Polarisation degree and angle are derived from the relative intensity differences measured with three different polariser settings at 0, 120 and 240 deg orientation w.r.t. the focal plane. A key parameter for the achievable polarimetric accuracies is the source-to-background contrast. The background subtraction for each polariser setting has a significant impact on the S/N of the signal by the source alone, beside the relative stability of the detector during the measurement. The uncertainty values are derived from the reproducibility of the intensity per polariser setting in the different polariser cycles per measurement.

Instrumental polarisations are listed in Table 3.2. The uncertainties are a combination of the reproducibility of the intensity per measurement and the scatter in the set of measurements used to derive the instrumental polarisation.

**Table 3.1:** Polarimetric Accuracies for Various Source-to-background Contrasts

detector	target	source	pol. deg.	source/	S/N	1 $\sigma$ pol. error
		flux		backg	source	per cycle
		[Jy]	[%]			[%]

P2, P_25 (79'')	XXX	400	$\sim$ 3	17	150	0.8
	XXX	$\sim$ 30	< 2	6.3	120	0.8
	XXX	$\sim$ 20	< 2	2	130	0.7
	XXX	$\sim$ 7	8	0.7	60	1.7
C200, C_160	3C 279	2.5	6	1.5 - 2	70 - 80	1.5 - 1.7
	3C 279	2.5	6	0.4	10 - 20	5 - 9
	3C 279	2.5	22	0.25	20 - 30	3 - 6
	3C 279	2.5	6	0.25	10 - 15	7 - 12
	$\alpha$ Boo	2.6		0.025	2	50 - 70

Note: Targets labelled XXX are from scientific programs which are not yet public domain.

**Table 3.2:** Instrumental Polarisation of ISOPHOT-P2 and ISOPHOT-C200

detector	filter	inst. pol. degree	uncertainty	inst. pol. angle	uncertainty
		[%]	[%]	[ $\deg$ ]	[ $\deg$ ]

P2	P_25 (79'')	3.0	0.2	-18.6	1.5
C200, pixel 1	C_160	4.9	0.3	-57.1	0.8
C200, pixel 2	C_160	5.5	0.3	+87.7	1.1
C200, pixel 3	C_160	6.1	0.3	-39.5	0.8
C200, pixel 4	C_160	3.4	0.3	-76.5	1.8

$\fbox {\fbox{\parbox{15.5cm}{For the ISO Post Operations Phase no improvement of... ...proved by the increase of the measurement data base used for its derivation.}}}$

3 Polarimetric Accuracies