The C-BASS receiver is a hybrid of a broad-band correlation polarimeter and a correlation radiometer. The polarimeter design is optimised for extremely low cross-polarisation, while the radiometer uses a pair of temperature-stabilised 4 K loads to allow accurate total power measurements.
To achieve our science goals, the minimum requirement is an rms noise of <0.1 mK per pixel, and an accuracy of 5% on scales up to 10 degrees. In practice the noise will be substantially lower. Our goal is to reduce systematic errors well below the 5% level, and on angular scales up to the quadrupole, in order to complement future B-mode satellites (e.g. CMBPol).
|Angular resolution:||0.73 deg (43.8 arcmin)|
|Sensitivity:||<0.1 mK/beam r.m.s.|
|Stokes coverage:||I, Q and U|
|Tsys:||<20 K, including sky|
|Frequency/bandwidth:||1 GHz bandwidth, centered on 5.0GHz|
|Northern site:||OVRO, California, latitude 37.2 deg., 6.1m dish|
|Southern site:||meerKAT Karoo site, South Africa, latitude -30.7deg., 7.6m dish|
The schematic layout of the C-BASS receiver is shown below. The receiver is a hybrid of a correlation radiometer and a correlation polarimeter, which share balanced LNA and warm amplifier chains to reduce production costs.
Some of the features of the receiver which are intended to reduce systematics are:
- Temperature-stabilised 4 K loads are used to obtain accurate total power measurements.
- Each circular polarisation is spread over two amplifier chains to lessen the effects of gain drift (a balanced receiver).
- All the signal paths are phase switched with orthogonal Walsh functions.
We have designed all of the components in the receiver with low cross-polarisation as our primary consideration. One of the major technical hurdles we have overcome was designing a broadband, compact, low cross-polarisation Orthomode Transducer (OMT).
C-BASS will have a bandwidth of about 1 GHz around 5 GHz. All the receiver components have been designed to cover the 4 to 8 GHz band where possible, and the WG13 band (4.6 to 7.1 GHz) otherwise. The actual survey band will then be selected by tuning the bandpass filters in the amplifier chains to the appropriate frequency. This allows us great flexibility in responding to conditions on site, such as local sources of man-made radio interference.
In addition to the use of cold loads and other receiver design features, we will reduce systematics by:
- Using ground screens to shield the receiver from polarised ground-reflected radiation.
- Rapid scanning of the telescope.
- Highly-redundant coverage at a variety of scan crossing angles.
The optical layout and feed-horn have been optimised for minimal sidelobes and cross-polarisation. Through the judicious use of absorbing tunnels it should be possible to reduce the sidelobes to more than 40 dB below the main beam, with the absorbing tunnels contributing about 0.8 K to the system temperature.
If we interpolate between the polarised surveys at 1.4 GHz (Wollenben et al., 2006) and 23 GHz (Page et al., 2007) using a polarised spectral index of β=-3.2, we can predict that 90% of the sky at 5 GHz will have a polarised intensity of greater than 0.59 mK. To satisfy signal-to-noise requirements on the data at CMB frequencies (nominally, 60 GHz) we require our C-Band data to have a signal-to-noise ratio of at least 2√5:1. Thus we require a sensitivity of < 0.13 mK.
The sensitivity of the C-BASS receiver to Q and U is given by the radiometer equation for a correlation receiver:
σ= Tsys / sqrt(2Δντ)
where Tsys ~ 20 K, Δν~ 1 GHz, and σ< 0.13 mK. Crunching the numbers suggests that we need about 20 s of integration time per pixel to achieve the necessary sensitivity. Observing for 9 months in each hemisphere, with a night-time observing efficiency of 50%, should easily achieve this sensitivity.