APQ Technical Glossary
The APQ Technical Glossary explains key terms used in optical design, optical and mechanical manufacturing, and astronomical applications. The glossary will be expanded continuously.
Entries are arranged alphabetically by the internationally used English technical terms.
A–Z: A | C | F | M | O | P | Q | S | W
A
Abbe number
A dimensionless measure of the dispersion of an optical material. A higher Abbe number generally indicates lower dispersion in the visible spectrum.
Apochromat
An optical system with strongly reduced secondary spectrum. In the classical definition, three wavelengths are brought to a common focus; actual polychromatic performance depends on the complete design and spectral range.
C
Chromatic focal shift
The change in the best-focus position as a function of wavelength. It is a key measure of longitudinal chromatic correction.
F
Field flattener
An optical group designed primarily to correct field curvature so that the image surface is flatter across the field.
Fluorite (CaF2)
Crystalline calcium fluoride with very low dispersion and high transmission over a broad spectral range. It is used as a premium optical material in selected APQ objectives.
Focal reducer
An optical group that reduces the effective focal length and focal ratio of a telescope; it may also incorporate field correction.
M
Magnetorheological Finishing (MRF)
A deterministic polishing process that uses a magnetically controlled abrasive fluid to correct optical surfaces with high precision.
O
Oil spacing
A design in which a thin optical fluid layer is placed between adjacent lens surfaces. It can reduce internal reflections and provides specific optical and mechanical design advantages.
P
Polychromat
At APQ, Polychromat denotes an objective designed for high image quality and chromatic correction over an unusually broad spectral range.
Q
Quadruplet
An optical system consisting of four lens elements. APQ Fluorite Quadruplet Polychromats combine four elements in a broadband corrected objective.
S
Spot diagram
A ray-trace diagram showing the geometric distribution of image points for selected field positions and wavelengths. It is used to assess geometrical image quality.
Strehl ratio
The ratio of the peak intensity of the actual point-spread function to that of an ideal diffraction-limited system under the same conditions.
W
Wavefront error
The deviation of an actual optical wavefront from the ideal reference wavefront, commonly specified as RMS or peak-to-valley (PV).