During the recent years, a series of steps have been taken to improve the image quality on the Danish 1.54m telescope. Among these are the installation of side ports, insulation of the dome floor and walls and installation of main mirror ventilation and cooling. The most recent effort is to replace the Loral 2k3eb CCD detector for the DFOSC with an EEV/Marconi CCD 44-82. This happened during September 2000.
Both of these detectors have 15um square pixels, spanning 0"40 on the sky. Nevertheless, a dramatic improvement in image quality resulted from the detector change. The Loral CCD suffers from charge diffusion, where photo-electrons wanders about before they are collected in the potential wells. This was first realised when inspecting trails from cosmic particle hits and subsequently measured directly in the laboratory. The same measurements have shown that the EEV CCD was practically free from this problem.
The image quality was tracked by logging observers' measurements of FWHM. Using only the smallest FWHM measured every night, a histogram from two years of Loral data and one year of EEV observations is shown. From changing the CCD, the median FWHM improves from 1"3 to 1"05, and the sharpest images recorded from 0"85 to 0"60. The median DIMM seeing during the test period was 0"91.
We are very grateful for ESO to offer the EEV detector on loan, and for the 2p2team to provide the data that made this evaluation possible.
Last updated November 30, 2001