Telescope Basics

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What exactly is a telescope?

The simplest definition is; an instrument that gathers light and focuses that light into an image. In turn, this image can be magnified. This instrument is also mounted in such a way that allows you to swing it from object to object. Let's break this down further.


A telescope's ability to gather light is dependent on:

Aperture ... the larger the primary optics (the largest lens or mirror) the more light is captured.

Optical quality ... the more reflective a mirror and the more a lens allows light to pass through it, the more light is transmitted to your eye.

Contrast ... the more absorptive the inside surface of the tube is, the less light will bounce around in that tube, which leads to improved contrast (darker backgrounds).  A darker background means that you see more of the object you are looking at. An improperly designed optical assembly can also rob you of contrast.

Collimation (alignment) ... even if you have the best optics, if they are not properly aligned, your light gathering capacity will be diminished.


A telescope's ability to focus properly is dependent on:

Collimation ... how well the optics are aligned.

Optical quality of the eyepieces

Focusing mechanism ... a focuser that is smooth and allows for minute adjustments is necessary. This focuser must also be aligned properly to perform at its best.


A telescope's mount needs at a minimum the following characteristics:

Stability ... the telescope should not sway or bounce on the mount.

Rigidity ... when you move or focus the telescope, the stand should be solid enough to quickly dampen out any vibrations, otherwise these vibrations will be seen in the eyepiece. A poorly designed mount or a mount that is too small to handle the weight and size of a telescope can render even the best telescope useless.

Smooth motions ... the mount should move smoothly from object to object.


Clearing up the myth about power (magnification)
It slices     it dices     it has 675 power!!!!

Over the years, magnification has been the most misleading specification printed on those glossy color boxes of many department store telescopes. Exaggerated claims of high power are almost always a sure sign of an inferior product. The real important specifications in no particular order are as follows:

Stability of the mount ...  does not vibrate and moves smoothly

Size of aperture ... light gathering power

Quality of optics ...  reflectivity and transmission of light

Ease of use ...  point and look. The telescope needs a good finder and, in our opinion, especially for the beginner, it MUST have a reflex sighting device of some type. (see reflex sight in glossary)

Please understand, we are not saying that power is unimportant, but it should be one of the last criteria you consider.


The amount of  power you can apply is dependant upon several factors such as:

Focal Length ... focal length is the distance that the light in the telescope travels from the objective lens or mirror to the point where that light reaches focus (the focal plane). To determine how much power a telescope can deliver with any eyepiece, simply divide the focal length of the telescope by the focal length of the eyepiece.

For example: If you have a telescope with a 1000mm focal length and a 20mm eyepiece, that eyepiece will deliver 50 power (1000/20). The same eyepiece in a 2000mm focal length telescope delivers 100 power (2000/20).

Focal Ratio ... telescopes are rated with an "f" number. As you look at the telescope market, the products are given specification like (6" f/5). This means the telescope has a 6" aperture and a focal ratio of 5. As with cameras, focal ratio simply represents the speed of the optics. The smaller the "f" number, the faster the optics. But how does this relate to magnification?

With any given eyepiece, fast focal ratios (f/3 to f/5) deliver lower power, wider fields of view and usually brighter images. Because of these brighter images and wider fields of views, fast focal ratios excel at deep sky observing and can deliver stunning wide-angle views.  Conversely, slower focal ratios (f/6 and up) deliver higher power and increasingly narrower fields of views.  Slow ratios excel at observing the moon, the planets and binary stars.

Don't know your focal length?
If the focal length of your telescope is not printed on the telescope, you can determine it by multiplying the focal ratio by the aperture. If you have a 6" (152.4mm) f/5 telescope then 152.4 x 5 = 762mm focal length.

Don't know your focal ratio?
If the focal ratio of your telescope is not printed on the telescope, you can determine it by dividing the focal length by the aperture. If you have a 762mm focal length and an aperture of  6" (152.4mm) then 762 / 152.4 = 5 or f/5 focal ratio.

