Factors to Consider when Buying a Telescope

To help you choose the best Meade telescope for your needs, the following section explains key factors to consider in your purchase. While each of these technical factors may or may not play into your buying decision, all are worth understanding. Before you start examining the technical factors though, think about the three following general questions:

• What will I be using the telescope for? Astronomical observing, terrestrial observing or both?
• What is my experience level and how serious are my intended applications?
• What is the size of my budget?

While every Meade telescope is a high-quality, finely-tuned instrument, all are designed for different experience levels and applications. By answering these basic questions, you can evaluate the telescope factors below to find the perfect fit for your intended purpose. For example, if you want a medium-priced telescope that can be used for all viewing situations, is easy to use, and can see many celestial bodies in detail, you may want a mirror-lens model (see telescope type below). Begin your examination with a consideration of telescope type and aperture, and then weigh the other factors in terms of their importance to you.

TELESCOPE TYPE

Begin your evaluation by determining the type or design of telescope you want. This decision will be based on all three of the variables highlighted above: applications, experience level and price. Three telescope designs are available:

• Refractor Telescopes

  Refractor telescopes are the most familiar type to most people: a large lens at one end and an eyepiece at the other separated by a long tube. They function by passing light through a large objective lens and focusing it towards the eyepiece. This simple design requires virtually no adjustment, is equally ideal for land and astronomical subjects and is easy to use. The simple design also offers remarkable image detail, clarity and contrast and has no obstructions or internal parts to interfere with the image.
  
  The only disadvantage of refractors results from the tendency of inferior lenses to have color problems, such as colored rings around objects, when light is dispersed through them. The benefit of Meade refractor telescopes is that they are all of an achromatic (2-element) design that virtually eliminates false color when light passes through the telescope. The highest quality Meade refractors are of an apochromatic design that feature extra-low dispersion glass, which entirely eliminates color problems and aberrations. The result is a more accurate and sharper image that has the best quality possible of any telescope design. The only downside from these remarkable color-free technologies is they are somewhat expensive to produce, reducing the amount of overall aperture (see below) you can get per-dollar. The design also can make the telescope more difficult to transport because of its long optical tube.
   

• Reflector Telescopes

  Reflector telescopes are the best aperture-per-dollar (see below) telescope option. Operating through mirrors instead of lenses, light enters one end of the telescope and is reflected back-up by a focusing mirror to another mirror and into the eyepiece. By using mirrors instead of lenses, reflectors avoid the color problems and the somewhat expensive corrections featured in refractors and can achieve larger apertures for a lower cost. The placement of the eyepiece at the front of the reflecting telescope also provides a comfortable viewing position versus the traditional back of telescope location.
  
  The disadvantages of a Newtonian reflector relate to the mirror design. Most notably, reflectors produce images that are upside-down. This orientation is of little consequence during celestial observation, but does make reflectors incompatible with land viewing. In addition, with the secondary mirror suspended at the top of the telescope, some light is blocked, which can result in decreased contrast, although with proper mirror alignment the impact is generally negligible. This situation does mean however that the mirrors need simple centering adjustments from time to time, which are easy to do but may prove slightly challenging at first for some. As a result of these restrictions, image quality cannot completely match that of a refractor, but can still product remarkably clear, colorful and detailed imagery.
   

• Mirror Lens (Catadioptric) Telescopes

  Mirror Lens-designed telescopes allow for large apertures (see below) in a more compact package. After light enters the initial lens, it is reflected between a number of mirrors to gain a large focal length, a requirement to allow focusing of large aperture instruments. The light is then reflected into a traditional eyepiece where it produces images of extremely high contrast, color and quality. This compact design also makes the instrument highly portable and versatile allowing for more advanced mechanics and functionality than other telescope designs of similar aperture. All together, these features make mirror-lens telescopes a complete package capable of terrestrial viewing, advanced astrophotography, CCD imaging and more.
  
  Mirror-Lens telescopes have only a few disadvantages when compared with refractors and reflectors - they do cost more per-aperture than reflectors and can't duplicate the perfect image quality of high-end apochromatic refractors. Otherwise, mirror-lens telescopes combine features and capabilities that make them excellent telescopes for all applications. Two mirror lens designs are available at DiscoveryStore: Schmidt-Cassegrains, the standard mirror-lens design, and Maksutov-Cassegrains, which are more expensive per-aperture given their extremely high-quality mirrors and lenses.

APERTURE

Aperture is the most important element of your telescope purchase as it determines what you see and the detail with which you see it. Measured in millimeters or inches, aperture is the diameter of the front end of the telescope where light is collected. Because the primary purpose of a telescope is to collect light (not to magnify as commonly thought), a telescope that gathers more light performs better and can show greater detail of astronomical and terrestrial subjects. For example, a 2" aperture telescope may show the cloud belts of Jupiter, but a 4" model will show added structure, color and smaller cloud belts not previously visible.

In theory, telescopes of similar aperture will collect the same amount of light and produce similar images. However, differences in materials, coatings, telescope type or eyepiece use may produce different results even at the same aperture. In addition, small changes in aperture can have drastic effects on light collection. For example, a 5" version of a telescope can collect 56% more light than the same telescope with a 4" aperture. The term "aperture-per-dollar" is often used to compare telescopes at different price ranges. Telescopes that have a higher aperture-per-dollar allow you to see more objects and detail for comparably less monetary cost.

