MITO telescope:
 

The telescope is an Aplanatic Cassegrain (R-C) configuration.The diameter of the primary mirror is 2.6 meters while the subreflector is 41 cm in diameter. Differential measurements at MITO are obtained by wobbling the subreflector. A large corrected focal plane, diffraction limited until 350 micron, equals to 45 mm in diameter, also for the tilted subreflector positions (beamthrow of 1 degree) allows an efficient 3-field observations.The optical design of the MITO telescope is described in De Petris M. et al. (AO, Vol.28, 10, 1989).

A high thermal stability and uniformity on the surface of the primary mirror is dictated in order to avoid measuring the different emission from the primary mirror during modulation. High thermal conductivity mirrors are necessary to quickly achieve good thermalization.The two monolithic mirrors were manufactured by Officine Ottico-Meccaniche Marcon (Italy). The mirrors are realized in an aluminum alloy (G Al Si 7 Ti UNI 7257-73 Avio). The structure is a light and stiff ribwork.Thin panels were thermally shaped, finished by a numerically controlled machine and finally hand polished. A metal ribwork was welded onto the back. With this procedure it was possible to limit the weights: the primary mirror is only 115 kg in weight while the subreflector is 1.8 kg.

  

The final quality of the 2.6-m primary mirror surface resulted in its worldwide application. The same primary mirror, with different subreflectors, can be found in other mm-experiments like: OASI (Osservatorio Antartico Submillimetrico e Infrarosso, Mario Zucchelli Station - Antarctica), OLIMPO (Osservatorio nel Lontano Infrarosso Montato su Pallone Orientabile), COMPASS (Cosmic Microwave Polarization at Small Scales), COCHISE(Cosmological Observations at Concord with High-sensitivity Instrument for Sources Extraction) and QUaD = QUEST (Q and U Extra-Galactic Sub-mm Telescope) at DASI.

In both the mirrors the r.m.s. surface roughness is lower than 0.1 um. This surface requirement is dictated by a low emissivity constraint of the mirrors and in order to have easy alignment procedures with visible light.

The modulator is based on the diffuse idea of two shakers in a ‘‘push-pull’’ configuration to wobble the mirror minimizing the microphonics induced on the detectors. It is possible to oscillate the secondary mirror with different waveforms at different frequencies from 0.5 Hz to a maximum value depending on the type of waveform.The more common waveforms are the sinusoidal, the 2 and 3 field square waves and the triangular (or linear) one. The user can create digitally new waveforms to fulfill his specific measurement.

The subreflector is tilted around the neutral point to minimize the geometrical aberrations due to the off axis position. In this way we can reach large angle separation in the sky, up to 1 degree, mantaining diffraction limited the optical performances. All the details of the modulation system are described in Mainella G. et al. (AO, Vol.35, 13, 1996).

 

The telescope has an altazimuthal mount. To have an efficient subtraction of the atmospherical contribution, a horizontal modulation in the sky is necessary.

Only an altazimuthal mount satisfies this requirement during the tracking of a source. With this kind of mount, the complete instrument fits a small dome: the ratio between the telescope diameter and dome size is almost 1! In fact local environmental laws have limited the maxima sizes of the dome. From the laboratory it is possible to communicate with all the instrument subsystems in the dome.

The telescope is shielded from local environment background by a shield with vanes in the inner surface. The vanes ensure an optimal rejection of off-axis radiation.