Telescope Basics

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Miscellaneous

Why buy a telescope? (Because it's a star-splitter.)
What can I see with a telescope?
- Nothing like NASA photographs!
- Here's how one experienced observer, Paul Baughman, described the capability of an 8-inch reflecting telescope: "Lunar features under one mile across; detail in Jupiter's cloud belts; cloud belts on Saturn's disk; hundreds of star clusters, nebulae, and galaxies; some globulars resolved almost to the core. Most galaxies and nebulas will appear silvery grey and if you're lucky some nebulae may look pale blue green. Unfortunately our eyes aren't very sensitive to color when the light is dim. Stars, on the other hand, show color quite well. The subtle blue, yellow, green and red colors of individual stars can be seen. Some people are more color sensitive than others. Don't despair if you can't see color. The planets show colors, but the colors are subtle and muted. Getting the most out of a telescope is a learning experience and takes time. Don't expect to find everything you search for. You may find star gazing and the telescope boring! That's OK. Life would be boring if we all had the same interests and opinions."
- Read "Seeing what you observe...," ch. 9 in Ken Fulton, The Light-Hearted Astronomer.
- Read the chapter "Seeing," in Annie Dillard, Pilgrim at Tinker Creek.
- Read the chapter on "Observation" in W. I. B. Beveridge, The Art of Scientific Investigation.
Which telescope should I buy? Our firm advice is:
- Start out right: Don't buy one!
- Starting out wrong will lead to frustration, disappointment, and wasted money. First get a reclining lawn chair and lay on your back for a year. Use our constellation web pages as a guide to the stars.
- Above all, don't buy a telescope from a toy or department store. If you are tempted, go to the Legos aisle instead and buy some of them--that way you'll be able to build something of higher quality. Save your money until you have at least $500 to invest in a real scope.
- Get a planisphere and use it regularly.
- Get a book like Chet Raymo's 365 Starry Nights, and become familiar with the night sky each night of the year.
- Find out what you can see with binoculars and become familiar with finding things in the sky with them.
- Subscribe to Sky and Telescope.
- Find a local astronomy club, attend star parties, and look through a variety of telescopes. Ask their owners what they like about their scopes. Then you'll know what to look for (having experience in naked-eye and binocular observation) and what not to get.
What should I do next?
- Let us know if you're interested in a family sky-watch seminar, where we'll show you how to get to know the night sky.
- Try to get a copy of the now hard-to-find book, The Light-Hearted Astronomer, by Ken Fulton. Read chapter 4, "The Ads," before responding to any telescope advertisements! Especially full-page glossy ads! The better telescopes aren't mass-produced in China, but sell on their merits and don't need whiz-bang gismo advertising.
- The Astronomics website has a very helpful guide to choosing a telescope, far more detailed than what is presented here. Quite informative; highly recommended. They also sell a variety of quality telescopes--and since they carry many different brands, they'll offer good advice about what's best for you.
- Sky and Telescope, January 2000, has an excellent review of small Dobsonian telescopes--a great choice for the beginner. Consider an 8-inch Dobsonian from Orion. For a cadillac Dobsonian check out Obsession.
- Telescope FAQ. Answers to frequently-asked questions not covered below.
- Browse the commercial sites mentioned below: Orion, Obsession, Meade and Celestron.

The Mount is as important as the telescope

Great optics on an unsteady mount provide images no better than cheap optics on an unsteady mount.
Altazimuth
1. Moves up and down (altitude) and side to side (azimuth).
2. Due to diurnal motion, objects move out of the field of view one degree every four minutes.
3. Incompatible with astrophotography.
4. Often used for terrestrial spotting scopes.
5. Inexpensive.
6. No alignments; simple to set up.
Image courtesy of Astronomics
Dobsonian

Image courtesy of Astronomics

Least expensive mount, most bang for the buck; a variation of the altazimuth mount.

Home assembly possible: made of wood, plastic, Teflon.

The telescope tube becomes part of the mount assembly, so that the center of gravity of the large reflector tube is always over the center of rotation on both axes. This eliminates the need for heavy counterweights, and is extremely stable.

Used with large reflectors (10 inch aperture or greater).

Incompatible with astrophotography, unless you're very serious. Yet Dobsonian telescopes, with their large apertures, are very compatible with real-time observation. Remember that the astrophotographs taken with smaller scopes are long-exposures, with detail and color never seen by the owner's eye. The alternative to astrophotography is actually seeing details with your eyes in real-time (as opposed to letting the camera "see" it for you).

