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Astron. Astrophys. 361, 895-900 (2000)

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2. Observations

Our observations were made with the mid-infrared camera, ISOCAM (Cesarsky C. et al. 1996). The telescope was pointed toward J2000 coordinates 00h45m32.5s, -73o18´46.3" on 5 July 1996. We used the 12" pixel-field-of-view lens, for which only the central [FORMULA] of the detector is illuminated. A first spectrum (July 5, 1996) was obtained by rotating the circular-variable filters (CVF) through the range 16.61 to 5.079 µm. A second spectrum (2 Oct 1997) was obtained by covering the same wavelengths but rotating the CVFs both forward and backward. During the second observation, we also observed a nearby reference position outside the SMC, at J2000 coordinates 00h45m32.5s, -73o18´46.3", to measure the Milky Way emission in the 10.74-11.79 µm range. For all observations, dark current was subtracted from the images using a library dark current image scaled such that it matches the level of the unilluminated edges of our images. The brightness level after dark current subtraction was compared to that expected from an interpolation of the COBE Diffuse Infrared Background Experiment (DIRBE) broad-band observations at 4.9 and 12 µm. At low levels of illumination, the ISOCAM detectors exhibit some transient response, and they take time to stabilize to the true sky brightness level. We applied a transient correction algorithm that takes into account the initial rapid rise of the ISOCAM gain and a single exponential rise thereafter. The transient corrections are only about 10% for wavelengths longer than 6 µm, because the change in brightness as the CVF rotates is small; but at the shortest wavelengths, the correction increases to 60%, because the sky brightness was decreasing more quickly than the ISOCAM detector could stabilize. The images at each wavelength were corrected for zodiacal light, stray light and vignetting by subtracting a special calibration observation of a blank field, scaled by a model spectrum of the zodiacal light that matches the COBE /DIRBE data for the same position and date (Reach et al. 1996a,b). To complement the CVF observations, we also present here a portion of an image made with the ISOCAM through the LW2 filter (5-8.5 µm) on March 13, 1996. This image was reduced using the CAM Interactive Analysis package and aligned to stars in the Palomar Digital Sky Survey.

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© European Southern Observatory (ESO) 2000

Online publication: October 10, 2000