The goal of this post is to give the reader a basic understanding of the terms and concepts surrounding amateur astronomy equipment so the reader can understand the purpose of each piece of equipment and how equipment choices affect one's observing and photography.
The terms covered here are focal length, aperture, focal ratio, eye relief, exit pupil, field of view, true field of view, and apparent field of view, telescope, eyepiece, barlow, diagonal, and filter. These definitions are meant to describe in a concise manner the most important aspect of the term being defined to a newcomer. In most cases there is a more formal definition which is strictly accurate, but which may be unnecessarily confusing if the goal is to convey a practical use of the term.
- Telescope - An instrument that gathers light and focuses it. Its primary function is to make dim objects brighter, and its secondary function is to make details more visible.
- Eyepiece - Used to magnify the telescope image. Eyepieces are primarily used by visual astronomers, but they are sometimes used in astrophotography.
- Barlow - Increases the focal length of a telescope.
- Diagonal - Used with some telescopes to divert the optical path to a more convenient angle.
- Filter - Used to reduce or eliminate undesired frequencies of light.
- Mount - This is the mechanism that holds the telescope and allows you to aim the telescope at different parts of the sky.
- Objective - The main light collecting element of a telescope.
- Aperture - The diameter of a telescope's objective.
- Focal Length - Focal length is used to characterize both telescopes and eyepieces.
- Focal Ratio - This is the ratio of the telescope's aperture to its focal length. Telescopes will commonly be specified by the aperture and either the focal ratio or the focal length.
- Eye Relief - This is the distance of your eye from an eyepiece.
- Exit Pupil - This is diameter of the image that exits the eyepiece.
- Field of View - This term is a bit ambiguous. If applied to a telescope it would probably describe the maximum angle of sky viewable through the instrument. If applied to an eyepiece, it would probably refer to the apparent field of view described below, but the user of the term may be implicitly referring to a true field of view which would apply to a particular eyepiece and telescope combination (see true field of view below).
- True Field of View - This is the angle of sky viewable with a particular eyepiece in a particular telescope.
- Apparent Field of View - A given telescope and eyepiece will show the user a specific area of the sky, but the design of the eyepiece may spread that area out over, for example, 30 or 80 degrees.
The next topic is a general discussion of
telescopes.
