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The design of the optics for LABOCA has been rendered by E. Kreysa at MPIfR with contributions from N. Halverson (UC Berkeley). The limitations on the space available in the Cassegrain cabin and the constraints due to sharing one large common mirror with the ASZCa experiment, required a long, accurate and laborious optimization of the optical design, carried out over several months with use of the software package ZEMAX.


The pictures below give an overview of the optics: on the left there is the APEX telescope, on the right a zoom on the tertiary optics installed in the Cassegrain cabin.


After the telescope's primary and secondary mirrors (here named M1 and M2), the optical path is actively shaped by other 5 mirrors (here named M3, M4, M5, M6, M7) plus a lens placed at the receiver's cryostat window.



Mirror M3 has been manufactured by a machine shop of LBNL (Berkeley) and is commonly used by LABOCA and the ASZCa experiment. M3 has a diameter of 1.6 m (LABOCA uses only the inner 80 cm circle) and surface accuracy of 20 µm (r.m.s.). It is attached to the floor of the Cassegrain cabin, in the central area, by means of a bearing which allows the operators to rotate the mirror to send the telescope beam to LABOCA or to ASZCa alternatively.

Mirrors M4 and M6 are flat, they have a diameter of 26 cm and 42 cm and have been manufactured by Kugler GmbH (Salem, Germany). They are both connected to the ceiling of the cabin and have visible ligth optical quality. In a near future, mirror M6 will be replaced by the reflection-type half-wave plate of the polarimeter.

Mirrors M4 (right) and M6 (left) at MPIfR

Mirrors M5 and M7 are active, 50 cm in diameter, and are attached to the floor. They have been designed and manufactured at MPIfR. They have a surface accuracy of 8 µm (r.m.s.).
Mirrors M5 and M7 in the machine shop at MPIfR

The receiver and all the mirrors (with exception of M3) are mounted on hexapods, provided by VERTEX Antennentechnik (the same company that built the APEX telescope). For the alignment, the length of the six legs of each hexapod need to be modified several times. In order to facilitate the operations, VERTEX has provided a software which helps to calculate the corrections to apply the legs of an hexapod to move the corresponding mirror as required.

Click on the picture below to get a high-resolution WMF with a CAD drawing of the Cassegrain cabin with LABOCA and its optics (1.64 MB).