Mechanics

This section describes the spectrometer mechanical systems.

The two main mechanical components of GIANO are the spectrometer cryostat and the optical bench, which also acts as tank for the liquid nitrogen, hence its name ``BeTank''. The direct contact with liquid nitrogen guarantees an uniform temperature distribution of the optical bench surface.
The BeTank is machined out of two blocks of Al-6082, whose thermal expansion coefficient was accurately measured and found to be equal to that of Al-6061, the same alloy used to manufacture the mirrors.

The inner part of the BeTank is divided into six pairs of vertically symmetric cells fed by vacuum-tight welded pipelines which carry the liquid nitrogen and evacuate the out-boiling gas. The cells and pipelines are organized so that the liquid nitrogen is poured and stored in the middle part of the tank. The liquid goes to the lower chambers only after the upper cells are full.

The aluminum optical bench is isostatically mounted on a hexapod system. The hexapod arms and joints are made of stainless steel to achieve a reasonably low heat conduction, while maintaining a rigid and easy to manufacture system.

The mirror support and alignment system has been developed in collaboration with Tomelleri s.r.l.. A very accurate (few microns) positioning and stability even for the remarkably large and heavy (20-30 kg) TMA1/3 mirrors has been achieved.
The optics are aligned at room temperature using six Al-7075 micrometers each of them acting on a spring-loaded steel ball inside a steel cell. This system has been extensively and successfully tested in the lab, results are published in Mochi et al. (2006).

The spectrometer has selectable short (6.5" sky-projected angle) slits and an open position for the IMA mode. All these elements are laser-cut steel foils with a fixed pattern of reference holes. They are mounted inside slit-holders with accurately machined needles matched to the slit frame holes. The holders are elastically mounted in the wheel sockets.

To simplify the calibration-plan of the instrument, the positioning of each slit should be repeatable with an accuracy of <1/10 of the slit width, i.e. a few microns. This is achieved using a wheel driven by a worm gear with large backlash in combination with an IN/OUT mechanism (the ``fork''), which extracts the slit-holder from its socket and presses it against three fixed kinematic points. This system has been thoroughly tested at cryogenic temperatures, the measured re-positioning accuracy is 2 microns rms.

The filter wheel is mounted on the same structure as the slit wheel. It does not require an accurate positioning and consists of a simple worm gear drive with large backlash driven by a stepper motor.

The spectrometer dewar is permanently mounted onto a structure (``cart'') together with all the cryogenic control systems. Four load cells are included to provide a direct measurement of the cryostat weight, which in turn is used to infer the amount of liquid nitrogen present in the BeTank. The cart, which also hosts the racks with the array and motor controllers, can be easily displaced and transported using retractable wheels.

Once mounted on the cart, the spectrometer becomes a standalone, complete system which can be operated anywhere close to a suitable supply of liquid nitrogen. The setup and alignment of the optics are made directly on the BeTank whose size and height relative to the ground (1 m) are similar to a standard optical bench. A dedicated elevator for lifting and removing the cryostat cover has also been developed to simplify the operations in the lab.