Single-Channel Photometer at the ESO 0.5-m

It was a conventional single-channel photometer — an instrument used to measure the intensity of light — consisting of the several components. As light travelled through the photometer, it first encountered a viewfinder. This allowed astronomers to focus on a desired star from a relatively large field of view. The light then passed through a diaphragm wheel, which constricted the field of view of the instrument to a single star. Finally, light passed through a filter wheel and photomultiplier, which together transformed starlight at a certain wavelength into an electrical signal. The photomultiplier tubes operated at ambient temperature, but could be cooled either thermo-electrically or using dry-ice.

The viewfinder had a field of view of 15 arcminutes, an area roughly a quarter of the size of the full Moon, and the diaphragm wheel used to constrict the view to a single star had apertures ranging from 10 to 80 arcseconds in diameter. The filter wheel had spaces for 12 different filters, and allowed astronomers to perform both Strömgren and Johnson photometry.

The Single-Channel Photometer was designed to allow fast daytime changeovers of photomultipliers and cold boxes, allowing an observer to choose the photomultiplier most suitable for their programme from a range of tubes.

A HP 1000 computer in the telescope dome controlled the Single-Channel Photometer using customised software, and allowed the instrument to be operated in manual, automatic, repeat and high speed modes.

The Single-Channel Photometer was decommissioned from the ESO 0.5-metre telescope in the 1990s.