Precautions in using CVF'S

• LOW FLUX LEVEL IMAGES: For the 3 CVF elements, and particularly CVFSW and CVFLW1, the background flux on the array is quite low, and the stabilization time can be a problem limiting the photometric accuracy. The smaller the PFOV, the worse the situation.
• LONG SCANS: Within an AOT CAM04, splitting a scan over a long wavelength range into several shorter sub-scans can save some time for several reasons:
  1. Some a priori knowledge of the source spectrum may indicate a dramatic change in the flux level along the scan. Applying to the whole spectrum the same value of derived for the faintest part of the spectrum is likely to produce a waste of time in the brightest part.
  2. The zodiacal background level also may change, so that the long stabilization time required at shorter wavelengths may not be needed at longer wavelengths.
  3. From one end to the other of a CVF element, the quantities per step and FWHM, and consequently the resolution, vary as indicated in Tables to , so that the wavelength sampling of a long scan with a fixed interval can change significantly along the scan. Splitting the scan in two parts of different may save time, still providing a satisfactory sampling of the spectrum.

• FLATFIELDS: There is a slight dependance of the center of the band pass of the CVF with the position inside the FOV of the camera. This is due to the field mirror, used with a large off-axis angle, which produces aberrations in the image of the pupil in the plane of the CVF. This phenomenon has been calibrated (TBW in a later issue).

  For a source located at 1.5' on the Y axis (perpendicular to the plane defined by the optical axes of ISOCAM) the central wavelength is shifted by /5. The effect is symmetrical with respect to the Z axis. For a line, this introduces a variation of the flat-field along the Y axis 5% with respect to the center line, due to the shift in wavelength of the transmission curve. For a continuum, this also introduces a shift of the effective wavelength by .

• STRAYLIGHT: Due to the less efficient aperture stop and to the parallelism between the plane of the CVF and the plane of the array, observations with a CVF are more sensitive to straylight than observations with the bandpass filters (See section ).