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2.1 Photometric accuracy
The uncertainties for the flux accuracy in Table 1 are given
for sources observed in the central part of the detector array. The values are
based on the analysis of observations of standard stars.
Note that although Polarization Observations (CAM05) are handled by the current
version of the off-line software, the corresponding products have not been
formally validated yet.
Table 1:
Photometric Accuracy of V9.5
| CAM AOT |
 mJy |
 mJy |
Notes |
| CAM01 (staring) |
20% |
30% |
- |
| CAM01 (micro raster) |
10% |
20% |
1 |
| CAM03 |
20% |
30% |
- |
| CAM04 (one way scan) |
40% |
40% |
2 |
| CAM04 (back and forth scan) |
30% |
30% |
2 |
| CAM05 |
- |
- |
|
Table NOTES:
- The better accuracies can be achieved by the SLICE package within CIA
(results soon to become available as post-processed archived products)
- The distinction between one way and back-and-forth scans are nolonger
relevant to LWS data processed using the OLP9.5 transient corrector
General notes:
- The fluxes for point sources in the CPSL (CAM Point Source List FITS
file) are derived by PSF-fitting in the
image. These photometry values can have larger errors than
stated in Table 1 if the fitting is incorrect.
- Because of wheel-jitter the ``library'' FlatField may not cover the same
detector region as the sky image does. The effect is small in the central
region of the array where the uncertainty is less than 1%. The
uncertainty becomes however very large near the edges (up to a factor of
5 in extreme cases), where vignetting is important (for example in the
case of a
PFOV measurement).
- Most CVF observations are performed after a discrete filter observation
thus implying a large decrease in the incident flux on the detector. This
may lead to important transient effects that may last several readouts and
hence several wavelength steps. The down transient is not always well
corrected by the transient algorithm in OLP9.5; the result is a larger
than true flux for the first wavelength steps.
- CVF observations are affected by the existence of ``ghosts'', i.e.
bogus images generated by multiple reflections between the detector and
the CVF surface. Ghosts can appear away from the source image but may
also fall onto (and surround) the sky image, depending on the actual
position of the source on the detector.
2.2 Astrometric accuracy
The most obvious source of astrometric errors is the absolute pointing error
of the ISO spacecraft (roughly 1.5 arcsec). This error will affect all
pixels of the detector equally.
CAM's astrometrical calibration is based on the premise that the optical axis
of the telescope intersects the detector at pixels [16.5,16.5]. This is true
when the lenses are centered with the optical axis. However, the lens wheels
have a small amount of play within each motor step and hence a lens wheel
may not come to rest exactly at its nominal position to better than a fraction
of a step (one wheel turn equals 480 motor steps). This may lead to a bending
of the optical axis along a direction perpendicular to the detector's
columns, i.e. along the direction of the spacecraft Y-axis. As with the
pointing error, all pixels are affected in the same manner.
A third error element is due to the ``pin-cushion'' effect: the lens
magnification changes as a function of the distance from the optical axis on
the detector plane. The distortion is about 1 PFOV in the corners of the array
for a
PFOV measurement. The distortion is negligible in the
case of a
PFOV measurement.
The estimates of these errors are shown in Table 2:
Table 2:
Astrometry Accuracy of V9.5
| Channel |
Pointing error |
Wheel Jitter |
| SW |
 arcsec |
 PFOV |
| LW |
arcsec |
PFOV |
Table 3 gives the estimated error, in microns, between the
indicated wavelength and the true wavelength. The indication
``negligible'' means that the error, if any, is at most a tenth of a CVF step.
The wavelength depends weakly on the column on which the source falls, with a
maximum shift of 1 step at the edges of the array. There is no array
line-dependence observed.
Table 3:
Spectral Accuracy of V9.5
| CVF Segment |
Error central pixels |
Error border pixels |
| SW CVF |
negligible |
 Wavelength step |
| LW short CVF |
negligible |
Wavelength step |
| LW long CVF |
negligible |
Wavelength step |
Next: 3. Improvements of the
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ISOCAM Calibration Accuracies Document