Which is better, fast or slow? ... well, that's like asking which is better, a sailboat or a power boat? Both have their own unique advantages and requirements. One could argue that for versatility, a powered sailboat is the best. In telescope parlance, this could be defined as a focal ratio of f/6 as it will yield nice low power wide fields of view and the ability to apply sufficient power for most observing situations. Please note that this is "highly" subjective.

One last point on this. Fast focal ratios = smaller focal lengths = shorter tubes. Shorter tubes mean the mounting need not be as large. A 6" f/6 telescope on a solid mount is easier to set up and transport than a 6" of  large focal ratio.  When you apply this thinking to larger aperture telescopes, you will find that the focal ratio also dictates whether you can keep both feet on the ground or have to climb a ladder to get to the eyepiece. ( The author hates ladders)

Seeing ... Seeing is the term used by astronomers to describe the quality (transparency) of the atmosphere on any given night. Our atmosphere is extremely dynamic. Most nights, the stars twinkle. Twinkling is caused by the light of the stars bending through our turbulent atmosphere. In general, twinkling = poor seeing.  Only on nights of good seeing, characterized by steady stars, can one apply high magnifications.

Relationship of power to aperture ... there is a simple formula used to determine just how much "useable" power can be applied. In general, given good seeing, 50X per inch of aperture is the limit. So a 6" aperture would allow up to 300 power (6 x 50).  However, at 300X in a 6" telescope, it becomes hard to focus and unless you have a rock solid mount, it becomes increasingly difficult to observe an object as even the tiniest of vibrations will cause it to dance around your eyepiece. It is true that if you use a Barlow lens (a lens that doubles or triples your magnification) you could go to 600X or 900X but these powers are completely unusable. (A side note here is that for many, this has put Barlow lenses in a bad light. This is unfortunate, as a good Barlow lens is a very useful accessory when used properly.)

In practice, on most nights, a formula of 25X to 30X per inch is far more realistic.

So, as you can see, many factors dictate useable power (magnification). In the final analysis, like most observers, you will find that the most pleasing and beautiful images often come from low power wide field views. 


Telescope types and how they work

The Refractor

The refractor telescope has been around for centuries. In 1610, Galileo used a small refractor to watch the phases of Venus and observe craters on the moon. With it he discovered the first 4 moons of Jupiter and observed Saturn's odd shape. Galileo's telescope had a 2" aperture, quite small by today's standards, and was, shall we say, optically challenged. Because of this lack in optical quality, he could not resolve that Saturn's strange shape was actually caused by its rings. Approximately 50 years later, improvements in optical quality allowed observers to determine that Saturn's odd shape was actually a big ring (rings) around the planet.

To refract means to bend. Light enters through the lens at the front of the tube. This lens is called the objective. The light is refracted down the length of the tube where it eventually reaches its focal plane ( where the light becomes focused at a specific point). There, an eyepiece mounted in a focuser, allows that light to be magnified into an observable image. Simply put, the better the quality and alignment of the optics, the better the image produced by a refractor will be. Actually, this statement holds true for all telescopes.

Refractors are a very versatile instrument. With the right eyepieces, they can be used for both daytime and nighttime observing and given good optics, deliver superb detail. Small ones are also very portable. Unfortunately, high quality refractors tend to be very pricey due to the cost associated with producing high quality optics. One of the downfalls of inexpensive refractors is that for astronomical viewing, they have the tendency to add false color to images. This is called chromatic aberration. These colors usually take the form of a pale violet halo around the observed object. Don't let this scare you off though. There are many excellent entry level refractors that will perform beautifully. Many observers swear by their refractors and believe them to be unrivaled for sharp lunar, planetary and binary star observing.

The Newtonian Reflector 

The reflector is a telescope design invented by Isaac Newton in the 1660s. Rather than lenses, the reflector  uses 2 mirrors to bring light to the eyepiece. Light travels down the tube to the primary mirror, which is the larger of the 2 mirrors. The primary mirror is generally a paraboloid (concave) mirror. It reflects the light back up the tube to the secondary mirror. The secondary mirror is an ellipse with a flat surface which is mounted at a 45 degree angle on a device usually called a spider. The light is reflected from the secondary mirror to the eyepiece where it can be magnified into an observable image.

