Reticle

A reticle is a reference pattern positioned in an optical instrument's viewing or imaging path. Crosshairs can mark a reference position; repeated lines or dots can provide a scale or comparison pattern. The pattern may be viewed along with an image or illuminated and projected by the instrument.^[ref]

In an autocollimator, an illuminated reticle is placed so the collimator projects its image at infinity. An external reflector returns the light, and the returned pattern's position is compared with a reference in the image plane. The reticle supplies the recognizable pattern; the optical geometry connects its displacement with angle.^[ref]

For an ideal focal-plane example, suppose adjacent marks are $p=10\mu \mathrm{m}$ apart behind optics with focal length $f=100\mathrm{mm}$. Their small angular separation is $p/f=10^{-4}\mathrm{rad}$, about20.6 arcseconds. In a double-pass mirror-angle measurement, the corresponding mirror tilt is half the return-beam angular change. The numbers are chosen to illustrate the distinction between pattern spacing and the physical quantity ultimately measured.

A reticle is therefore not automatically an absolute measuring scale. Scale interpretation depends on its location, the effective focal length or image magnification, and measurement calibration. Crosshair coincidence alone establishes a position comparison relative to a chosen reference.