Astron. Astrophys. 327, 1206-1214 (1997)

2. Observation and reduction technique

The images were obtained on 13-15 December 1995, at the ESO/MPI 2,2m telescope at La Silla, Chile, with the IRAC2 infrared camera and the associated Fabry-Pérot interferometer (FP). The IRAC 2 camera is equipped with a pixel NICMOS array and was used with optics that yield a scale and a field of view. The FP is built by Queensgate Instruments Ltd. (serial number ET70WF-923). The etalon has a measured finesse , and a transmission of 70%-80% in the 2.04-2.46 µm wavelength range (measured by Anton van Dijsseldonk, 1992, ESO internal memorandum). The announced resolution is 1000, although we measured it to be closer to 1500 (200 km/s at 2 microns). At each wavelength observed, narrow-band "blocking" filters were used to isolate the orders (see Table 1).

Table 1. Fabry-Pérot settings and order sorting filters used

In each of the H₂ lines observed, we imaged OMC-1 with the FP tuned, first to the line rest wavelength, then to a "continuum wavelength" (typically 0.004 - 0.007 µm from the line wavelength, avoiding bright airglow lines; see Table 1), and then to the blue- and red-shifted wings of the FP profile (i.e. roughly 0.001 µm, or 140 km s^-1). These images allowed us to check the accuracy of the tuning of the FP: by fitting gaussians to photometric measurements from the line and wing images (calibration frames were obtained for each of the 5 lines observed), we established that the FP does indeed accurately tune to the desired line wavelength.

Once we had "peaked-up" on the line flux in this way, we observed, consecutively, line continuum source and line continuum sky frames. Typically, sky frames were obtained every 60-120 secs. The ESO-MIDAS reduction software was used to reduce the data. Each frame was "sky-subtracted" and flat-fielded in the normal way, using the sky frames observed just before or after each object frame, and dome flat fields taken at the end of each night (for the flat-fielding, images of the inside of the telescope dome, illuminated with a halogen lamp, were observed at each FP wavelength setting). Frames were linearized before averaging. The same source that appears in all of our images, IRc9 , was used to flux calibrate the data. A black-body fit to the J, H and K magnitudes of this standard (N. Minchen, 1996, priv. comm) was used to predict the flux density at each wavelength observed: these data were then used to calibrate the response of the system to photons at each wavelength.

Finally, before subtracting continuum frames from line emission frames, the images were smoothed with a 2-D gaussian function so that the mean point-spread-function in each pair of images was as closely matched as possible. Residual artifacts in the differenced images were removed by hand.

© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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