Beginner Level - Suited for a wide range of uses, these products are simple to operate and set up. Some initial assembly may be required depending on the type of product. Optical instruments with this rating will have very good optical and mechanical quality. Great for families, young people, and folks who don't want to mess with equipment and complexity. Beginning level telescopes will show you countless lunar craters, Saturn's rings and a myriad of star clusters and nebulae! Referring to the manual is recommended.

Intermediate Level - These products offer higher performance and more advanced features than Level 1: Beginner models. They typically take a bit longer to learn and need some set-up or adjustments. But anyone with the slightest technical bent will have no problem getting familiar with these models. Referring to the manual is recommended.

Advanced Level - These products provide the best performance but may require more skill to master and appreciate. Optical instruments with this rating have exceptionally fine optics and mechanics. Some are easy to use but might be on the large or heavy side. Some are intended for specialized uses. These products will appeal to the more technically inclined. Referring to the manual is highly recommended.

For telescopes, the optical diameter (also known as aperture) is the size of a telescope's main light-collecting lens or primary mirror, measured in millimeters or inches. Telescopes with larger optical diameters collect more light, which leads to an increase in brightness and image resolution compared to smaller instruments.

For binoculars, the optical diameter (also known as objective lens diameter) is the size of each of the front-facing objective lenses of a binocular measured in millimeters. Binoculars with larger objective lenses collect more light, which increases image resolution and brightness. Binoculars with larger objective lenses are recommended for low light situations, and binoculars with at least 50mm or larger objective lenses are recommended for pleasing astronomical observations at night.

The focal ratio of an optical system is the ratio of a telescope's focal length to its aperture. Short focal ratios (f/5, f/4.5) produce wide fields of view and small image scales, while long focal lengths produce narrower fields of views and larger image scales.

Newtonian reflectors will have either a spherical shaped mirror, which is less expensive to produce, or a higher quality parabola, which does not result in spherical aberration. Cassegrain telescopes routinely use spheres in addition to other lenses in the optical path to correct for residual spherical aberration. Refractors use a series of lenses to provide a clear image. Designs range from a standard air-spaced doublet (two lenses in a row) to exotic designs such as oil-spaced triplets and 4-element multi group lenses.

The theoretical resolving power of a telescope can be calculated with the following formula: Resolving power (in arc seconds) = 4.56 divided by aperture of telescope (in inches). In metric units, this is: Resolving power (in arc seconds) = 116 divided by aperture of telescope (in millimeters). Note that the formula is independent of the telescope type or model, and is based only upon the aperture of the telescope. So the larger the telescope's aperture, the more it is capable of resolving. This is important to keep in mind when observing astronomical objects which require high resolution for best viewing, such as planets and double stars. However, it is usually atmospheric seeing conditions (not the telescope) which limits the actual resolving power on a given night; rarely is resolution less than one arc-second possible from even the best viewing locations on Earth.