The cryostat head is the nexus of all interfaces of the detector system. Conflicting requirements from different domains had to be accommodated since the instrument was not developed within ESO. The outside space limitations from the surrounding instrument together with the circular field flattener imposed a high filling factor and several form factors onto the cryostat. Following the symmetrical focal plane layout with 'left, left, right, right' CCD output register location (per mosaic column), the mechanics of the detector head was designed with square housing. But the instrument housing (above) and the cooling system (below) are both circular. Symmetrical mechanical design, wherever possible, helped the modularity, the thermal properties of the CCD mosaic table, as well as the space, flexure and earthquake safety constraints.
Due to the limited access to all parts inside the cryostat and the fact that all mechanical and electronics parts had to be mounted without prior prototypes, the system was integrated without ultra-cleaning. After a few minor modifications the final cleaning was applied with strategies suitable to the types of materials in question. Owing to the sheer size of the parts conventional cleaning methods could not always be applied. Therefore, amongst others, plasma cleaning was employed in-situ in the cryostat vessel. . Thereafter the clean cryostat head was integrated inside the clean room with the other cleaned component groups. Over some months, it was gradually put into cryogenic and electrical operation. The CCD mosaic was stepwise populated and tested with different numbers and different grades (mechanical, engineering, science) of detectors to reduce the possible impact of mechanical and chemical risks, and electrical damage.
Key design drivers of the cryostat detector head were therefore symmetry, modularity, and easy access to all parts during all phases of assembly, integration, and testing. On the outside this principally concerned an easily separable but precisely tilt-aligned and centered optical field flattener with integrated defogging system at the top and the cooling system at the bottom with a special interface for the easy lift-off of the detector head. Inside the detector head, cryogenic connections and electronic boards had to be designed to ensure that they functioned independently and that each part inside the detector head can be integrated or disassembled at any time without removing others. Despite an external diameter of 70 cm, the overall alignment error budget required all critical parts to be machined with a precision of a few microns.
To ensure effective cooling of the CCD mosaic, all intermediate levels of cooling were largely abandoned. The aluminum-based CCD mosaic table was based on previous experience and the ability to distribute thermal energy quickly and uniformly. Following detailed calculations, this base plate was designed as a light-weight yet stiff 3D structure by integrating it into the outer frame. On the top it resembles Swiss cheese with mounting openings for CCDs and ZIF sockets, the bottom side interfaces with its cold fingers directly to the clamps that connect it both mechanically and thermally to the cooling system. After application of different machining technologies and many thermal cycles to release mechanical stress, a flatness of a few |m was achieved. It is held and thermally isolated by fiberglass parts which are dimensioned according to the flexure budget and earthquake safety. All fiberglass parts have been Parylene coated to reduce outgassing in the vacuum.
The integrated electronics have approximately 1400 contacts to the outside world and are designed to have low thermal conductivity, high modularity, and good signal separation. A 'four-in-one bus board' was developed to optimize both manufacturing cost and routing space. It is mounted in the bottom cavities of the mosaic table. Per quadrant (see Fig. 1), two of these boards support eight CCDs and interface with one flexrigid interface board per signal group to a total of three vacuum connectors with 128 pins each. The connections for each of the four auxiliary CCDs are fed through this board set to the nearest quadrant. The symmetry of the focal plane in connection with the symmetry of the mechanics permits the electronics of two quadrants to be identical. All quadrants link to identical outside cabling for testability and ease of cabling. A total of 28 flexrigid boards have been designed and hand routed in complex 3 D shapes for the cryostat system. The use of glue-free materials was mandatory to avoid contamination. Figure 2 shows a bottom view of the cryostat head, while Fig. 3 shows the 16K*16K mosaic. .
All parts in the light entrance between auxiliary CCDs and the CCD mosaic were blackened with Keplacoating to avoid contamination and straylight (see Fig. 4). An actively cooled shield acts as an ice barrier. The shiny bond wires of the CCDs were masked with both cold and warm shields. After stepwise qualification of all alignment-critical parts, the CCD mosaic was laser-triangulated at -120 C (and through a special dewar entrance window without optical power); a flatness value of 25 ^ (pp) had been achieved.
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