If you are already an astronomy buff, you may have purchased a telescope for your own personal use. But if this is your first telescope purchase, then you and your students have reason to be excited! There's a vast, mysterious universe waiting your discovery.
The most important feature of a telescope is its aperture size—the diameter of its main lens or mirror. A large aperture collects more light and produces a better-focused image. When using a 4-inch telescope you will see star clusters as fuzzy spheres of light. If you use a 6-inch telescope, the fuzzy spheres become individual faint stars. With a 12-inch telescope the stars become clearly defined.
For best viewing, the maximum magnification of a telescope should be no greater than 50x per inch of aperture. If you have a 3-inch telescope, 150x should be its maximum magnification. For a 4-inch telescope, 200x should be its maximum magnification. Magnification beyond the range of 50x per inch results in fuzzy imaging.
Once you've decided on the size of the telescope you want, you need to choose the type of telescope: refractor, reflector, or Schmidt-Cassegrain. The refractor telescope, used by Galileo, has a front-mounted lens that gathers and focuses light. The reflector telescope, or Newtonian telescope, uses a rear-mounted concave mirror to gather and focus light. The Schmidt-Cassegrain, or catadioptic telescope, is a combination of a reflector and refractor. The Schmidt-Cassegrain uses a front-end refractive lens plus a combination of mirrors to fold a long focal length into a fairly short tube.
If you compare equally sized (by aperture) refractor and reflector telescopes, the refractor will have the sharper image.
If you compare a refractor and a reflector by price, the reflector gives you a better telescope for an equal amount of money.
A Schmidt-Cassegrain telescope is perfect for general purpose viewing, but is not as good as a refractor or reflector for specialized viewing. A Schmidt-Cassegrain is also ideal for astrophotography.
Coating & Focal Length
Coated mirrors or lenses, which are available for some telescopes, increase light transmission and give images up to 15% brighter.
Another feature to consider is the telescope's focal length. (The focal length, which is measured in millimeters, is the distance light will travel from the main lens or mirror to the eyepiece. Camera buffs may already know that the f/ratio is the focal length divided by the aperture size.) In general, a telescope with a short focal length (400-700 mm) will have a wider field of view than one with a long focal length. A telescope with a short focal length and low viewing power is preferred for wide-angle viewing of the Milky Way and Deep Space. Telescopes with a longer focal length and high viewing power (100X-200X) are better for planetary studies. Most telescopes can be used at either high or low power by changing the eyepiece, and so good quality eyepieces are useful additions for any telescope.
To determine the magnification of an eyepiece, divide the focal length of the telescope by the focal length of the eyepiece. A good quality Barlow lens (which costs between $100 and $200) can double or triple the telescope's magnification range. Be careful though—poor quality eyepieces will add distortions such as spherical or chromatic (color) aberrations to the image.
The finder scope is the small scope on the side of a telescope and is used to aim the telescope. The typical finder scope provides a low power (5X-8X) wide field viewing area. Without a finder scope, even locating the Moon through a telescope can be difficult!
Many finder scopes are identified as being 5 x 24. This means the finder scope magnifies 5x and has a 24 mm aperture. Since a diaphragm inside the front lens of the finder scope often reduces the aperture to 10 mm, a 6 x 30 finder scope is a better choice.
Finder scopes attach to telescopes with a bracket. During viewing, a 6-point bracket is easier to adjust than a 3-point bracket.
Straight tube finder scopes are less confusing to use than right angle finder scopes. The right angle finder scope gives a mirror image view of the sky, and this can be confusing to telescope users. Image-correcting lenses for finder scopes are available, but they can dim the view or add imperfections such as ghost images.
Don't forget to consider portability when buying your telescope. After all, the larger the telescope, the heavier it will be. And the more complex the telescope's design, the longer it will take to assemble. A smaller telescope is portable and lightweight and can offer hours of planetary study. A good quality telescope with an aperture of 3-8 inches is quite suitable for studying the Moon and the planets. After all, not even a large-aperture telescope will give you good deep space viewing if you live in or near a large city.
When selecting a telescope, examine the mounting as well as the optics. Basically, there are two types of telescope mounts, the altazimuth and the equatorial. The altazimuth mount uses up/down and side/side movements to aim the telescope. A good altazimuth mount will have slow-motion controls for fine-tuning. But you will need an equatorial mount if you plan to track stars as they move across the sky. Since the apparent motion of the sky causes stars to move out of the field of view within seconds, a motorized equatorial mount that can automatically track star movement is a very nice feature to have.
Setting circles are generally not worth the investment. It is difficult for amateurs to properly align the telescope and calibrate the circles to give accurate coordinates. Instead, purchase a good star chart and use that as your guide.
Square-legged tripods are usually sturdier than round-legged ones. Vibration pads that fit underneath the tripod's legs will reduce vibrations and help keep the telescope steady. "Damping" refers to how long it takes the telescope to stop vibrating after being lightly tapped. A 5-6 second delay is okay; a 2-3 second delay is even better. Any delay longer than 6 seconds is not good. After all, you will have to wait for the vibrations to stop each time you touch the telescope.
Spotting scopes with interchangeable eyepieces can be used as telescopes, but they must be mounted on tripods. Since camera tripods lack slow motion controls and can be difficult to aim precisely enough for astronomy studies, the usefulness of a spotting scope as a telescope is quite limited. Spotting scopes with zoom lenses are perfect for bird and nature studies but aren't meant for stargazing. If you really want to use your telescope for nature studies, invest in a 4-inch refractor or Smith-Cassegrain telescope.
Suggestion for Beginners
For beginners, an 80 mm refractor or a 4.5-inch reflector telescope will make an excellent telescope. A 6-inch equatorially mounted reflector telescope would also be a good choice, and offers more features. An 8-inch Schmidt-Cassegrain combines the best features of both the refractor and reflector telescopes and is perfect for serious fieldwork. Before assembling make sure the school cleaning services are up to par. You wouldn't want to scratch any valuable glass pieces.
_____ Type (Refractor, reflector, or Schmidt-Cassegrain)
_____ Aperture size
_____ Magnification strength
_____ Focal length
_____ Quality of optics (Coated lenses and mirrors)
_____ Quality of eyepiece (Orthoscopic and Plossl are better than Kellner.)
_____ Eyepiece diameter
_____ Number of eyepieces included
_____ Any special lenses included (Barlow)
_____ Dimensions of finder scope
_____ Straight tube or right angle
_____ Type of finder scope bracket
_____ Damping time
_____ Type of mounting (altazimuth or equatorial)
_____ Motorized - AC or DC (Can it run off a battery?)
_____ Slow motion controls
_____ Type of tripod (Square or round leg)
_____ Portability and weight
_____ Ease of set-up
_____ Carrying case
_____ Accessories available
© 2004 Anne Wallingford. All rights reserved.