An 8-inch Dobsonian makes a great first telescope for the beginner (see the review in the January 2000 issue of Sky and Telescope).

Except for altazimuth and Dobsonian mounts, expect to pay as much for a good, sturdy mount as for the telescope assembly itself.
Equatorial
1. With proper set up, the telescope turns to follow any star with only one movement instead of two, as with the altazimuth mount.
2. A "polar axis" of the mount is aligned with Polaris. Then the telescope tube is set perpendicular to the polar axis, so that it swings east or west in the plane of the celestial equator.
3. Essential for astrophotography with smaller scopes.
4. Useful for public viewing, especially with clock drive.
5. Very convenient for high magnification observing, which makes objects tend to go out of the field of view very quickly.
6. German Equatorial Mount
  1. Combines a tripod with a heavy, movable counterweight.
  2. More difficult to track objects as they cross the meridian. (The scope may need to be "tumbled" or rotated 180 degrees.)
  Image courtesy of Astronomics
7. Fork and Wedge Mount
  1. Works best with short tubes.
  2. A "fork" assembly attaches around the telescope tube, which is secured to an inclined platform or "equatorial wedge," which sits on top of a tripod. Both fork and wedge must be sturdy.
  3. Rotation of the fork upon the wedge is obtained through a clock drive.
  4. Clock (or worm) drives can be attached to automatically track an object. Once located, the object will remain in the center of the field of view. Without a clock drive objects quickly move out of any telescope's field of view due to the rapid motion of the earth.
Image courtesy of Astronomics

General features of telescopes

Aperture
- Diameter of objective lens or mirror.
- Most important feature of any telescope.
- How wide an aperture should I get? As wide as you can afford! It varies as to which type of telescope you choose, as explained below (a 4-inch refractor costs as much as a 10-inch reflector with comparable performance). Yet no matter how wide your telescope is, most objects will remain "barely visible." To enjoy a telescope requires contentment with visual subtleties and the patient control of one's "aperture fever."
Objective lens or mirror
- Largest diameter lens at the star-end of a refractor.
- Largest diameter mirror at the bottom or base of a reflector.
- Although there are many variations, the basic idea is that the objective creates a cone of light that comes to a focus at the focal point. The focus is then magnified for viewing by an eyepiece.
Light Gathering power
- Depends on aperture.
- Allows clear, detailed viewing of faint objects.
- Telescopes are light buckets, or light funnels, more than image magnifiers.
Resolution
- Refers to crisp, distinct images. Poor resolution means fuzzy images.
- Greater the larger the aperture.
Magnification
- Usually expressed with a number and an "x" for "times." For example, a magnification of 20x means that an image appears twenty times larger than actual size.
- A rule of thumb for practical performance is to expect a maximum of 50x of useful magnification per inch of telescope aperture.
- Equals focal length of objective lens/mirror divided by focal length of the eyepiece.
  - Sample calculation: A refracting telescope has an objective lens with a focal length of 50 cm. The eyepiece used has a focal length of 10 cm. What is the magnification?
- A higher magnification:
  - Dims images. Twice the power, one-fourth the brightness; half the power means four times brighter.
  - Decreases field of view.
  - Requires better viewing conditions (air stability).
  - Larger apertures compensate for these effects.
- When shopping for a telescope, ignore claims about magnification. Buy aperture, not magnification.
- Answer to sample calculation above: 5x.

Eyepiece

Use eyepieces to obtain the desired total magnification as suggested in the table below. Calculate total magnification by dividing the focal length of the objective by the focal length of the eyepiece, as noted above.
Don't buy a telescope without reserving some money for good eyepieces!

Magnification	Use
40-75	Wide field of view (the upper end of this range will be about half a degree, or about as wide as the apparent diameter of the moon). Useful for sweeping Milky Way; greatest ease in identifying star fields.
80-150	General observing; rich field observing of nebulas, open clusters, galaxies.
150-250	Globular clusters and smaller nebulae
300-500	Narrowest field of view. Detail of moon, planets, close double stars. Requires better than usual viewing conditions.

