4.13 Global Error Budget

The uncertainties in the ISOCAM measurements depend on a variety of factors. Actually, there should be a specific error budget for each observing mode, depending on a number of parameters such as exposure time or optical configuration. The specific error budget will of course depend on the version of calibration libraries and on the signal processing algorithms used. In Table 4.6, a global list of instrumental effects contributing to the uncertainty budget is compiled. The estimated magnitudes of the various effects are given for typical circumstances. Column 2 refers to the relative error before data are corrected. In column 3 typical magnitudes are given of residuals still present after using available calibration methods.

Table 4.6: Summary of the error budget for ISOCAM calibration, from Biviano et al. 1998d, [8]

Noise origin	Magnitude	Residual After Correction
Stabilisation (systematic)	40% source flux	15% source flux (point sources)
		1% source flux (extended emission)
Saturation	$\geq$30% decrease in responsivity
Read-out noise	LW: 0.7 ADU/G
	SW: 2 ADU/G
Gain	0.15% output signal
Dark current	LW: 0.5 ADU/G	0.25 ADU/G
	SW: 0.5 ADU/G
Flat-field	LW: 0.5% (high background)
	SW: $\leq$1%
Sensitiv	5%
PSF (photometric correction)	20%	few %
PSF (pixel position)	$\leq$3%
CVF SRF	3%
CVF spectral purity	$\leq$1 CVF step
Ghost	$\leq$25%
Straylight	10% sky flux
Astrometry (pointing)	3 $^{\prime \prime }$
Astrometry (jitter)	$\pm$2 columns
Astrometry (field distortion)	1 pixel	$1/3$ pixel

Non-ideal behaviours of the instrument include:

• the long stabilisation times after flux changes
• the memory effect of the detectors
• the response variations after glitches
• the astrometric uncertainties
• the straylight induced by CVF filters
• the field distortion.

One may crudely estimate basic uncertainties of ISOCAM observations by using numbers as specified in Table 4.6. This table should make it obvious that any ISOCAM observer should try to correct for these various effects to the best of their knowledge. For a detailed review of the error budget see the `ISOCAM Calibration Error Budget Report (Biviano et al. 1998d, [8]).