Dollar for dollar, reflectors offer the most aperture. They produce sharp images that are free of any added color.  Optically speaking however, they have 2 things going against them:

1) The spider holding the secondary mirror forms a central obstruction that produces a diffraction pattern. This is most noticeable at high magnifications on bright objects. Look at the image below. Notice the 4 spikes coming away from the bright star in the center? This is the diffraction pattern caused by a 4-vane spider.

2) Because of the shape of the primary mirror, reflectors suffer from a condition called coma. This has the effect of making objects at the outside edge of the field of view to have the appearance of being wedge shaped or look like little comets. This really is not that big a concern for visual observation as the effect is most noticeable only at the outside most part of the field of view and not present at all in the central field of view which is where most of your observing is done anyway. Also, some eyepiece designs work well to counteract this effect.

Again don't let these characteristics scare you away from a reflector. Consider this, many big telescopes in observatories around the world are reflectors.

While there is no such thing as a perfect telescope, for visual observing a well designed 6"  f/5  to f/7 reflector with good mirrors that are kept in alignment (collimated) on a Dobsonian mount (see mounts section bellow) is, in our opinion, a superb instrument that will give you years of use and not break the bank. It will deliver good views of the moon and planets and has enough light gathering power to reveal many deep sky objects given dark skies and good seeing.

Catadioptric Telescopes

Catadioptric telescopes are essentially a combination of a refractor and a reflector. There are 2 very popular flavors widely available in today's telescope marketplace. One is the Schmidt-Cassegrain, and the other is the Maksutov-Cassegrain (see images below). These telescopes fold the light path 3 times allowing for a much shorter tube. Because of their clever use of corrective lenses and lack of a spider to hold the secondary mirror, they are free of many of the optical defects present in refractors and reflectors.

Light enters the front of the tube through a corrective lens. The light then travels down the tube to the primary mirror. From there, it is reflected up the tube to a secondary mirror which in turn reflects the light back down the tube to the focal plane.

The focusing mechanisms of Catadioptric telescopes are different than in reflectors and refractors. Instead of moving the eyepiece in and out of the focal plane, the whole primary mirror is moved in and out. Because of their more complex design, their tube construction and mount are generally very well thought out and implemented. Most of them are wonderful telescopes.

Catadioptric telescopes can be very expensive, but a well-built Catadioptric telescope is quite simply a joy to own and use.


The Resolving Power of Aperture
How much can you see with a telescope?

How much you can see with a telescope has everything to do with the size of your aperture. Smaller telescopes (4 inches and under) will not reveal very much in the way of nebulas and galaxies. That being said, bear in mind that somewhere between 1750 - 1800, Charles Messier compiled a list of approximately 100 diffuse objects now known as the Messier Catalog.  These objects were difficult to distinguish from comets through the telescopes of the day. So don't write off that small scope.

For the record, when comet Shoe Maker-Levy 9 slammed into Jupiter in 1994, I used a very modest Bushnell 565 refractor telescope with an aperture of only 60mm and could make out the dark smudges in the Jovian atmosphere left by the comet fragments .. so you see, there are things you can see with even a small telescope. I REALLY wish I had already built my 12.5" Newtonian back then.

The chart below will give you a good approximation of the effect that aperture has relative to how much can be seen. I have used stars to demonstrate the increase in visible objects and have listed the naked eye for reference. Limiting Magnitude refers to the faintest object visible with a given size of aperture. Please bear in mind that this number is also affected by local seeing conditions, the quality of the telescope and eyepiece's optics as well as the eye of the observer. More information on magnitude can be found in the glossary.

Number of
Visible Stars
Naked eye Naked eye 6 (maximum) approx 8,500
4 102 13 approx 15 million
6 152 13.5 approx 30 million
8 203 14 approx 45 million
10 254 14.5 approx 85 million
12.5 318 15 approx 130 million


See the 12.5" Dobsonian telescope I built

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