A

Achromatic

Non-chromatic; lens combination correcting primary color errors. Secondary color errors may still occur at high magnification.

Amici prism

Prism with 45° or 90° deflection producing an upright, correct image.

Aperture

Opening.

Apochromatic

Lens combination correcting all color errors including secondary ones.

Equatorial operation

Alignment of mount to celestial pole so RA axis is parallel to Earth axis.

Aspherical

Non-spherical.

Resolution

Ability to distinguish fine details (angular resolution).

Damping time

Time needed for mount to stabilize after vibration.

Autoguider

Device automatically correcting tracking during astrophotography.

Autocollimation

Optical testing method using double-pass light beam.

Alt-azimuth operation

Horizontal/vertical mount alignment.

Aphelion

Farthest point from the Sun.

Asteroid

Small body orbiting the Sun.

Astronomical unit

1.49 × 10¹¹ meters; Earth-Sun distance.

B

Barlow lens

Increases focal length and magnification.

Arcminute

1/60 of a degree.

Arcsecond

1/60 of an arcminute.

Focal length

Distance from lens to focal point.

Field rotation

Image rotation due to improper mount alignment.

Field derotator

Device compensating field rotation.

BK-7

Optical glass type.

C

Calcium fluoride

Optical mineral.

Chromatic aberration

Color error.

Clear Skies (CS)

Common astronomer greeting.

D

Declination (DEC)

Celestial coordinate similar to latitude.

Deep-sky

Objects outside solar system.

German mount

Equatorial mount type.

Dark adaptation

Eye adjustment to darkness.

E

ED glass

Extra-low dispersion glass.

Ecliptic

Earth’s orbital plane.

F

Fluorite

See calcium fluoride.

Prime focus photography

Imaging without eyepiece.

Focusing

Adjusting sharpness.

G

Degree

Angular unit.

Limiting magnitude

Faintest visible brightness.

GoTo

Automatic object positioning.

GPS

Positioning system.

H

Primary mirror focusing

Focusing via mirror movement.

Heavy-duty

Robust construction.

Altitude

Height above horizon.

I

Interference filter

Selective light filter.

Initialization

Calibration process.

K

Kellner eyepiece

Three-element eyepiece.

Contrast

Difference in brightness.

Conjunction

Closest approach of objects.

Comets

Solar system objects.

L

Light gathering power

Ability to collect light.

Transmission

Light throughput.

M

Messier objects

Catalog of deep-sky objects.

Maksutov

Mirror telescope type.

N

Tracking motor

Compensates Earth rotation.

Nadir

Point opposite zenith.

O

Obstruction

Light blockage.

Eyepiece

Viewing lens.

P

Parallax

Apparent shift due to Earth motion.

Parsec

Distance unit.

Planet

Large celestial body orbiting a star.

R

Reflector

Mirror telescope.

Refractor

Lens telescope.

S

Seeing

Atmospheric conditions.

Sidereal rate

Earth rotation tracking speed.

T

T2 ring

Camera adapter.

V

Magnification range

Usable magnification limits.

W

White dwarf

Collapsed stellar core.

Z

Zenith prism

90° viewing prism.

Inch

25.4 mm.