TERMINOLOGY

Aperture: This is the diameter of the telescope's objective ( front lens, corrector or primary mirror depending on the type of telescope). A telescope's aperture determines its light-gathering power and resolution limit.

Binocular Terminology: Binocular specifications are given by specifying the magnification and aperture (in millimeters) separated by an "x". For example, 20x80 binoculars have a magnification of 20x and front lenses (objectives) that are 80 millimeters in diameter.

Date: Dates within postings are provided in Universal Time (corresponding to the date in Greenwich, England at the time of the observation) and are followed by "U.T."

FOV: This is an abbreviated term for "Field Of View". It's used to communicate the diameter of the area of sky visible when looking through an optical instrument. It may be specified in degrees and fractions of a degree, as in 3.1 degrees (three and one-tenth degrees); or in minutes and/or seconds of arc as in 32' 15" (thirty-two minutes fifteen seconds). There are 60 minutes of arc in one degree, and 60 seconds of arc in one minute of arc.

Magnification: This is how many times larger an object appears in a telescope than it would appear to the unaided-eye. An "x" appears after the number. For a magnification of fifty-six I would type: 56x. A telescope's magnification is easily changed by changing to a different eyepiece.

NELM: I use this term as an abbreviation for "Naked-Eye Limiting Magnitude". It's the magnitude (brightness) of the faintest star visible to the unaided-eye at the zenith (directly overhead). A larger number signifies a fainter star. NELM is used as a measure of sky darkness. In general I make use of a group of stars of known brightness in the vicinity of Polaris, the North Star. A minor adjustment is made to arrive at the value for the zenith.

Newtonian: A telescope type invented by Sir Isaac Newton that utilizes a parabolic mirror at the back end of the telescope tube to collect and concentrate light. A small, flat, diagonal mirror in the center of the front of the tube redirects the light to the side of the tube where the eyepiece is located. A Newtonian is a type of reflecting telescope.

Refractor: A telescope that utilizes a front, objective lens (or lenses) to collect and concentrate light is a refractor. A "spyglass" is a refractor. Achromats and apochromats are types of refractors.

SCT: "Schmidt Cassegrain Telescope" -- One of several types of telescopes that utilize a combination of mirrors and lenses to collect and concentrate light. The secondary mirror of an SCT is in the center of the front lens (corrector plate).

Seeing: This is a measure of atmospheric steadiness. In this blog I'll use the terms: Excellent, Good, Fair, Poor, and Bad. When appropriate I'll use the appearance of the highly magnified image of a star to determine night-time seeing conditions. This scale (not fully explained here) is dependent upon a telescope's aperture. For example, a night of Good seeing with a small telescope might be a night of equal or worse seeing for a larger telescope.

Telescope: When describing which telescope I used, I first provide the telescope's Aperture (see above) followed by the type of telescope (Newtonian, Refractor, SCT, etc.).

Time: All times, unless otherwise specified, will be Universal Time (basically Greenwich Mean Time). I'll try to add a "U.T." as a reminder, as in 23:45 U.T.