Site menu:


Ouch! A spiral galaxy as viewed edge-on collides with a small blue galaxy. From the Hubble Ultra Deep Field. Photo by NASA, ESA, S. Beckwith (STScI) and the HUDF Team.


astro101 -- The Telescope Page

Understanding telescopes

After buying a telescope, many spend a lifetime exploring the cosmos. Others become frustrated and leave the telescope to gather dust. Often the conclusion is reached because of the type of telescope purchased.

Refracting vs. Reflecting Telescopes

There are two major telescope types. The oldest is the refracting telscope, dating to the early 1600's and used by Galileo. The refractor uses a large objective lens to focus images. The second type of telescope, a reflecting telescope, invented by Issac Newton, replaces the lens with an objective mirror. Some modern reflecting telescopes use a Schmidt Cassegrain (and the similar Maksutov-Cassegrain) design in which a secondary mirror is used. In Schmidt-Cassegrains, the light travels through the telescope twice. Because of this, they need only be half of the length of a traditional newtonian reflector, making them more portable.

A major advantage to the reflecting telescope is that it is easier and cheaper to build a large mirror than a large lens. As a result, all of the largest Earth-based telescopes and the Hubble Space telescope are reflectors.

What a Telescope does

A telescope had three main functions. First, it gathers more light than the human eye, which enables us to see fainter objects. Second, a telescope improves our ability to see detail. Finally, a telescope magnifies. Of the three things, most professional astronomers think of the third as the least important. In fact, with any telescope, we can get whatever magnification we want simply by changing the eyepiece. If we make an object look big but blurry, we haven't accomplished much.

For both reflectors and refractors, the larger the objective lens or mirror, the better light gathering ability and clarity. This is true as long as the optics are of sufficient quality.

Telescope Mounts

Another consideration for a telescope is the mount. One type of mount is an altitude-azimuth mount (also called alt-az or horizon). With this mount, the telescope can be moved in either of two directions: horizontal or vertical. Professional astronomers generally use an equatorial mount. One direction moves North or South (towards or away from the North star) while the other direction moves East or West, parallel to the equator. This type of mount makes it easier to "track" celestial objects by compensating for the Earth's motion. Some newer model telescopes, such as Meade's ETX series can track using an alt-az system.

Whatever the type of mount, it must be sturdy (which often meaans it will be heavy). If the telescope vibrates, the quality of the image will be poor, even with the best of optics. Some telecopes, like the Meade ETX series can be used without a tripod by sitting it on a table, thus providing an advantage of portability.

10 inch Dobsonian telescope by Orion

Dobsonian mounts

Both the equatorial and alt-az mounts are most often tripods. Another type of mount is a Dobsonian mount, which pivots at the base of the telescope. Generally, a Dobsonian reflector gives the largest mirror (and therefore the best light collecting ability) for the money. Some disadvantages are that they are bulky. Some find Dobsonians difficult to maneuver, while others are thrilled with them.

Telescope Bells & Whistles

Some telescopes have a built in "go-to" feature in which the telescope can find deep space objects from a computerized data base of thousands. As a caution, some telescopes are too small to gather enough light to see all of the objects in the data bases. There are also telescopes with GPS sensors so the telescope can figure out where it is and then set up automatically.

Telescope Eyepieces

Celestron NexStar

The eyepiece is as important a consideration as the rest of the telescope. A cheap eyepiece on the finest telescope will result in a poor image. Good eyepieces contain multiple elements. Some common designs are the Plossl and the Nagler. The focal length of the lens will dictate the magnification of the telescope. Smaller focal lengths mean more magnification. For a starter, we recommend using low magnification. Higher magnification makes the field of view smaller, making it more difficult to find objects. For reflecting telescopes, typical eyepiece focal lengths of 40 mm and 26 mm are good. For some refracting telescopes, smaller focal lengths may be needed to obtain the same magnificaiton. A good eyepiece can easily run over $100.

Another way of increasing the magnification is by the addition of a Barlow lens. It is probably best to master the lower magnifications before purchasing one.

Summary & Recommendations:

Meade ETX125

The most popular type of telescope today is the Schmidt-Cassegrain (or Maksutov-Cassegrain). There are many good models by Meade and Celestron. A telesope with an 5 to 8 mirror gives a good combination of portability (although 8 inches gets on the heavy side) and good light gathering ability. With tripods, the 8" telescopes are mostly over $1000. Many of the new Meades feature GPS and run around $2000. Meade even makes a huge 16"reflector -- a true research grade instrument, with a price of $16,000 and a weight of over 300 pounds. The new Maksutov-Cassegrains such as the Meade® ETX-90PE Telescope with UHTC are the most portable telescopes around and can be set up easily on a table. Although the ETX60 is a nice unit, it's aperature is too small to detect faint objects.

The traditional newtonian reflector maintains some popularity. The tubes are longer, so they are less portable. A Newtonian with a Dobsonian mount gives the largest aperature per dollar, but is large and awkward.

<b></b>Orion Skyquest The smallest of these,the Orion SkyQuest XT4.5 (4.5" diameter mirror) sells for $230. The SkyQuest XT12, which is a huge telescope sells for $870. Lugging this huge instrument anywhere presents a challenge. Celestron's Dobsonians include the "Starhopper"series. Celestron Stahopper 10 can be found for about $500. Meade m akes a Dobsonian telescope with a truss instead of a tube (Lightbridge series). A Lightbridge 8" costs $469.00. The absense of a tube makes the telescope lighter. However, it should only be used in very dark locations, since there is nothing to block stray light.

Meade ETX125

With modern glass optics, refractors produce clear images with excellent contrast between a celestial object and the dark sky. We do not recommend anything smaller than a 90 mm opening. Telescopes with a 60mm aperature are popular, but the light gathering ability is insufficient to see the fainter objects. For a refracting telescope, the key is to have good optics. Meade, Celestron, and Orion all make high quality refracting telescopes. There are, however, many cheaper brands on the market that do contain quality optics and often lead to frustration. Large aperature refractors (6" or more) are long, heavy, and pricey (but provide excellent images). Orion's Astroview 90mm telescope is around $300,while a Skyview 120mm is around $600.00. The better telescopes are apochromatic and do have the problem of color separation. Orion's Skyview 100 EQ apochromatic is around $1200.00. With added "Go-to" features, these can top $3000.

Where to buy telescopes

Retail telescope stores that specialize in quality instruments are generally rare in most parts of the country, although if you happen to have one in your neighborhood, it is a good place to browse and talk to someone knowledgeable about telescopes. Local department stores are generally not good places to buy telescopes. Although the telescopes look similar, they often have inferior optics and flimsy mounts. People who start with these telecopes frequently give up on astronomy