| Aberration: Any optical defect and/or design error which causes any of the processed light to deviate from reaching the focal point, therefore reducing the quality of the image. |
The maximum view angle of an optical instrument. The number, in degrees, supplied by the manufacturer is the Apparent Field of View. To find the Actual Field of View ( also known as the Actual Field of View ), divide the Apparent Field of View by the Magnification. |
| Absolute Magnitude: The apparent brightness a star would have if placed at a distance of 10 parsecs from the earth. |
Filter: This is usually a disk of coloured glass or film that sits in front of the telescope eyepiece or objective. It transmits only certain wavelengths of light while rejecting others. ( It is important to remember that a solar filter must always be placed in front of the objective ). |
| Achromatic Lens: A refractor lens, made of two or sometimes three separate lenses, which has the effect of bringing most of the viewed colours to a sharp focus, thus reducing chromatic aberration. |
Finder Scope: A low power telescope attached parallel to the main instrument which provides easy object locating and telescope aiming. |
| Alt-azimuth: A simple mount that allows movement in altitude ( up and down ) and in azimuth ( side to side ). |
Focal Length: The distance of the light path from the objective ( primary lens or mirror ) to the convergence of the beam. T he convergence is known as the Focus, or Focal Point. |
| Antireflection Coating: A thin layer of film applied to an optical surface that reduces the loss of transmission of light. |
Focal Ratio: This is found by dividing an optical system’s Focal Length by it’s Aperture. The resulting value is sometimes referred to the system’s “speed”. |
| Aperture: The diameter of a primary mirror or lens. |
Focuser: A device which brings the light rays in a telescope to a precise focus. Common designs include geared ( rack and pinion ), gearless ( Crayford style ), and helical. |
| Barlow Lens: A “negative” lens which, when placed in front of the eyepiece, increases the focal length and magnification and decreases the field. |
A system of latitude and longitude defined by the plane of our galaxy rather than the equatorial system ( RA and DEC ), based on the celestial equator. Coordinates can also be specified locally, for example by Altitude and Azimuth. |
| Catadiotric System: A system using both lens and mirror components to produce an image, allowing the telescope to be more compact than other designs. |
Lens: A transparent optical element consisting of one or more pieces of glass. A lens has curved surfaces that bring distant light to focus. |
| Celestial Sphere: An imaginary ball with the Earth at it’s centre. All astronomical bodies, disregarding their true distance, are assigned a two-dimensional location on the surface of this ball`. |
The amount by which a system increases the apparent size of objects. Magnification is determined by dividing the Focal Length of the telescope by the Focal Length of the eyepiece. |
| Chromatic Aberration: The tendency of a lens to bend light of different colours by unequal amounts. It can produce nasty haloes around bright objects. A well made achromatic lens reduces this problem. |
In a telescope, it is a highly polished surface made to reflect light. Primary mirrors are usually made spherical or paraboloidal ( parabolic ) to focus the light rays. |
| Coated Optics: In lenses this is an antireflection coating. In mirrors a coating is applied that preserves the aluminum mirror surface. |
The primary or largest element in an optical system; sometimes called the “fixes optics”. |
| Collimation: The process of aligning all the elements of an optical system. Collimation is routinely needed in reflectors, often in Catadioptric systems, but seldom in a refractor. |
The housing and optical train of a telescope; not including the mount, diagonal, eyepiece, or accessories. |
| Coma: An off-axis aberration that results in blurry star images near the edge of the field of view. It is most common in fast f-ratio reflecting telescopes. |
A parabolic or more accurately a “paraboloidal” mirror, is ground to a shape which brings all incoming light rays to a perfect focus , on axis. |
| Constellation: A randomly defined group of stars within a boundry, that is said to make a picture or diagramin the nighttime sky. The sky is currently divided into 88 Constellations. |
A telescope mount’s axis that is parallel with the Earth’s axis . With a drive motor, the motion of stars due to the Earth’s movement can be counteracted so that they remain in the field of view. |
| Contrast: The difference between the brightness of the object and it’s surroundings. The higher the contrast the easier the object is to view. |
See Magnifying Power. |
| Declination: Similar to Latitude on Earth’s surface, it is the distance in degrees North or South of the Celestial Equator ( the projection of the Earth’s Equator onto the Celestial Sphere ).The degrees can be subdivided into minutes and seconds. |
The focal point of the objective mirror or lens. |
| Deep Sky: A name given by amateur astronomers to objects beyond our Sun and it’s planets. |
The ability of an optical system to reveal daetails. |
| Dew Cap: A tube extending forward from the lens of a telescope. It prevents dew from forming on the lens as it cools down, and acts as a shade to reduce reflections during the day. |
The ability of a telescope to separate closely positioned objects. |
| Diagonal: A mirror or prism system which changes the angle and orientation of the light rays coming from the telescope to the eyepiece. |
Similar to but not the same as Latitude on the Earth’s surface. It is the position eastwards from the Vernal Equinox, in 24 one-hour units. The hours can be sub-divided into minutes and seconds. |
| Double Star: Two or more stars that appear very close in their positions. True double stars are in orbit about one another, while optical doubles simply seem close from our point of view. |
Circular scales attached to the telescope. They are marked off in degrees of Declination and hours of Right Ascension. Together, the circles allow the position of a known object to be found by setting the dials to the equatorial coordinates. |
The blocking of one astronomical body by another as seen from Earth. The most common of these events are the Solar and Lunar eclipses. |
A blurring of the image caused by the inability of a spherical mirror to focus all light from infinity to one focal point. Light rays from the edge of the spherical mirror focus to different points than those from the centre. |
| Equatorial Mount: A telescope mount with an axis parallel to the axis of the Earth. This provides easy tracking of sky objects and for photography when combined with a clock drive. |
True Field: How much sky, in angular measure, is available at the eyepiece. It is contrasted with Apparent Field, which measures the field of the eyepiece alone. |
| Eyepiece: Also called an ocular. This is a small tube that contains the lenses needed to bring a telescope’s focus to a final image in the eye. Telescopes usually come with at least two eyepieces; one for low power viewing, the second for a higher power view. |
An eyepiece with an Apparent Field of more than 50 degrees. |
| Eye Relief: This is the distance between the eyepiece lens and the position in which the eye must be placed to see through the telescope. Telescope users who wear glasses while observing , appreciate the benefits of longer eye relief. |
The point in the sky that is directly overhead. |
| Exit Pupil: This is the diameter of the beam of light from the eyepiece which reaches the pupil of the eye. It is usually expressed in mm, and is determined by dividing the diameter of the primary ( in mm ) by the Magnification. Knowing this value and the diameter of your diluted pupil allows you to choose the eyepieces which will work best for you with a specific telescope. |
Zoom Lens: An optical system which provides a variable focal length. |
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