We (F.-Xavier Désert from LAOG, Martin Giard from CESR & Alain Benoit from CRTBT) have built a photometer dedicated to the Sunyaev-Zel'dovich effect and other diffuse sources at 2.1 mm. Its name is DIABOLO. The first results obtained with a preliminary version of this instrument (two co-aligned channels, centred at 1.2 and 2.1 mm, respectively) were obtained during the winter 1995-6 and are reported in Désert et al 1998 (submitted to New Astronomy; see http://www-laog.obs.ujf-grenoble.fr/ desert/diabolo/diabolo.html).
The DIABOLO photometer in its current configuration was installed on the 30-m telescope and fully tested in December 1997. It consists of two co-aligned arrays of 3 channels forming an equilateral triangle, operating at 1.2 and 2.1 mm, respectively. The spatial channels have a FWHM beam of 20'' and are positionned with an accuracy of 1.5'', r.m.s. Their nominal offset from the telescope axis is 22''. Because the bolometer is located at the Nasmyth focus, the array projection on the sky rotate with elevation. The detectors are cooled down to 100 milliKelvin.
We have mapped with DIABOLO the brightest X-ray cluster (RXJ1347.5-1145 - see Schindler et al, AA, 317, 646) at 1.2 and 2.1 mm, and detected its SZ signature at 2.1 mm with a S/N ratio . For this type of observations, the subtraction of the 1.2 mm channel output from that of the 2.1 mm channel offers a powerful way to decrease the sky noise.
We think that the instrument can now be used by experienced radioastronomers external to our team, and encourage the 30-m telescope users to apply, via the IRAM Program Committee. Observers will be helped by the DIABOLO team members on a shared risk & benefit basis. Please contact F-X. Désert (see address below) well before the September 7th, 1998 deadline if you intend to request telescope time with this instrument.
In the normal observing mode, drift scans at constant
elevation are made across the target source, while the secondary mirror is
nutated in azimuth (typical rate: 1 Hz, beamthrow: 180''). The
sensitivity achieved in good weather conditions on the sky is given:
- for a point source (using only one pointed bolometer): 230 and 86 mJy s , i.e. roughly 4 and 1.3 mJy ( ) in one hour;
- for an extended source (averaging all 3 bolometers): 220 and 250 in one hour, i.e. 0.4 and 0.16 MJy/sr. At 2.1 mm or of can be achieved per sky pixel.
At 2.1 mm, a factor of of improvement can be achieved for some programs by subtracting the 1.3 mm channel output from the 2.1 mm output.
The total observing time, including dead-time, pointing, focussing, skydips, and photometry calibrations, is about the double of these integration times.
The raw data will be prereduced with a custom-made software (based on IDL) which yields uncalibrated FITs-format maps.
Although it is less sensitive at 1.2 mm than the MPIfR diffraction-limited bolometer array, DIABOLO is well suited for studies of the SZ effect, of microwave background anisotropies, and of the cold dust properties. The 1.2 mm and 2.1 mm bands are optimized for the SZ effect and measurements of dust temperatures of the order of 15 K or lower.
Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 414 rue de la piscine, 38041 Grenoble Cedex 9 FRANCE Tel: (+33) 4 7663 5512, Fax : (+33) 4 7644 8821 email: Francois-Xavier.Desert@obs.ujf-grenoble.fr