sapienza asi cardiff cea chalmers iaps ifac ingv inpe neel  nnsu wits

  OLIMPO: a mm and submm telescope on a stratospheric balloon

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Instrument Overview:


OLIMPO is a complex stratospheric balloon payload, including a large-aperture telescope, a differential Fourier-transform spectrometer, and four arrays of cryogenic detectors cooled at 0.3K by a large cryostat. The system is complemented by a precision attitude control system, able to point the telescope with sub-arcmin accuracy, tracking the target source during flight operations, by large solar panels and power converters, and a world-wide coverage telemetry. The full payload is 5 m tall and has a mass of about 2 tons.  Click here for the expected performance .


telescope test

telescope optics

The optical system of OLIMPO

The mm-wave telescope of OLIMPO has a 2.6 m diameter primary and a 0.6 m diameter secondary. Starting from a classical Ritchey-Chretien configuration, the telescope has been optimized to scan the sky by tilting the primary mirror around a cross-elevation axis. A description of this class of telescopes can be found here. The telescope is protected by a set of large, optimized ground-shields and sun-shields, to limit the effect of straylight and sidelobes. The focus of the telescope is located inside the cryostat. The beam is reimaged through a set of 3 cold mirrors, the second one being the cold stop of the system, and split by 3 dychroic beamsplitters into 4 photometric channels, each one containing one array of bolometers. 

cold optics

 gondola

cryostat

The cryogenic system of OLIMPO

Is 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.

detectors

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 references therein)


The attitude control system

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).



Expected performance