The optical design of the camera (Figure 2.1) was based on an imaging lens focussing the sky image provided by the ISO telescope onto the detector. At the focal plane of the ISO telescope, a field mirror (called `Fabry mirror' in Figure 2.1) imaged the telescope exit pupil onto the camera pupil. The filters and the CVF were located at the camera pupil position. An aperture stop was placed inside the filter mounts. For the CVF, an aperture stop was located before the precise pupil location. The field mirror was also used as a field stop.

Radiation from the sky entered the camera via a pyramidal mirror and first encountered an entrance wheel offering the alternatives of a clear aperture or a set of three polarising grids with position angles 120 degrees apart; the zero orientation was defined as the spacecraft y- axis (for a description of the spacecraft axes, see the ISO Handbook Volume I, [40]). The selection between the two channels of the camera, SW and LW, was achieved by opting to use one of four tilted field mirrors mounted on the selection wheel. These field mirrors were placed in the focal plane of the telescope, and depending upon which of them was in position, the telescope beam was fed into one of the two channels.

Each channel included a filter wheel, with bandpass filters and Circular Variable Filters (CVFs). The SW channel contained 13 filters (including two redundant ones), and a CVF for the 2.273 to 5.122 $\mu$ m range. The LW channel contained 10 filters, and 2 CVF's covering the 4.956 to 17.34 $\mu$ m ranges. The spectral resolution was about 40 for the CVF's and ranged from 2 to 30 for the bandpass filters.

In each channel a lens, mounted on a wheel, re-imaged the focal plane of the telescope onto the array. Four different lenses on the wheel provided four different magnification factors matching the fixed physical pixel size to the desired pixel field of view (pfov hereafter) on the sky. The pfov for the different lenses were: 1.5, 3, 6 or 12 $^{\prime \prime }$ (see Table 2.1).

Each of the field mirrors, described earlier, yielded an image of the telescope pupil. This image was located between the field mirror and the lens, in the plane of the filter wheel. Each filter carried a diaphragm, actually a hole punched through a sheet of metal, which acted as an aperture stop. For the CVF, of course, such a stop could not be implemented at this location (i.e. precisely at the filter location). Instead, a stop was placed 6 mm ahead of the plane of the pupil image, slightly oversized because of the beam aperture. The fixed filters were tilted with respect to the optical axis, to avoid ghosting due to back-reflected light. For mechanical reasons, the CVF had its plane normal to the axis. This, together with the poorer aperture stop, produced more straylight for the CVF than for the fixed filters (see Section 4.9).

The field mirror acted as a field stop for the light coming from the telescope. For each channel, there were actually two possible field mirrors providing two different fields of view: $198\times 198$ arcsec

, and $87\times 87$ arcsec

With the 1.5 $^{\prime \prime }$ and 3 $^{\prime \prime }$ pfov, only the central part of the 3 $^{\prime}$ diameter unvignetted field of view provided by the telescope beam was used by ISOCAM; the field of view of the camera was then limited by the size of the detector array.

In the 6 $^{\prime \prime }$ pfov, there was some vignetting in the corners of the $3\times 3$ arcmin

square field of view covered by the array, since the system was designed to match the 3 $^{\prime}$ diameter unvignetted field of view of the telescope. The vignetting can be up to 20% in extreme cases because of the wheel repositioning jitter (see Section 2.2.2).

With the 12 $^{\prime \prime }$ pfov, the field stop was always the field mirror, and the outer part of the detector array was not illuminated.

**Table 2.1:** *Efective field of view for different combinations of pfov and lenses*
pfov	Field mirror	Effective field of view
1.5 $^{\prime \prime }$	small	$45^{\prime \prime} \times 45^{\prime \prime}$
3 $^{\prime \prime }$	small	$87^{\prime \prime} \times 87^{\prime \prime}$
3 $^{\prime \prime }$	large	$1.5^{\prime} \times 1.5^{\prime}$
6 $^{\prime \prime }$	large	$3^{\prime} \times 3^{\prime}$
12 $^{\prime \prime }$	large	$3.3^{\prime} \times 3.3^{\prime}$

The Point Spread Function (PSF) is a convolution of the diffraction figure of the 60 cm telescope with the sky sampling of the pfov. The PSF (see Section 4.6) was undersampled with the 12 $^{\prime \prime }$ and the 6 $^{\prime \prime }$ pfov; it covered about 2 pixels for the 3 $^{\prime \prime }$ pfov, and it was better sampled with the 1.5 $^{\prime \prime }$ pfov. The PSF was uniform over the circular 3 $^{\prime}$ field of view, but was somewhat degraded in the corners of the square 3 $^{\prime}$ field of view.

Due to the tilted field mirror, there was a small field distortion. For the 6 $^{\prime \prime }$ pfov, an object which would have given an image of 32 pixels on the top of the array gave an image of only about 31 pixels at the bottom of the array (see Section 4.10).

Since it was not possible to have a broad-band anti-reflection coating on the detector, light was reflected back from the detector. This reflection was a source of straylight. To avoid strong ghosts the fixed filters were tilted with respect to the optical axis. Nevertheless, secondary reflections on the wheels, and on the filter mounts caused residual ghost images (see Section 4.9). The worst case arose for the 3 $^{\prime \prime }$ pfov where light coming from the whole 3 $^{\prime}$ field of view fell on the golden connecting strips at the edges of the photosensitive part of the detectors. To reduce this effect in the specific case of the 3 $^{\prime \prime }$ pfov, the optional small field mirror was provided to define a field better matched to the array size in this optical configuration. This field mirror was undersized to take into account positioning tolerances. Only $29 \times 29$ pixels were illuminated with this small field mirror and the $3^{\prime \prime}$ pfov lens. When the CVF was used in combination with the 3 $^{\prime \prime }$ pfov, the use of the small field mirror was mandatory. The most frequently applied optical configurations, as recommended by the instrument team and selected by the observers, were:

2.2.2 The edge columns

There was a certain amount of play in ISOCAM's wheels. This caused the field mirror to occasionally shift the field of view away from column 0 or 31, the edge-columns of the detector array. Column 0 was more often affected than column 31. The vignetting can be seen in LW measurements when the background is strong. The shift of the field of view also occured in the SW channel, as evidenced in the trend analysis of the CAM daily calibration measurements (Gallais & Boulade 1998, [35]).

2.2.3 Polariser displacement

Sources observed through the three polarisers were displaced by several pixels on the detector array as compared to the source position obtained through the entrance hole. The displacement was strongly dependent upon the polariser in place. Taking the source position obtained through the entrance hole as the nominal (zero) position and measuring the relative displacement

of the source centre for each polariser, the results given in Table 2.2 were obtained. For polarisation observations using the observing mode designed for extended sources, the displacements of the different polarisers were compensated for by offsetting the spacecraft pointing (Section 3.4) in opposition to the numbers given in Table 2.2. As a consequence of this approach it was possible to take, as a first order approximation, the polariser images directly obtained, and to calculate the Stokes vector without applying further registration techniques.

**Table 2.2:** *Polariser displacements.*
Polariser	$x \pm \Delta x$	$y \pm \Delta y$
	[ $^{\prime \prime }$ ]	[ $^{\prime \prime }$ ]
P1	$-3.4 \pm 0.1$	$-11.2 \pm 0.3$
P2	$-8.9 \pm 0.1$	$+0.6 \pm 0.3$
P3	$-3.0 \pm 0.1$	$-4.7 \pm 0.3$

2.2 Optical Design

2.2.1 General description

2.2.2 The edge columns

2.2.3 Polariser displacement