The telescope's ability to collect light is directly related to the optics that are used. Telescopes with poor quality optics can be very frustrating to use. Here are some optical considerations to think about when buying a telescope:
- Focal length
- Focal ratio (f/number)
- Wave number
The telescope's ability to collect light is directly related to the size (diameter) of the objective lens or primary mirror. Generally, the bigger the lens or mirror, the more light the telescope collects and brings to focus, and the brighter the final image. Aperture is probably the most important consideration when buying a telescope, but it is not the only consideration. You want to try to purchase as much aperture as you can reasonably afford; however, you should also consider other factors that will be discussed below, including size, weight, storage space and portability. The biggest telescope is not always the best one for you!
This consideration is perhaps the most misleading to novice telescope buyers. Often, manufacturers of "cheap, department store" telescopes will display "200x power or more" on the boxes of their products. The magnification or power has little to do with the optical performance of the telescope, and is not a primary consideration. The telescope's ability to enlarge an image (magnification) depends upon the combination of the lenses used, usually a long focal length objective lens or primary mirror in combination with a short focal length eyepiece. As the magnification of an image increases, the field of view and the brightness of the image decrease. A general rule about magnification is that the telescope's maximum magnification is 40x to 60x (average = 50x) per inch of aperture. Since any magnification can be achieved for almost any telescope by using different eyepieces, aperture becomes a more important feature than magnification. Furthermore, most astronomical objects are best viewed on a low magnification or power to gather the most light possible.
Focal length is the distance required by the objective lens or primary mirror to bring all of the light collected to one point (the focus or focal point). The focal length of the lens or mirror is usually printed somewhere on the telescope's tube; if not, it should be found in the instructions or on the box. The focal length is an important number to know. As discussed above, magnification depends upon the focal length of the objective lens or primary mirror and the focal length of the eyepiece. Generally, long focal length telescopes are capable of delivering higher magnifications than short focal lengths. However, do not mistake the length of the telescope tube for the focal length, because compound telescopes have a folded light path, which delivers a long focal length in a short tube.
Focal Ratio (f/number)
Focal ratio or f/number relates to the brightness of the image and the width of the field of view. The focal ratio is the focal length of the objective lens or primary mirror divided by the aperture. The focal ratio concept comes from the camera world, where a small focal ratio means a short exposure time for the film, and was said to be "fast." Although the same is true for a telescope, if a "fast" and a "slow" telescope are compared at the same magnification for visual rather than photographic viewing, then both telescopes will have the same quality image. Generally, the following information about focal ratios can be helpful:
- f/10 or higher - good for observing the moon, planets and double stars (high power)
- f/8 - good for all-around viewing
- f/6 or lower - good for viewing deep-sky objects (low power)
Wave Number or Wave Error
No glass lens or mirror is perfect! The wave number or wave error is an indication of how well the mirror or lens was ground compared to an ideal surface. This number is probably listed in the instrument's specifications, somewhere in the instructions (it is never prominently displayed, so you may have to ask the dealer or manufacturer). Wave numbers are expressed as fractions of a wavelength. The smaller the wave number, the better the mirror or lens. While the absolute minimum acceptable wave number is one-fourth, good telescopes generally have wave numbers of one-eighth or less. Also, the overall performance of the telescope is an accumulation of the wave errors of each optical component. For example, a telescope with two mirrors (each with a one-eighth wave number) will have an overall wave error of one-fourth.
The ability to see fine detail in an image of the moon, a planet or a double star depends upon how well the telescope can separate, or resolve, two close objects. Resolution depends upon the aperture of the telescope, the quality of the optics, and the observing conditions (e.g. light-polluted vs. dark, dry vs. humid atmosphere). Generally, the ideal limit, also called the Dawes limit (arc-seconds), is 4.46/aperture in inches or 116/aperture in millimeters.