AbstractTheCOSMOSmachine is an automatic high-speed micro densitometer. Although specifically designed for high precision measurements in astronomy it has in practice found use in many other branches of science (e.g. medicine, aerial photography, sideways looking radar pictures and geology). For stability and accuracy the machine is maintained in an air-conditioned and temperature controlled (20°±0.5°C) environment. Thex,ycarriage is digitised to 0.5 micron in each co-ordinate. The plate holder will accept plates up to a maximum size of 356×356 mm. The transmission of each pixel is digitised into 256 values. This gives a dynamic range in density of over 2. The zero point of the dynamic range is adjustable subject to a maximum density measurable by the machine of 3.