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how to select your first telescope


key features every new backyard astronomer should know

purchasing a new telescope can be very confusing. it's almost like you need to learn another language to understand all the various types of telescopes and accessories. choosing the right telescope for you depends on your individual needs including cost, portability, versatility, usability, appearance, etc.

before we jump into the different types of telescopes here are some important ground rules to be aware of. knowing these tips will help you better understand the different types of telescopes and help you choose what is best for you.


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.

aperture – bigger is better…sometimes. the most important specification on a telescope is its aperture. aperture is the size of the telescopes light gathering lens or mirror. that lens or mirror is often referred to as the telescopes objective. aperture is usually measured in millimeters or inches. it is aperture that will determine how much light gathering power the telescope will have. the telescopes ability to gather light is what will determine how well you will be able to view objects.

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.

your first major decision – what type of telescope? refractors – reflectors and catadioptrics

there are three basic types of telescopes – refractors, reflectors, and catadioptrics. all these designs have the same purpose, to collect light and bring it to a point of focus so it can be magnified and examined with an eyepiece, but each does it differently. below we will discuss the most popular types of telescopes and describe advantages and disadvantages of each.

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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.

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» 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


» 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


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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.

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» lowest cost per inch of aperture compared to refractors and catadioptrics since mirrors can be produced at less cost than lenses in refractors in medium to large apertures.
» reasonably compact and portable
» excellent for faint deep sky objects such as remote galaxies, nebulae and star clusters because of their larger apertures for light gathering.
» low in optical aberrations and deliver very bright images


» generally not suited for terrestrial applications
» slight light loss due to secondary obstruction when compared with refractors

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catadioptrics use a combination of mirrors and lenses to fold the optics and form an image. there are two popular designs: the schmidt-cassegrain and the maksutov-cassegrain. in the schmidt-cassegrain the light enters through a thin aspheric schmidt correcting lens, then strikes the spherical primary mirror and is reflected back up the tube and intercepted by a small secondary mirror which reflects the light out an opening in the rear of the instrument where the image is formed at the eyepiece. catadioptrics are the most popular type of instrument, with the most modern design, marketed throughout the world in 3 ½” and larger apertures.

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schmidt-cassegrain advantages

» best all around, all purpose telescope design. combines the optical advantages of both lenses and mirror while canceling their disadvantages.
» excellent optics with razor sharp images over a wide field
» excellent for deep sky observing or astrophotography with fast films or ccd's
» very good for lunar, planetary and binary star observing or photography.
» excellent for terrestrial viewing or photography
» closed tube design reduces image degrading air currents
» most are extremely compact and portable
» easy to use
» durable and virtually maintenance free
» large apertures at reasonable prices and less expensive than equivalent aperture refractors.
» most versatile type of telescope
» more accessories available than with other types of telescopes
» best near focus capability of any type telescope

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schmidt-cassegrain disadvantages

» more expensive than reflectors of equal aperture
» it is not what people expect a telescope to look like
» slight light loss due to secondary mirror obstruction compared to refractors


the maksutov design is a catadioptric (using both mirrors and lenses) design with basically the same advantages and disadvantages as the schmidt. it uses a thick meniscus-correcting lens with a strong curvature and a secondary mirror that is usually an aluminized spot on the corrector. the maksutov secondary mirror is typically smaller than the schmidt's giving it slightly better resolution for planetary observing.

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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.

the maksutov optical design typically is easier to make but requires more material for the corrector lens than the schmidt cassegrain.

now that you have read about magnification, aperture and the different types of telescopes, you will have more knowledge to browse our site more informed.

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telescope 101 - telescope selection - copyright © 2005 james eaton - site email address is

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this site was last updated 05/11/2008