What exactly is a telescope?
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:
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.
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.
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 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.
Catadioptric Telescopes
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.You Like This Topic? Then Share It With Your Friends
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