12/6/2023 0 Comments Key parts of a telescope![]() Once the fairing was closed before launch this auxiliary helium tank could no longer be used and boil-off began. To maintain a cold environment inside the cryostat during the last few days before launch in Kourou, an auxiliary liquid Helium tank was used. 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.ĭuring ground operations, the vacuum vessel was closed by the means of a cover, located at its top, which was opened once in orbit. 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). The heat load on the tank evaporated the Helium over the mission lifetime at an estimated rate of about 200 grams per day. In orbit the liquid Helium was maintained inside the main tank by means of a phase separator (a sintered steel plug). 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. The cryostat provided 1.7K as its lowest service temperature to the instruments. This is achieved with a total amount of 2160 litres of helium cryogen. 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. The cooling concept for the Herschel instruments was based on the proven principle used for the ISO mission. ![]() The Herschel cryostat housed the focal plane units of the three scientific instruments depicted in Figure 2.3. ![]() Key telescope data are summarised in Table 2.2. 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. The M1 and M2 optical surfaces were coated with a reflective aluminium layer, covered by a thin protective "plasil" (silicon oxide) coating. In-flight results confirmed the correctness of the focus position, although there was no possibility of in-flight adjustments such as focusing. The measured wavefront performance in cold was in line with the requirements. The proper telescope alignment and optical performance were measured on the ground in cold conditions. The focus was approximately one metre below the vertex of M1, inside the cryostat. Finally, three quasi-isostatic bipods, made of titanium, supported the primary mirror and interfaced with the cryostat. The hexapod structure (also made of SiC) supported M2 in a stable position with respect to M1. 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. ![]() It was adjusted on the SiC barrel by tilt and focus adjustment shims. The secondary mirror (M2), with 308-mm diameter, was manufactured in a single SiC piece. 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 telescope was constructed almost entirely of silicon carbide (SiC). ![]() The chosen optical design was a classical Cassegrain with a 3.5-m diameter primary and an "undersized" secondary. Figure 2.2. The Herschel telescope flight model seen in the clean room at ESTEC, prior to transport to Kourou. ![]()
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