The Telescope and Baffling System

**Figure:** A sketch of the configuration of the telescope main and secondary mirrors. Linear dimensions are in mm and angular dimensions in degrees (exact dimensions may be subject to change). In this sketch, the telescope boresight is in the horizontal plane.

The optical design of the telescope has been carefully studied, and the resulting parameters are summarized in Table 3.1. One of the main goals is to obtain diffraction limited performance at wavelengths longer than $\sim$ 800 $\mu$ m. Achieving this goal requires that the (on-axis) in-flight wavefront error (WFE) be less than $\sim$ 60 $\mu$ m; this requirement sets the accuracy which must be achieved by the reflector surfaces (see Table 3.1). At wavelengths shorter than $\sim$ 800 $\mu$ m the beam size will be set by the detector size rather than the diffraction limit (see HFI); the detectors at these wavelengths are oversized (with respect to the diffraction pattern) in order to collect efficiently the flux of the degraded image.

A second goal is to achieve good control over the shape of the radiation patterns at all frequencies (in order to reject straylight effectively, see Straylight Rejection). In order not to degrade the far side-lobes (which are the main causes of straylight contamination), the micro-roughness of the reflector surfaces must be kept below 1 $\mu$ m (rms), implying that the total integrated scatter produced by the mirrors will be less than 0.2% at $\lambda =$ 350 $\mu$ m. Finally, due to the multi-beam nature of the payload, careful attention must be paid to the minimization of off-axis aberrations. The mapping nature of the mission requires that the beam pattern shapes for all array pixels within each frequency channel be as uniform as possible. As shown in more detail under Optical Quality, these goals can be met by the Planck
payload.

**Table:** Telescope Parameters
Main reflector (M1)
shape	off axis paraboloid
physical size	1.492 $\times$ 1.292 m
focal length	0.72 m
surface accuracy $^\dagger$	<10 $\mu$ m rms
roughness $^\ddag$	<1 $\mu$ m rms
Sub-reflector (M2)
shape	off axis ellipsoid
physical size	0.845 $\times$ 0.796 m
focal length	0.514 m
f-number	1.36
surface accuracy $^\dagger$	<10 $\mu$ m rms
roughness $^\ddag$	<1 $\mu$ m rms
Telescope
focal length	1.8 m
main- to sub-reflector axis angle	14 $\ifmmode^\circ \else$
central feed to sub-reflector axis angle	34. $\ifmmode^\circ \else$ 129
Total Wavefront Error	<40 $\mu$ m rms
Total emissivity	0.01
$^\dagger$ Deviation from best paraboloid/hyperboloid
$^\ddag$ Average over spatial scales up to 0.8 mm

The design (see the Figure) consists of an off-axis tilted Gregorian system, offering the advantages of no blockage and compactness. The eccentricity and tilt angle of the secondary mirror, and the off-axis angle obey the so-called Dragone-Mizuguchi condition, which allows the system to operate without significant degradation in a large focal plane array, while simultaneously minimizing the polarization effects introduced by the telescope.

The baffling system is composed of two elements. The first (the ``shield") is a large, self-supporting, and roughly conical structure covered with MLI, which surrounds the telescope and focal plane instruments. Together with the optical bench, it defines the payload (or optical) ``enclosure". It has an important function both in reducing the level of straylight (which at the chosen orbit is in large part due to the spacecraft itself) and in promoting the radiative cooling of the enclosure towards deep space. The second element (the ``baffle") consists of one half of a conically shaped surface that links the focal plane instruments to the bottom edge of the subreflector; its function is to shield the detectors from thermal radiation originating within the enclosure.