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Telescopes – Beginners Guide – Basics December 15, 2009

Posted by aquillam in Astronomy.
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Basic Telescope Properties

Telescopes are basically light buckets.  Their real purpose is to gather as much light as possible to make images appear brighter and clearer. The light gathering power is a measure of the brightness, and resolution is a measure of how sharp the image is. In addition, telescopes usually magnify the image.

Light Gathering Power

LGP depends on width

A wider container can catch more rain. Similarly, a wider telescope catches more light.

The Light Gathering Power  (LGP) is just what it sounds like, a measure of how much light the telescope can collect.

The amount of light a telescope can collect depends on the area of the primary mirror or lens. Since the primary is normally round, the area is proportional to the diameter squared. So, a telescope that is 2 times wider than another ‘scope will have a LGP of 22 = 4 times bigger.  If another telescope is 3 times widder, it will gather 9 times more light.

The bottom line: a little more width gets you a lot more light.


Resolution is a measure of how sharp an image is. It is usually given and the smallest feature you can distinguish, so the smaller the number, the better.

Sizes on the sky are measured in degrees, arcminutes, and arcseconds.  Degrees you’ve probably heard of. There are 360º in a circle, or about 180º from horizon to zenith to the opposite horizon. However, most astronomical objects are too small for degrees to make sense, so we need smaller units.  Each degree is divided into 60 “minutes of arc” or arcminutes (abbreviated with a double quote “), and each arcminute is divided into 60 seconds of arc or arcseconds (abbreviated with a single quote ‘).

The full Moon is (on average) about half a degree, or 31 arcminutes. The stars Alcor and Mizar, the second star in the handle of the big dipper, are 11 arcminutes and 48 arcseconds apart, and anyone with good vision can distinguish them. A good amateur telescope will resolve Mizar into two stars,14 arcseconds apart. If this is the smallest thing the telescope can resolve, than we say the telescope’s resolution in 14 arcseconds.

Alcor and Mizar as seen through a good 'scope

Alcor and Mizar as seen through a good telescope. Mizar can be resolved as two stars, 14 arcseconds apart.

The resolution of the telescope depends on the diameter, or aperature, of the telescope.  Photons come in to the telescope from all directions. The photons  that strike the edge of the mirror or lens are diffracted from the edge and interfere with the other photons trying to come through.  The wider the telescope, the less diffraction there is.

There is one caveat here though.  Although the diffraction limit of a telescope may be exceptionally small, most telescopes are still on Earth, where we have the atmosphere to deal with.  The atmosphere generally limits the resolution at most viewing sites to 1 arcminute, which is about the resolution of an 8″ telescope. Professional telescopes get around this with techniques like active or adaptive optics, interferometry, or by putting telescopes in space.

The bottom line: unless you are dealing with a professional observatory, wider is better up until 8 inches. After that, it doesn’t really matter much.


Magnification is a measure of how big something looks through the telescope.

Magnification is a comparison of how much bigger something looks through the telescope compared to the original, un-magnified view.

Magnification is really a comparison between how it looks through the telescope to how it looks to the unaided eye. If something looks twice as big through the telescope, the magnification is 2.  If it looks 100 times bigger, the magnification is 100.

To magnify an image you need an eyepiece.  You can calculate the the theoretical magnification by dividing the focal length of the primary by the focal length of the eyepiece.To change the magnification, change the eyepiece.  Professional telescopes usually completely ignore magnification since in most cases they don’t use eyepieces.

The bottom Line: magnification doesn’t really matter.

Field of View

Related to magnification is the field of view. This is the area on the sky that you can see through the telescope.  For example, if you can just barely see all of the Moon through a telescope, it would have a field of view of about half a degree.  If you switch to an eyepiece that magnifies the moon 4 times more, you will only see 1/4 of the moon, or about 1/8º.


You may also encounter the term “f-ratio”.  This is the  focal length of the primary divided by the aperature. A large f-ratio indicates the telescope is lang and narrow.  A long reflector may be an f/5, but a refractor of the same length may be an f/12, and will be less than half the width.



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