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 3.5). 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. This is due to 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 5.2.2). 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.

It is possible to separate the straylight and dark signal components in the absolute photometry AOTs P05 and P25 to increase the observational accuracy. The observer has the option including an FCS measurement with no FCS heating power applied to it as well as a measurement when a special configuration of the filter wheel blocks the sky light.

The PHT instrument team is currently investigating how the FCS straylight can be corrected for the other AOTs.

4.7.2 Instrumental Straylight

Dedicated in-orbit calibration measurements were performed to determine the straylight from sources outside the ISOPHOT field of view. The off-axis straylight rejection requirement is that the parasitic light level in the focal plane should not exceed 10% of the diffuse astronomical background at any wavelength. The thermal self-emission from the optical system should also be less than 10% of the diffuse astronomical background. Main straylight sources are the Sun, Earth, Moon, and Jupiter. 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, o Ceti (Mira) with P2 in P_25, and $\gamma$ Dra (HR 6705) with PHT-S. The target fluxes in the given filter bands are 32 kJy, 1.8 kJy, and between 3 kJy (at 3 ${\mu}$ m) and 140 Jy (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 has 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. Further near-field measurements are not required.

**Table 4.7:** Near-field off-axis rejection at 160-200 ${\mu}$ m predicted by an APART study of Breault Research, Tucson
Angle (arcmin)	Level
	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 Straylight from Sun, Earth and Moon

Straylight measurements of Sun and Moon were performed in Autumn 1997 during eclipses of ISO. The results will be described in a later version of this manual. A description of the results can be found in Klaas et al. 1998 [13].

4.7.3 Spectral Straylight

At the time of writing of this document, there is no indication of any spectral impurity as well as 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 Straylight from Sun, Earth and Moon

4.7.3 Spectral Straylight