Evolution of the Astronomical Eyepiece - Brayebrook Observatory

SURFACE QUALITY OF OPTICAL ELEMENTS
Surface Quality refers to the cosmetic features of an optical element in terms of the
amount of defects that can be visually inspected on the element’s surface. A scratch is
any mark or tear on the surface and a dig is any pit or divot in the surface. The Scratch-
Dig specification defines the quality of a polished surface.
Scratches and Digs are defined by the U.S. Mil.SPEC. MIL-0-13810A. Scratch numbers
are essentially defined by the width of a scratch in 1/10,000mm (0.1µm). However the
lengths of scratches and combinations of smaller scratches also contribute to the scratch
number. Dig numbers are defined by the actual diameter of a dig in 1/100mm (10µm).
Smaller digs and irregularly shaped digs also affect the dig number. Specification of optical
surface quality is denoted by the scratch number followed by the dig number. For
example a scratch-dig ratio of 60-40 is acceptable in commer cial eyepiece optics.
Typical scratch-dig ratios, from the highest quality essential for scatter free lenses,
through to the minimum acceptable quality suitable in emitter-detector optics are:
SCRATCH No.: 10; 20; 40; 60; 80; 120; 160.
DIG No.: 05; 10; 20; 40; 50; 70; 100.
the higher the scratch and dig numbers the lower the surface quality.
54