ROYAL ASTRONOMICAL SOCIETY / SCIENCE PHOTO LIBRARY ROYAL ASTRONOMICAL SOCIETY / SCIENCE PHOTO LIBRARY
Iris photometer. This instrument is used to measure the relative brightness of stars. An adjustable aperture (the iris) is adjusted around the star being measured until the light matches a reference value. This is seen on the screen at left. This is a digitised iris photometer, with the digital readings displayed on the machine at centre. Digital astronomical equipment was developed in the mid-20th century. This photometer is located at the Royal Observatory, Edinburgh, Scotland.
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