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Astron. Astrophys. 361, 895-900 (2000)
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]](img2.gif)
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.
© European Southern Observatory (ESO) 2000
Online publication: October 10, 2000
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