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How To Select Your First Telescope First Things First – A Word About Magnification. Magnification or “Power” is the least important factor when choosing a telescope. The single greatest misconception about telescopes is that you need to have lots of magnification to see objects. It is the telescopes light gathering ability, referred to as Aperture that will determine how much you can see. Be cautious of outrageous claims of 500x or 600x magnification on inexpensive telescopes – these often department-store brand telescopes, are taking advantage of the number one falsehood in telescopes – that magnification is everything. Ok. So a telescopes ability to gather light is the most important thing. So I should look for the largest telescope? Not necessarily. You don't want to forget about portability. What good is a monster telescope if you never want to lug it around? You will need to ask yourself “Where will I want to use my new telescope?” If the answer is nearby in the backyard then having a large telescope will be of great advantage to you. If you need to bring the telescope to darker skies away from city lights – will you be able to handle the size and weight of a large telescope? It is often overlooked that you will have to assemble and set up your telescope in the dark making it even more challenging. Avoid the thought of just getting the biggest scope you can get. You will have little enjoyment of your new telescope if it sits in the attic because every time you think about using it you cringe at the though of moving it. Refractor Refractors are what the average person identifies with the word “telescope”, a long, thin tube where light passes in a straight line from the front objective lens directly to the eyepiece at the opposite end of the tube. Advantages » Easy to use and reliable due to the simplicity of design. » Little or no maintenance » Excellent for lunar, planetary and binary star observing especially in larger apertures » Good for distant terrestrial viewing » High contrast images with no secondary mirror or diagonal obstruction » Color correction is good in achromatic designs and excellent in apochromatic, fluorite and ED designs » Sealed optical tube reduces image degrading air currents and protects optics » Objective lens is permanently mounted and aligned Disadvantages » More expensive per inch of aperture than Reflectors or Catadioptrics » Heavier, longer and bulkier than equivalent aperture Reflectors and Catadioptrics » The cost and bulk factors limit the practical useful maximum size objective to small apertures » Less suited for viewing small and faint deep sky objects such as distant galaxies and nebulae because of practical aperture limitations Reflector Reflectors usually use a concave parabolic primary mirror to collect and focus incoming light onto a flat secondary mirror that in turn reflects the image out of an opening at the side of the main tube and into the eyepiece. Advantages
Catadioptrics
Maksutov-Cassegrain The Maksutov is heavier than the Schmidt and because of the thick correcting lens takes a long time to reach thermal stability at night in larger apertures. Understanding how power, or magnification, is calculated when using a telescope will require the understanding of a relationship between two independent optical systems - the telescope itself and the eyepiece you are using. To understand this we must first understand the term Focal Length. Focal Length Focal Length is the distance measured in millimeters (mm) in an optical system from the lens or primary mirror to the point where the telescope is in focus. This point is called the Focal Point. The longer the focal length of the telescope, generally the more power it has, the larger the image and the smaller the field of view. For example, a telescope with a focal length of 2000mm has twice the power and half the field of view of a 1000mm telescope. Calculating Magnification (power) To determine power in a telescope, divide the focal length of the telescope by the focal length of the eyepiece. By exchanging an eyepiece of one focal length for another, you can increase or decrease the power of the telescope. For example, a 25mm eyepiece used on a telescope with a 1000mm focal length would yield a power of 40x (1000 / 25 = 40) and a 10mm eyepiece used on the same telescope would yield a power of 100x (1000 / 10 = 100). Since eyepieces are interchangeable, a telescope can be used at a variety of powers for different applications. There are practical lower and upper limits of power for telescopes. These are determined by the laws of optics and the nature of the human eye. As a rule of thumb, the maximum usable power is equal to 60 times the aperture of the telescope (in inches) under ideal conditions. Powers higher than this usually give you a dim, lower contrast image. For example, the maximum power on a 60mm telescope (2.4” aperture) is 142x. As power increases, the sharpness and detail seen will be diminished. The higher powers are mainly used for lunar, planetary, and binary star observations. Be very cautious of manufacturers who advertise a 375 or 750 power telescope which is only 60mm in aperture, as this is false and misleading. Many department store brand telescopes know that customers are not informed how telescopes operate. These manufacturers of telescopes market their products to the misconception that magnification is the most important feature on a telescope. Most of your observing will be done with lower powers – 6 to 25 times the aperture of the telescope (in inches). With these lower powers, the images will be much brighter and crisper, providing more enjoyment and satisfaction with the wider fields of view. There is also a lower limit of power which is between 3 to 4 times the aperture of the telescope at night. During the day the lower limit is about 8 to 10 times the aperture. Powers lower than this are not useful with most telescope and a dark spot may appear in the center of the eyepiece in a Catadioptric or Newtonian Reflector telescope due to the secondary or diagonal mirrors shadow. Using Eyepieces
Most telescopes will come with 1 or more eyepieces. It is good to have a wide selection of eyepieces for various types of observation. To truly get the most out of your telescope it is a good idea to have a wide selection of eyepieces. You will find that most telescopes will come with a 25mm (sometimes 20 or 26 depending on the telescope) because this is one of the most common eyepieces focal lengths to use to get the optimum magnification and field of view from your telescope. It is good to spread out your selection of eyepieces. A good selection would be a 5mm – 10mm – 15mm – 25mm – 32mm as an example. You will find that many companies offer kits of eyepieces that will give you a wide selection in one package. When choosing an eyepiece it is good to remember this rule of thumb. The telescope is only as good as the eyepiece. You could have the most amazing telescope quality, but if you use a poorly manufactured eyepiece, you are not getting the advantage of the telescope. There should be a balance between the quality of your telescope and the quality of your eyepiece. If you have a top-of-the-line telescope, it is wise to spend the extra money on a suburb quality eyepiece. Here is a list of manufacturers that make exceptionally high quality eyepieces. Although these eyepieces are amazing in quality - some of the prices for an eyepiece alone could purchase a complete telescope! » TeleVue Here is a list of manufacturers that make some very good eyepieces at a good price. » Celestron |