Refractors

Like binoculars, refractors use lenses; not mirrors. Type of telescope used by Galileo.
Provides the highest quality images aperture for aperture. Unequaled resolution. Images are crisp and well-defined even at large magnifications.
Much higher cost aperture-for-aperture.
Best choice for observing high resolution objects such as double stars, small details of planetary features, globular clusters, small faint galaxies.
Low power eyepieces widen field of view.
Images typically reversed left-to-right.
Rugged, maintenance free; closed tube.
May require an altazimuth mount to be affordable.
Tubes are long; may not fit into a car's trunk; transport may be difficult.
If any part is made of plastic, buy legos instead. But the better refractors give razor-sharp images.
Suggested aperture for a beginning refractor: 3 or 4-inch.
- Look at the ads by Orion in any Sky and Telescope issue. It's hard to go wrong with a good refractor. Spend $600 and you'll get something of permanent utility. If I had a lot of money I'd buy a 5-inch refractor.

Reflectors

Reflectors use a primary and secondary mirror. Type of telescope used by Newton.
Most aperture per dollar. (Only one optical surface on a mirror instead of two surfaces as with lenses.)
Suggested aperture for a beginning reflector: 8 to 18 inches.
Bright images and a wider field of view.
Images typically upside down.
Best choice for deep space "Big Faint Fuzzies" such as large galaxies, faint nebulae, scattered open clusters, dark nebulae.
Poorer resolution than refractors of equal aperture.
Shorter tubes than refractors of equal aperture.
No chromatic aberration as with refractors.
Occasional maintenance includes cleaning (rarely) and realignment (mirror collimation; easy). Open tube.
Since larger apertures are affordable, reflectors tend to be big and bulky; transport may be difficult.
Equatorial mounts expensive and heavy for big reflectors.
Easily combined with Dobsonian design and mount.
- Suggested models: Look in any Sky and Telescope issue for ads by Obsession and Orion. $600 will buy a good quality Dobsonian reflector.
- Personally my sympathies lie here. Just to dream a bit: if I had $6000, I would follow my cousin Paul Baughman's example and buy an 18-inch Obsession reflector with a Dobsonian mount, NGC-Max digital setting circles, TeleVue Nagler eyepieces and a Kendrick dew remover system. There's no such thing as a perfect telescope, and a good case easily may be made for different components, but this is an example of what one might do. An 18-inch reflector can resolve objects to an apparent magnitude of 15.
- An Obsession 15-inch reflector and an Obsession 18-inch reflector
  on either side of Paul Baughman.

Catadioptrics (Schmidt-Cassegrain)

Image courtesy of Celestron

Uses combination of lenses and mirrors.
Compact, portable, quick to set up.
Most popular backyard telescope of last 20 years.
Easily combined with either German equatorial mount or a Fork and Wedge equatorial mount. Often sold with clock drives.
Images typically right-side-up, not reversed. Useful for terrestrial viewing (e.g., wildlife photography).
Maintenance free, closed tube.
Priced midway between reflectors and refractors aperture-for-aperture.
33 percent central obstruction decreases effective aperture. Images seem dim and dull to experienced users of refractors and reflectors.
Suggested beginning aperture: 8 inches, or more if you can afford it.
- Mass-produced by Meade and Celestron with all kinds of whiz-bang bells and whistles (e.g., automatic location of thousands of objects in the computerized database). $2000 will get you a good Schmidt-Cassegrain. See any Sky and Telescope issue for full-page or multi-page ads. Beware! Oftentimes these scopes are cheaply made. Compare the optics of these scopes with Dobsonians and refractors before you buy, and buy locally where you can test the optics of the scope before you take it out of the store.
The Maksutov design is similar to the Schmidt Cassegrain. The OBU Planetarium has a Questar Maksutov telescope which we regularly use to view the planets after Friday night shows.

Coda: The Joy of Amateur Astronomy

Spend half of my money on the mount? You must be kidding!!

Nope. As an example, here's how Paul Baughman prepared his backyard to receive a 5-inch refractor.

Above: Note the solid pillar in the center. That's Joey Baughman and their cat standing beside it.

Below: Joey Baughman and his grandmother Carolyn Baughman survey the 600-E German Equatorial mount fastened to the top of the pillar.

Above: The 5-inch refracting telescope from Astro-Physics has found a home in Strawberry Point, Iowa. Compare its length to Joey.

Paul says: "Since I'm too old and too fat to become an astronaut this is the closest I'll get to outer space." Thanks, cousin, for leading the way to the stars!