be able to try some different types out and ask questions. talking with amature astronomers that have used different types of telescopes will certainly help you make a good choice when choosing a telescope to fit your needs.

Amateur astronomy clubs exist in every state. Every club I have visited has several telescopes for use by members. Clubs also have "public nights" where they bring the telescopes out for the public to use. If you attend, don't be afraid to ask questions about the equipment.

Usually there are telescopes outside with volunteers ready to assist you. If it is not crowded, ask if you can't try finding something in the scope. Ask if they have a suggestion for an object to find. Ask them to step you through the process of hunting for the object and zeroing in on it. Ask them what types of telescopes they own and which ones they use most often.

Becoming a member of a club also has it's rewards. Clubs have "member nights" which are less crowded and give you an opportunity to work hands on with a seasoned astronomer.

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