The Infrared Photometer at the ESO 1-m

The Infrared Photometer was mounted at the Cassegrain focus of the ESO 1-metre telescope at the La Silla Observatory, Chile, in 1983.

The Infrared Photometer could do photometry in a selection of broad-band filters between 1.2 and 25 μm, narrow band spectrophotometry using circular variable filters (CVF) between 1.4 and 5.2 μm and between 8 and 14 μm and mapping with both broad band filters and CVFs.

The optical configuration of the photometer included different types of mirrors. An off-axis elliptical mirror converted the f/13.5 telescope beam to f/35 and formed an image of the secondary mirror on a small flat mirror. This flat mirror could be wobbled to provide sky chopping through any angle of up to 2.5 arcminutes (peak to peak) on the sky. It also had a central hole, sized to match the central obscuration of the telescope, behind which was located a spherical mirror, whose centre of curvature fell on the diaphragm wheel inside the detector cryostat. This mirror reduced the background radiation from the central obscuration which would otherwise be seen by the detectors. The central position of the wobbling mirror could be adjusted to cancel any chopping offset. The last mirror in the photometer mount was a 45 degrees beam diverter that could be rotated to direct the input beam to the required detector. It also included an eyepiece for guiding. The guiding eyepiece field was 5 arcmin and the magnitude limit which could be seen was V ~ 14 in night-time and V ~ 5 in day-time.

The Infrared Photometer was decommissioned in 1994.