The wavelength calibration obtained on ground was checked against the
observed spectra of a planetary nebula which has a few bright and well
identified emission lines. The wavelength calibration was found to be
adequate, with no shifts observed between measured and predicted
values. Cautions for using the CVF are documented in the ISOCAM
Observer's Manual in section 5.5.6; a general discussion can be found
in section 4.3. In addition:
As discussed in section 5.5.6, transient behaviour of the array causes the
accuracy of the fluxes measured in the CVF positions to increase with
increasing number of exposures taken at each step position. It should also
be noted that the accuracy will decrease when the wavelength increments are
larger. The best observing mode is :
: at least 10 images at each CVF step (+ 2 to cope with wheel motion) Such observations provide spectra with 15% accuracy. If a higher accuracy is needed at least 20 exposures or two scans (one in each direction) should be used.
The observer is reminded of the straylight problem described in section 2.1
of the ISOCAM Observer's Manual. Ghost images may be present in some cases.
With the 3 arcsec lens, the observer must use the small field mirror,
otherwise, the observations will be spoiled by straylight. With the 6 arcsec
pfov lens, multiple reflections between the detector and the CVF produce
straylight which can be removed by observing zodiacal light. A calibration
file will be provided to remove this straylight.
It may be useful for observers to bracket their CVF scans
with short staring measurements through a fixed filter, if
the source is not too bright. This should help with the discrimination
of straylight and ghost features. It is necessary, however, to ensure that
the source will not saturate in the fixed filters.