A chronograph is a device used to measure the velocity of an object passing above it's photo sensors. The sensors are mounted at a fixed distance and the onboard computer calculates the speed of the projectile. On sunny days the unit is used with translucent white diffusers to help the sensors see the silhouette of the projectile, on cloudy days the diffusers can be removed. Pictured here is a model F1 chronograph from Shooting Chrony fitted with an indoor lighting kit.

The projectile is fired above the sensors and the velocity appears on the display. For accurate measurements, many shots are fired then the results are calculated, compared and averaged. Common ballistic calculations include; AVG (average), HI (fastest shot), LO (slowest shot), ES (extreme spread, HI minus LO), SD and AD (standard deviation and average deviation, used to measure variance). Chronograph data is indispensable for accurate fine tuning of any launcher.