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Astron. Astrophys. 336, 662-666 (1998)

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2. Observations and reductions

A [FORMULA] map centered on the main H II   region N 4A was obtained in each of the LW2 and LW3 filters, with [FORMULA] pixels. The 32[FORMULA]32 pixel long-wavelength camera of ISOCAM was used to make a square 3[FORMULA]3-step raster map with a shift (then overlap) of 16 pixels between successive positions. The single read-out integration time was 2 seconds. The 25 and 34 first reads-out obtained with the LW2 and the LW3 filter respectively were eliminated for the first step of the raster map in order to insure stabilization of the detectors, which show memory effects of their previous history. The useful total integration time at 7 and 15 µm respectively was 618 and 584 seconds. The map at each position was dark-current subtracted and flat-field ed in the usual way. Then an automatic software was used to detect and eliminate the parts of the records affected by glitches due to the impact of charged particles, and also to correct for the transient response of the detectors when submitted to changes in incident flux. Corrections for field distortion have been applied to images in both filters before combining the images of each raster, providing a better sensitivity on the final maps. A second-order flat-field correction was finally used to match the levels on contiguous edges of the elementary maps of the raster. This correction is only a few per cent and affects the photometry in a negligible way.

The images were calibrated using the library of calibration files associated with version 4.3 of the off-line software in Villafranca, Spain. One should be aware that a definitive reduction of the ISOCAM images is not yet possible. At the present stage, imperfect correction for the transient behavior of the detectors is likely to yield systematic errors in the flux determinations up to 30%. We cannot at this stage assign errors to the fluxes or flux ratios.

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998