So that the favourable conditions offered by being in space could be exploited to the full, Herschel carried a precision, stable, low background telescope (Figure 2.2). The Herschel telescope was passively cooled by the large sunshade, allowing the size limitations imposed by active cooling using a helium jacket to be overcome. Thus its diameter was only limited by the size of the fairing on the Ariane 5-ECA rocket. The Herschel telescope had a total wavefront error (WFE) of less than 6 μm (corresponding to "diffraction-limited" operation at < 90 μm) during operations. It also had a low emissivity to minimise the background signal, and the whole optical chain was optimised for a high degree of straylight rejection. In space, the telescope cooled radiatively, protected by the fixed sunshade, to an operational temperature in the vicinity of 85 K, with a uniform and very slowly changing temperature distribution.

Figure 2.2. The Herschel telescope flight model seen in the clean room at ESTEC, prior to transport to Kourou.

The chosen optical design was a classical Cassegrain with a 3.5-m diameter primary and an "undersized" secondary. The telescope was constructed almost entirely of silicon carbide (SiC). The primary mirror (M1) was made out of 12 segments that were brazed together to form a monolithic mirror, which was machined and polished to the required thickness (~3-mm) and accuracy. The secondary mirror (M2), with 308-mm diameter, was manufactured in a single SiC piece. It was adjusted on the SiC barrel by tilt and focus adjustment shims. In order to avoid the Narcissus effect on the detectors, the central part of the secondary mirror was shaped in such a way that no parasitic reflected beam could enter the focal plane.

The hexapod structure (also made of SiC) supported M2 in a stable position with respect to M1. Finally, three quasi-isostatic bipods, made of titanium, supported the primary mirror and interfaced with the cryostat. The focus was approximately one metre below the vertex of M1, inside the cryostat.

The proper telescope alignment and optical performance were measured on the ground in cold conditions. The measured wavefront performance in cold was in line with the requirements. In-flight results confirmed the correctness of the focus position, although there was no possibility of in-flight adjustments such as focusing.

The M1 and M2 optical surfaces were coated with a reflective aluminium layer, covered by a thin protective "plasil" (silicon oxide) coating. The telescope was initially kept warm after launch into space to prevent it acting as a cold trap for outgassed volatiles while the rest of the spacecraft was cooling down.

Key telescope data are summarised in Table 2.2.

The Herschel cryostat housed the focal plane units of the three scientific instruments depicted in Figure 2.3. The cooling concept for the Herschel instruments was based on the proven principle used for the ISO mission. The temperature required in the instrument focal plane was provided down to 1.7K by a large superfluid helium dewar (helium at 1.6K), sized for a scientific mission of 3.5 years. This is achieved with a total amount of 2160 litres of helium cryogen. The cryostat provided 1.7K as its lowest service temperature to the instruments. Further cooling down to 0.3K, required for two instruments (the SPIRE and PACS bolometers), was achieved by dedicated ³He sorption coolers that were part of the respective instrument focal plane unit. In orbit the liquid Helium was maintained inside the main tank by means of a phase separator (a sintered steel plug). The heat load on the tank evaporated the Helium over the mission lifetime at an estimated rate of about 200 grams per day. The enthalpy of the gas was used efficiently to cool parts of the instruments that did not require the low temperature of the tank (two temperature levels, at around 4K and around 10K). After leaving the instruments the evaporated gas was further used to cool the 3 thermal shields of the cryostat, before venting into space: the helium venting effectively counterbalanced almost exactly the light pressure on the sun shield.

During ground operations, the vacuum vessel was closed by the means of a cover, located at its top, which was opened once in orbit. To maintain a cold environment inside the cryostat during the last few days before launch in Kourou, an auxiliary liquid Helium tank was used. Once the fairing was closed before launch this auxiliary helium tank could no longer be used and boil-off began. The space side of the Cryostat Vacuum Vessel (CVV) was used as a radiator area to cool the CVV on orbit to a final equilibrium temperature of about 85K. This radiator area was coated with high emissive coating to achieve low temperatures in the L2 orbit. Multi-Layer-Insulation (MLI) covered the outer CVV-surfaces, in order to insulate it from the warm items (satellite bus and Sunshield). The outer layer of the MLI was optimised for the lowest temperature of the CVV. The outside of the cryostat was the mechanical and thermal mounting base for the Herschel telescope, the local oscillator unit of HIFI, the Bolometer Amplifier Unit of PACS and the large sunshield protecting the CVV from the sun.

Figure 2.3. The Herschel cryostat.

The science payload was accommodated both in the "cold" (CVV) and "warm" (SVM) parts of the satellite. The instrument FPUs were located in the "cold" part, inside the CVV mounted on the optical bench, which was sitting on top of the superfluid helium tank. They were provided with a range of interface temperatures from about 1.7 K by a direct connection to the liquid superfluid helium, and additionally to approximately 4 K and 10 K by connections to the helium gas produced by the boil-off of liquid helium, which was used efficiently to provide the thermal environment necessary for their proper functioning. The "warm" - mainly electronics - parts of the instruments were located in the SVM. The following instruments were provided within the Herschel spacecraft:

The instruments are described in their respective users' manuals

The electrical power of the satellite was produced by the solar array. This was placed in front of the cryostat to protect the vacuum vessel from solar radiation. The rear of the sunshield was covered with multi layer insulation, as was the part of the cryostat facing this warm part of the system. The geometrical design had to consider the size of the cryostat and the telescope, the required solar aspect angles of the s/c in orbit and the limited diameter of the fairing of the launcher. For Herschel, a relatively simple system with a fixed solar array was selected. Only the lower part of the sunshield actually carried the solar cells. The upper part was free of solar cells to allow it to be at a lower temperature, which in turn helped the telescope to stay at the required temperature. The height of the sunshield was driven by the need to shade the entire telescope when the spacecraft was pointed closest to the sun (60° Sun aspect angle).