OPTICAL SPECIFICATIONS

As important as aperture is, a telescope's optical performance also depends on the materials and coatings used in its manufacture. For example, as discussed above, refractors with apochromatic lenses perform better than achromatic versions because they completely eliminate color aberrations. The highest quality optical components result in telescopes that are rated as diffraction-limited. Simply, this means that the optical system's performance is of professional quality and is limited only by the principles of physics, with no additional performance improvements technically possible. Other features include BK7 superclear optical glass that permits observations into the ultraviolet region and EMC super-multi coatings that maximize light transmission through the optical system.

MAGNIFICATION POWER

Power is one of the least important factors to consider in a telescope purchase, but is included here for an explanation. Viewing power for each telescope varies based on the eyepiece and barlow lens in use. To determine the viewing power, divide the telescope's focal length in millimeters by the eyepiece size in millimeters. If you are also using a barlow lens, multiply the result by the barlow factor.

For example, using the ETX-70AT (372 focal length) with a 5mm eyepiece would yield 74x power. If a 3x barlow lens was also used, the magnification power would increase to 222x.

Too often, first-time buyers try and overpower their telescope in an effort to gain greater detail and instead get images that are fuzzy, ill-defined or poorly resolved through no fault of the telescope. The term Maximum Practical Power indicates the highest recommended power level to use with your telescope, based on its aperture. At powers above this limit, images will tend to appear blurry or fuzzy as the telescope's aperture isn't large enough to collect the light needed at that power. Dawes' Limit provides a general calculation of 50x the aperture in inches to determine the maximum practical power for a telescope. For example, a 60mm telescope would have a maximum power of 2.4" x 50 = 120x. This limit can be improved upon if a telescope's components and/or optics maximize viewing ability, but Dawes' Limit is general starting point. Dawes' Limit also re-emphasizes that aperture--not power--is the primary means of observing finely detailed and bright images.

FOCAL LENGTH AND FOCAL RATIO

Focal length is a simple measure in millimeters of the path light takes before its focused in the eyepiece and is important to know when determining the magnification power. It also contributes to something called the focal ratio, which is the ratio of focal length to aperture. Long focal ratios (e.g. f/16) yield narrower fields of view, but with higher-contrast images desired by planetary observers. Shorter focal ratios (e.g. f/4) yield extremely wide fields and faster photographic speeds, but generally with a lower level of image corrections at the edge of the field. Most telescopes compromise at about f/10, a ratio that permits comfortably wide fields, reasonable photographic speed and very good image contrast. If you have specific applications in mind, you may want to choose a telescope with a specific focal ratio.

MOUNTINGS

While all the above considerations focus on the quality of the images being produced by the telescope, mountings are solely concerned with making the telescope stable and easy to use. There are a number of mounts available, each with specific advantages and applications in mind:

• Altazimuth Mount

  Altazimuth mounts move the telescope in the vertical and horizontal directions independently. While this provides the user with great freedom for terrestrial observations, celestial viewing can be somewhat more challenging. Because objects move across the sky diagonally, tracking them as they move requires physical motion adjustments on both axes instead of just one. This extra effort is greatly simplified with the addition of electronic controls that let you make these adjustments at the push of a button.
   

• Equatorial Mount

  The equatorial mount allows easier tracking of celestial objects by aligning one telescope axis with the Earth's polar (rotational) axis. When aligned, tracking of astronomical sights can be achieved by moving the telescope right to left, instead of the two simultaneous motions required of the altazimuth mount. Before each use, equatorial mounts must first be aligned with Polaris (the North Star), a simple set-up step that takes about 2 minutes time. Another advantage of the equatorial mount is that a small electric motor may be connected to the telescope's polar axis, for fully automatic tracking of astronomical objects.
   

• Dobsonian Mount

  Dobsonian mounts come exclusively on reflector telescopes and are extremely simple, which helps reduce the overall cost of the telescope. The mount sits on the ground and two pads on the telescope sit in the cut-outs of the mount. When set-up, the telescope moves up, down, left and right with ease, while providing solid stability. Dobsonian mounts are priced well below other mount types, freeing up your budget to spend on more accessories or a larger aperture.
   

• Computer-Controlled Mounts

  Fork mounts with dual-axis drive systems represent the newest developments in telescope mounting. Although they operate like altazimuth mounts in vertical and horizontal directions, they use electronic and mechanical controls to allow push-button locating and tracking. When equipped with Autostar controllers or other advanced computer systems, computer-controlled mounts can automatically track celestial objects on both axes simultaneously, negating the need for an equatorial mount. Equally as important, these advanced mountings allow automatic "GO TO" object location at the push of a button with the proper equipment. Worm gears on these mounts increase the control needed to precisely move heavy telescopes.

ELECTRONIC CONTROLS AND "GO TO" DEVICES

As discussed above, many Meade telescopes come with electronic controls and mechanics to greatly facilitate the location, centering and tracking of astronomical subjects. These telescopes vary in how many speeds they feature (from 4-9) and whether or not they track objects once centered in the field of view. More advanced models will automatically follow the objects you center the telescope on by correcting for the earth's rotation, while simpler models may require the addition of an equatorial wedge, mount or Autostar for the same effect.

Many Meade models use their electronic controls with computer databases to achieve automatic "GO TO" object location. In these instances, the user can select an object from a database catalog of the computer, and the telescope will automatically locate, center and track it as it moves. The number of objects available depends on the telescope model and range from 1,400 to 65,000. For less expensive models, this feature usually requires the addition of an optional Autostar computer controller while high-end models come with this feature already installed. This ability is a great feature for all astronomers, but is especially useful for beginners who need help locating celestial objects. Guided tours, programmable features and other extras make them great tools for stellar exploration.