Astron. Astrophys. 318, L13-L16 (1997)

2. Observations

All the observations have been carried out in the frame of a survey at 15µm of the inner galaxy, the ISOGAL project (Pérault et al. 1996). In particular the observations used in this work are those of the field at . Both the radio and NIR observations are part of a preparatory/followup program for the ISO data. Here we will briefly summarize the fundamental observational parameters.

2.1. ISO observations

The ISO observations where carried out during April 1996. ISOCAM was used to observe a field around the position l = b = . The filter used was the 15µm broadband LW3 filter (passband from 12 to 18 m), and the pixel field of view . After image reduction (Pérault et al. 1996) about 700 point sources were extracted with a preliminary photometry. The completeness limiting flux achieved was approximately . The magnitude at 15µm () has been computed assuming =9 for a source.

2.2. NIR observations

The K-band observations of a slightly larger field than that observed by ISOCAM were carried out during October 1995, using ARNICA (Lisi et al. 1996) mounted at the TIRGO ¹ telescope. ARNICA is equipped with a NICMOS3 detector, the plate scale at the TIRGO is . For a comprehensive description of the instrument performance at the TIRGO see Hunt et al. (1996a). Photometric calibration was performed by observing a set of the ARNICA standard stars (Hunt et al. 1996b), the calibration accuracy is about 10%. The limiting magnitude achieved in the observations ( aperture) was (0.46 mJy).

The K-band data was astrometrically calibrated using a large set of optical stars extracted from the Digitized Sky Survey as described by Testi (1993). K stars positions then were used to astrometrically calibrate the ISO images. The present astrometric precision achieved is for the K-band data and for the ISO data.

On the July 1996, a small region () around the position of the H₂ O maser was observed at UKIRT ² at K and nbL ( m). The m data are about two magnitude deeper than the TIRGO data. All the sources previously detected are confirmed, and a background of fainter objects suggests that these images are confusion limited at K . In the nbL image stars corrensponding to the brightest K sources are detected, plus a faint object () without a bright K counterpart. The limiting magnitude of the m image is .

2.3. H₂ O maser observations

A new H₂ O maser was discovered during an unbiased survey of the field, which is presently on-going at the Medicina ³ radio telescope. The observing setup, the sensitivity and the spectral resolution of the observations closely resemble those described by Brand et al. 1994. The Full Width at Half Power (FWHP) of the Medicina antenna at 22 GHz is . Up to now of the field has been observed. Taking into account the beamwidth and the pointing accuracy, we estimate an error of on the maser position.

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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