OLIMPO: a mm and submm telescope on a stratospheric balloon
The cryogenic system of OLIMPOIs a classical LN/LHe aluminum cryostat with a 3He refrigerator. Both are improved versions of the BOOMERanG ones (see links A and B). Its purpose is to cool the detector arrays at 0.3K, and the reimaging optics (filters, mirrors, cold Lyot stop) at 1.6K. The hold time of the 3He refrigerator at 0.3K has been measured to be 12 days, with a re-cycling time of 8 hours. The hold time of the LHe and LN fluids has been measured to be 14 days.
The detectors of OLIMPO
We use two different detector technologies for OLIMPO: Ge-thermistor bolometers made in Grenoble for the high frequency channels (348 and 483 GHz) and TES bolometers made in Cardiff for the low frequency channels (148 and 215 GHz). These detectors are close to be photon noise limited in the stratospheric environment with a room-temperature telescope.
New detectors, much less
sensitive to cosmic rays, are being developed in
Chalmers and are currently tested in Rome. These are
Cold Electron Bolometers (see e.g. Salatino et al.
astro-ph/1403.7779, astro-ph/1410.5870, and
OLIMPO is composed of a tiltable inner frame, including the telescope, the spectrometer and the detectors cryostat, and an outer frame containing all the other systems and connected to the balloon through an azimuth pivot and the flight chain. OLIMPO points the desired sky region using a combination of azimuth and elevation movements. The azimuth actuators are a flywheel and a motor torquing aginst the flight chain. The elevation actuator is a linear motor modifying the tilt of the inner frame with respect to the outer frame. The attitude sensors for real-time control are a set of 3-axis laser gyroscopes, while the elevation sensor is a 16-bit optical encoder. The main sensor for attitude reconstruction is a red/IR star camera, able to detect all mag.7 stars in a 2 deg x 2 deg field. The tests carried out show that arcmin stability and sub-arcmin reconstruction can be obtained, as required for spectroscopic observations. Photometric observations can also be carried out in scanning mode, tilting periodically the primary mirror in cross-elevation (20s typical period, with an amplitude of about 1 deg max).