The FCS signal is obtained by deflecting the focal plane chopper outside the sky field of view to one of the FCS positions (see Section 2.7). In-orbit tests have shown that FCS measurements do not only contain signals from the FCS proper and dark signal of the detectors but also a straylight component from the sky entering the instrument. The FCS straylight component was determined by performing an FCS measurement for which the FCS heating power was switched off. The FCS straylight component arises probably from the absence of a mechanical shutter to block the external beam. Both the dark signal as well as this straylight signal should be removed from the total FCS signal before calibrating the target signal.

In the derivation of the FCS calibration tables, the straylight contribution in the signal has been eliminated by subtracting the sky straylight signal obtained while pointing at the calibration target (see Section 5.2.4). For C200 the straylight contribution to the FCS signal can be as high as 10% of the background signal in case of observations near the galactic plane. The other detectors show a lower straylight contribution.

In the absolute photometry AOTs PHT05 and PHT25 it is possible to separate the straylight and dark signal components to increase the observational accuracy. The observer had the option to include an FCS measurement with no FCS heating power applied to it as well as a measurement for which a special configuration of the change wheel blocks the sky light (see Table 3.1).

4.7.2 Instrumental straylight

In order to fulfil the scientific objectives of ISO, stringent straylight requirements were imposed to the optical system. First, the parasitic light level in the focal plane should not exceed 10% of the minimum diffuse astronomical background for the wavelength range from 2.5 to 240 $\mu$ m. Second, the thermal self-emission from the optical system should also be less than 10% of the minimum diffuse background. Main straylight sources are the Sun, Earth, Moon, and Jupiter.

Due to the many instrumental configurations available with PHT, in-orbit straylight verification measurements were mainly performed at 25 $\mu$ m where the straylight contribution from the Earth and Moon are largest and at 170 $\mu$ m which is the most sensitive band to detect straylight due to thermal self-emission.

The verification measurements have shown that straylight is a negligible contribution to most measurements, but it can be a small correction factor under rare observing conditions near the detection limit. An overview of the in-orbit results can be found in Klaas et al. 1998, [24] and Lemke et al. 2001, [38].

The observer is recommended to check the position of the brightest celestial sources with respect to the target position, particularly for absolute photometric observations.

4.7.2.1 Near-field straylight

The near-field straylight is the off-axis rejection within about 1 degree radius from the field of view of ISOPHOT. This was measured by performing cross-scans on Saturn using C200 in band C_160,

Ceti (Mira) with P2 in P_25, and ${\gamma}$ Dra (HR 6705) with PHT-S. The target fluxes in the given filter bands are 32kJy, 1.8kJy, and between 3kJy (at 3 $\mu$ m) and 140Jy (at 10 $\mu$ m) for C200, P2 and PHT-S, respectively.

Comparison between the measured profiles and the beam profiles shows small or negligible straylight contributions, even from very bright sources, out to off-axis angles where the natural sky brightness dominates. An in-depth study for C_160 carried out before the mission predicted straylight levels now being confirmed by the in-orbit measurements of Saturn. This proves that the design assumptions were correct.

The computed straylight levels in the 160-200 $\mu$ m range for a point source are listed in Table 4.7. Major straylight contributors are:

Because the 3 channels selected for the in-orbit verification of the straylight rejection are representative for the major subsystems of the instrument, it is concluded that straylight corrections can be made for all channels.

**Table 4.7:** *Near-field off-axis rejection at 160-200 $\mu$ m predicted by an APART study of Breault Research, Tucson.*
Angle	Level
[arcmin]
0	1
3	$3.6~10^{-3}$
10	$2.9~10^{-4}$
30	$6.2~10^{-6}$
60	$1.0~10^{-6}$
120	$1.6~10^{-7}$
300	$1.3~10^{-8}$

4.7.2.2 Far-field straylight of Sun, Earth and Moon

Straylight measurements of Sun and Moon were performed in autumn 1997 during eclipses of ISO by the Earth. The Sun straylight contribution could be measured directly by observing a reference target before, after, and during the eclipse. A Moon straylight test could be performed without interference from the Sun while measuring during the eclipse a low background reference field at certain angles from the Moon.

The straylight component from the Earth was investigated by measuring the level of a low background reference field while the Earth was shining into the inner side of ISO's sunshade. This measurement was possible when the Earth-ISO-Sun angle was around 170degrees and while the x-axis of ISO was pointing with a minimum inclination angle of 87 degrees w.r.t. Earth limb.

No detectable straylight levels from Sun, Earth, and Moon were measured. In Table 4.8 the measured 1 $\sigma$ upper flux limits are listed (Kranz 1998, [29]. The upper limit at 170 $\mu$ m is higher than the straylight requirement of 10% of the minimum astronomical background. The minimum brightness at 170 $\mu$ m is estimated to be approximately 2MJysr $^{-1}$ which implies a required upper limit of about 0.2MJysr $^{-1}$ . It should be noted that the upper limits presented in Table 4.8 apply to the most unfavourable straylight configuration.

**Table 4.8:** *Upper limits (1 $\sigma$ ) for far-field straylight of Sun, Moon, and Earth at 25 and 170 $\mu$ m, from Kranz 1998, [29].*
Source	$\lambda_{ref}$	Surface Brightness	Flux Density	% of sky background
	[ $\mu$ m]	[kJysr $^{-1}$ ]	[mJy]	in measurement
Sun	25	16	9.2	0.05
Moon	25	179	100	0.34
Earth	25	188	105	0.51
Sun	170	10	2.4	0.10
Moon	170	46	11	0.53
Earth	170	280	69	5.9

4.7.3 Spectral straylight

There is no indication of any spectral impurity and of the presence of ghosts in the PHT-S spectra.

4.7 Straylight

4.7.1 Straylight during FCS calibrations

4.7.2 Instrumental straylight

4.7.2.1 Near-field straylight

4.7.2.2 Far-field straylight of Sun, Earth and Moon

4.7.3 Spectral straylight