Abell: A catalog of rich clusters of galaxies. This catalog is an all sky catalog covering both northern and southern hemispheres containing 4,073 rich galaxy clusters. George Ogden Abell (1927-83) published the original catalog of 2,712 rich clusters of galaxies in 1958, the catalog has since been revised to include another 1,361

Absolute Magnitude: Absolute magnitude is the apparent magnitude an object would have if it were a standardized distance from our location. This allows the overall brightness of the object to be compared without regards to distance. The measure of absolute magnitude uses the same convention as the Visual magnitude, with a 2.512 difference in the brightness steps / rates. (because 2.5125 = 100) same as the Magnitude scale as already explained. For example: our galaxy, the Milky Way has an absolute magnitude of -20.5 So if a quasar has a absolute magnitude of -25.5, it would be 100 times brighter than the Milky Way.. Or by 5 magnitudes brighter!

ACHRO: Achromatic refractor

AFOV: (Apparent Field of View) Apparent field of view is the angular diameter, expressed in degrees, of the circle of light that the eye sees.

Airy Disk: This is the central spot of the image of a star, surrounded by diffraction rings, formed when viewed in a telescope. Noticeable before and after focus of a star. Due to diffraction and since stars are so far away in distance, no matter what magnification you use, it is impossible to resolve a star into a disk as you could a planet. The only star that can be seen as a large disk is our own sun. The disk formed by the diffraction of stellar images is known as an Airy Disk, after the English Astronomer Royal, George Airy.

Due to the wave nature of light, light passing through apertures is diffracted, and the diffraction increases with decreasing aperture size. The resulting diffraction pattern of a uniformly illuminated circular aperture has a bright region in the center, known as the Airy Disc, which is surrounded by concentric rings. The diameter of this disc is related to the wavelength of the illuminating light and the f-number of the circular aperture. The angle from the center at which the first minima occurs is.

ALT: Altitude

Aperture: Aperture is the diameter of the main / primary mirror or objective lens of the telescope. The larger the aperture, the better the resolution and the fainter the objects you can see. The more aperture a telescope has, the more light grasping ability it will have.. Thus allowing you to see farther and fainter objects.. Also allowing closer brighter objects to be seen in even greater clarity and detail

Most measurements used when referring to telescopes are in millimeters. Although, you will still find many that are in Inches.

1”in. = 25.4 millimeters
10mm = 1centimeter
2.54cm = 1”in
39”in. = 1meter

Aperture gain: Although not a commonly heard term, this one is still important and helpful to know. It is the gain in aperture size as compared to the diameter of your pupil. On average, the human eye when dilated and adapted to darkness, is approximately 7 to 8mm in diameter.. Aperture gain will give you an idea of the faintest stars visible to you through a telescope of a specific aperture.. For example, If your pupil is 8mm in diameter, a 80mm aperture telescope is 100 times the size of your pupil. 100 x gain in aperture. That is the same as a five-magnitude difference, So what does that mean to you? It means that if you can see a star of the 6th-magnitude with the naked eye, then you should be able to see a star of the 11th-magnitude in a 70-mm telescope. We will discuss magnitude later in this article. This reasoning ignores loss of in the optics and or due to the quality of optics. Also ignores atmospheric conditions effect on seeing...

To calculate what the aperture gain is for your telescope You would divide the aperture of your telescope by the diameter of your pupil squared. Or multiplied by itself, which ever you prefer. Unless you know the exact diameter you can only guess that you are an average person.. So we can say your pupil size when dark adapt is 7mm.. Let’s say you have a telescope with an aperture of 6”

Aperture gain = Objective Diameter / Eye Pupil Diameter 2

Since we know 6” is equal to 150mm you would take 150 / 7 = 21.42 or 462 So a 6” aperture telescope would have 462x aperture gain over the 7mm pupil of your eye.

APO: Apochromatic refractor

Apparent magnitude: Given the absolute magnitude , you can also calculate the apparent magnitude from any distance.

Absolute Magnitude for planets, comets and asteroids: A different definition of absolute magnitude is used for non stellar objects when we refer to the planets, Comets and asteroids. The absolute magnitude is defined as the apparent magnitude that the object would have if it was one (AS) astronomical unit from both the Sun and the Earth at a phase angle of zero degrees. Physically this is impossible, although convenient for the purpose calculation.

Calculations: Formula for H: (Absolute Magnitude)

where the apparent magnitude of the Sun at 1 AU is -26.73, is the geometric albedo of the body (a number between 0 and 1), is its radius. and is 1 au (˜149.6 Gm).

Example Moon: = 0.12, = 3476/2 km = 1738 km

Apparent magnitude The absolute magnitude can be used to help calculate the apparent magnitude of a body under different conditions.

where as 1 AU, is the phase angle, the angle between the Sun-Body and Body-Observer lines; by the law of cosines, we have:

as the phase integral (integration of reflected light; a number in the 0 to 1 range) Example: (An ideal diffuse reflecting sphere) - A reasonable first approximation for planetary bodies

A full-phase diffuse sphere reflects 2/3 as much light as a diffuse disc of the same diameter Distances: is the distance between the observer and the body is the distance between the Sun and the body is the distance between the observer and the Sun

Example: Moon = +0.25 = = 1 AU = 384.5 Mm = 2.57 mau How bright is the Moon from Earth? Full Moon: = 0, ( ˜ 2/3)

(Actual -12.7) A full Moon reflects 30% more light at full phase than a perfect diffuse reflector predicts. Quarter Moon: = 90°, (if diffuse reflector)

(Actual approximately -11.0) The diffuse reflector formula does better for smaller phases.