The camera and CCD entrance window were send to Copenhagen to get new optics with a better coating. The original new camera did not have a proper coating which caused a large dip in efficiency in the B-band with an overall decrease toward longer wavelengths. The new optics with the new coating did improve the efficiency in the B-band, but the efficiency toward longer wavelength decreased as well. It was found that this was because the company that made the optics did not dare re-try the multilayer coating that had failed before, but put on a single-layer coating. The overall change from the old optics with the old (Loral) CCD to the new optics with new coating and new (E2V) CCD range from +8% in the U-band to -30% in the I-band, where the biggest part is due to the coating of the optics.
The overall increase in the U-band is due to the new CCD, and also the optical quality in the blue has improved as the E2V CCD does not suffer from the charge-diffusion present in the Loral CCD. Tests with a pinhole mask has shown that with the new optics the distortions as a function of distance from the CCD center are within one pixel.
Possible improvements to the current situation should be considered.
Thomas Augusteijn 2006-05-08