The Planck LFI RCA flight model test campaign
Introduction
Planck is an ESA mission designed to map with high precision the angular distribution of the cosmic microwave background (CMB). The Planck observations are expected to produce major steps forward for precision cosmology as well as for Galactic and extragalactic millimeter-wave astrophysics. Two instruments, LFI (Mandolesi et al., 1998, Mandolesi et al., 2004) and HFI (Puget et al., 1998), are integrated at the focal plane of an off-axis 1.5 m telescope, each covering two different spectral regions.
LFI is an array of 22 HEMT-based pseudo-correlation receivers coupled to the telescope by eleven dual profiled corrugated feed horns. The radiation gathered by each feed horn is divided into two orthogonal linearly polarized signals by an Ortho Mode Transducer. Each signal is fed to a radiometer where it is mixed by an hybrid coupler with a reference signal coming from a 4 K load thermally connected to the HFI shield. The two output branches of the hybrid coupler are amplified by a couple of cryogenically cooled HEMTs low noise amplifiers, appropriately shifted in phase and in a synchronous way with the data acquisition, decoupled by another hybrid in order to remove common noise, and sent through waveguides, to a back end section where they are filtered, detected, and differenced. The geometry and materials of the waveguides are chosen to minimize the thermal link between the 20 K cold part of the instrument to the back end 300 K stage.
The instrument design and development were mainly driven by the high sensitivity performance of the detectors and to a strict control of the systematic effects (Mennella et al., 2004). After the single flight units qualification, each of the 11 radiometer chain assemblies, consisting of two pseudo-correlation receivers with four detectors, have been tested to verify and measure their basic performances. In this paper the RCA test campaign is discussed.
In Section 2, the experimental setup and cryogenic equipment are described. Section 3 reports the methods for the calibration tests used to evaluate the main radiometer parameters and performances. Finally, the main conclusions are drawn.
Section snippets
The RCA test facilities
The Planck LFI RCA calibration was performed with the radiometer integrated in two dedicated cryofacilities.
The 30 and 44 GHz radiometer chains (Fig. 1) were tested at Alcatel – Alenia Space in Italy, while the 70 GHz channels were calibrated at Elektrobit in Finland. The design of the RCA cryofacilities is such that it provides thermal interfaces and environment highly representative of the flight conditions, and it allows to perform testing and performance verification of the RCAs. The two
Main calibration tests
After functional warm tests, the RCAs are cooled to be tested at their nominal operating temperatures. We summarize here some of the main calibration tests performed. A user friendly data analysis software (RaNA) was developed, in IDL environment, to quickly display and analyze data, exploiting ad-hoc built modules for each kind of calibration test (see for instance Fig. 5).
Conclusions and next future
A description of the Planck LFI radiometer chains calibration tests is given. The experimental setup and the test methods were described. Preliminary data analysis confirms that the radiometers achieve the best performances ever reached at these frequencies for space missions, in relatively good agreement with the requirements. The entire Radiometer Array Assembly is then integrated and tested in a larger cryo facility to verify the absence of unwanted cross-talk among different chains and to
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LFI radiometric chain assembly (RCA) data handling ''Rachel''
2009, Journal of Instrumentation