7 Recent improvements

A number of improvements to the Diabolo instrument have been made since the original design. The changes made to the Diabolo setup are listed below:

• All quartz lenses have been replaced by polyethylene lenses, because the anti-reflection coatings had a tendency to fall off due to the stresses induced by temperature cycles;
• A new 0.1 K cryostat has been designed with a Joule-Thomson cycle on the mixed Helium output, which produces the 1.8 K stage, hence the main lHe vessel is now at 4 K. The major advantage is that refilling the cryostat with lHe is now faster because the 0.1 K and 1.8 K stays at the same temperature and no lHe pumping is needed any more. The cryogenic duty cycle of the instrument is now of half an hour refill every three days;
• One bandpass filter has been removed in channel 2, in order to increase the sensitivity by broadening the band. We have checked that the small leaks that appear at high frequencies have no effect on the detection of the SZ effect;
• New electronics, now fully digitally controlled with a computer interface, have been designed and used for subsequent observations. The new system is described in detail by Gaertner et al. (1997);
• The regulation of the temperature of the thermal bath has been improved. Thermometers attached to the 0.1 K stage provide temperature information, whereas resistances permit to heat up the 0.1 K stage by a feedback system to stabilise the temperature in a closed loop;
• Shock absorbers have been attached to the mount in order to minimize the microphonics induced by telescope motion (as seen by a general increase of the noise at all frequencies);
• Single-bolometer detectors for each channel have been replaced with bolometer arrays of three bolometers in each channel.

Subsequent, upgraded versions have been used at the IRAM 30 m antenna in Spain, and at the POM2 2.5 m telescope (without wobbling secondary mirror) in the French Alps, in the winters 1995 through 1999, yielding in particular, significant detections of the Sunyaev Zel'dovich effect towards several clusters of galaxies (Désert et al. 1998; Pointecouteau et al. 1999). The instrument has been open to the IRAM community since 1998.

Acknowledgements

We thank Louis d'Hendecourt and M. Gheudin for their help in measuring the transmission of the Diabolo filters at the IAS and DEMIRM. We thank the Programme National de Cosmologie (ex GdR), the INSU and the participating laboratories for their continued support of this experiment. We also thank Pierre Encrenaz and Claudine Laurent for their early support of the project. Part of us (M. de Petris, P. de Bernardis, S. Masi, G. Mainella) have been supported by Italian ASI and MURST. We thank Istituto di CosmoGeofisica (CNR) in Turin for logistic support. Finally, we wish to thank the referee, C.R. Cunningham, for having suggested several significant improvements to the manuscript.