The Single-Channel Photometer at the ESO 1-m

The Single-Channel Photometer was mounted at the Cassegrain focus of the ESO 1-metre telescope at the La Silla Observatory, Chile, in the 1970s. This 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. Strömgren and Johnson photometry were possible with the Single-Channel Photometer.

It was a conventional instrument that allowed for the intensity of light to be measured and was built of several components that were positioned along the optical axis. As light travelled through the photometer, it first encountered a viewfinder with a field of view of 7 arcminutes. This allowed astronomers to focus on the 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. In total, there were 10 diaphragm wheels available, which had a diameter range between 4 arcseconds and 88 arcseconds. Finally, light passed through a filter wheel and a photomultiplier, which together transformed the starlight at a certain wavelength into an electrical signal. Two filter wheels, each with 12 positions, were available. The photomultiplier tubes operated at ambient temperature but could be cooled either thermo-electrically or using dry-ice to minimise the impact of noise counts.

The Single-Channel Photometer, together with the telescope, was controllable through an HP 2100 computer from a single CRT console in the dome.

The stars to observe should not be brighter than V = 7.0 for UBV photometry, and V = 5.0 for uvby photometry. The purpose of these limitations was to avoid damage to the multiplier.

The observations could be stored on magnetic tape, paper tape, and/or line printer. The data saved on magnetic tape and paper tape used ASCII. In some cases, the magnetic tape could be edited and the data reduced at the observatory.

The Single-Channel Photometer was decommissioned in the 1990s.