3. Receivers and Correlators

3.1. Sensitivity

When preparing your observing proposal, you are required to estimate the expected brightness and sensitivity of your source for your particular correlator/receiver combinations. For spectral-line observations, sensitivity per bandwidth channel can be estimated from the following equations of line brightness and line flux respectively:

\[T_{rms} (mK) \sim \frac{ T_{sys} }{ \sqrt{npol \frac{BW}{chan} \delta T} }\]
\[S_{rms} (\frac{mJy}{beam}) \sim T_{rms} G \eta_b\]

In the above, G is the main-beam gain (Jy/K) for a receiver defined from \(G = \frac{ \Omega_{mb} }{ \Omega_{tot} }\), where \(\Omega_{mb}\) and \(\Omega_{tot}\) are the solid angles on the sky for the main beam and the total collecting are of the antenna, repsectively. npol is the number of polarisations (an average of two independent polarisation channels), BW is the bandwidth [MHz], nchan is the number of channels and \(\delta T\) is the on-source integration time in seconds. \(\eta_b\) is the beam efficiency factor \(\frac{\Omega_{mb}}{\Omega_{tot}}\) = 0.7, where \(\Omega_{mb}\) is the . For continuum, we need to calculate the sensitivity over the whole bandwidth. The continuum line brightness and line flux respectively become:

\[T_{rms} (mK) \sim \frac{ T_{sys} }{ \sqrt{npol BW \delta T} }\]
\[S_{rms} (\frac{mJy}{beam}) \sim T_{rms} G \eta_b\]

For both the line and continuum flux, the source is assumed to fill the main beam which has efficiency \(\frac{\Omega_{mb}}{\Omega_{tot}}\) = 0.7. The \(1\sigma\) theoretical RMS noise estimates for line and continuum observations can be estimated by using the on-line sensitivity calculator.

3.2. Parkes Receiver Fleet

The system temperature across the Parkes frequency range is shown below. For each receiver (identified by a coloured bar), the variation of \(T_{sys}\) is shown across the band. Additional information on available receivers is provided in the table below.

Range in system temperature for Parkes receivers.
Receiver Band [cm] Range [GHz] Diam [m] FWHP [‘] Tsys[K,a] Sens [Jy/K,b] Pols[c] BandW [MHz]
UWL 40-7.5 0.70-4.00 64 7 21 1.6 2xL 3300
1050CM 50 0.70-0.764 64 30 40 1.1? 2xL 64
10 2.60-3.60 64 6.4 35 1.1 2xL 1000
MULTI 21 1.23-1.53 64 14.2 28 1.1 26xL 300
H-OH 21/18 1.20-1.80 64 14.8 25 1.2 2xL 500
GALILEO 13 2.20-2.50 64 9.2 20 1.3 2xC 300
2.15-2.27 64 9.2? 20? 2.1? 2xC 120
2.29-2.30 64 9.2? 19? 1.4? 2xC 10
AT S-BAND[d] 13 2.20-2.50 64 9.2? 79? 1.9? 2xL 300
AT C-BAND[d] 6 4.50-5.10 64 4.5 50 1.3 C 500
AT X-BAND[d] 3 8.10.8.70 64 2.4 140? 1.2 2xL/C 500
3 8.10-8.70 64 2.4 140? 1.2 2xL 500
13 2.20-2.50 64 9.2? 79? 1.9? C 300
METHANOL 5 5.9-6.0 64 3.4 55 1.4 2xC 300
MARS[e] 3 8.0-8.9 55 2.45 30 1.7 2xC 500
KU-BAND 2.2 12.0-15.0 64 1.9 50? 1.6? 2xL 500
13MM 1.3 21.0-24.0 55 1.3 05? 2.0? 2xC 500
16.0-26.0 55 1.4 95 2.2 2xL 1000
21.0-22.3 55 2xC 1000

Notes:

  • [a] Includes typical atmospheric, ground and galactic contribution at Zenith.
  • [b] Calculated over main-beam and using \(\frac{\Omega_{mb}}{\Omega_A}\) = 0.7.
  • [c] L = linear, C = circular.
  • [d] AT S, C, X-BANDS: Dual linear feeds, \(\lambda\)/4 plates avaliable for band centers.
  • [e] Full bandwidth by special arrangement.

Most of the Parkes receivers allow injection of a calibration noise signal into the receiver waveguide ahead of the ortho-mode transducer (OMT). This is generally a more satisfactory method than injecting after the OMT or after the LNAs as these elements can then be modelled using the calibration signal. The calibration signal is generally injected through a coupler in the circular waveguide oriented at 45 degrees to the linear probes of the OMT. Thus the cal can be closely represented by a 100% linearly-polarised signal with an accurately known feed angle. The amplitude of the cal signal is adjustable by inserting or removing fixed attenuators between the noise source and the coupler. Changing the level requires access to the receiver in the focus cabin and takes of order 30 minutes. The cal can be switched on or off remotely as required, using an observer-selectable waveform. Typically the cal is run as a continuous low-level NAR (Noise-Adding Radiometer) with the cal level approximately 10% of Tsys, for a time-averaged increase of 5%. The frequency of the switching signal is typically between a few Hz and 500Hz. For more information on particular receivers, please refer below.

3.2.1. UWL

3.2.2. 1050CM

The 50cm receiver in the 1050CM package injects a cal signal using a directional coupler after each LNA (strictly, after the 4-port hybrid used to combine the signal from each pair of opposing probes and LNAs). A splitter is used to generate two identical cal signals from the same noise source. Further information on the receiver can be found in Granet et al. (2005) .

3.2.3. 20CM Multibeam

The Parkes telescope is equipped with a sensitive 13 beam receiver operating at 20cm, and a 26 channel spectral line correlator (13 beams by 2 polarisations). The Multibeam system comprises 13 identical dual-linear feeds, each with cryogenically-cooled HEMT LNAs, covering a frequency range of 1230-1530 MHz. The FWHM of the center beam is 14.0 arcmin, beams 2-6 14.1 arcmin and beams 7-13 14.5 arcmin. The thirteen horns are disposed in a hexagonal pattern, with the inner and outer rings of beams having a radii 29.1 arcmin and 50.8 arcmin respectively. The receiver package rotated at an angle of 15 degrees to the scan direction presents a nearly uniformly spaced “comb” of beams spanning approximately 96 arcmins. Adjacent scans of 35 arcmins (0.583 degrees) thus have an approximately two-thirds overlap. The package can be rotated in feed angle up to -70 degrees and +83.75 from its neutral position; rotation is in a positive direction corresponds to increasing position angle on the sky, or anti-clockwise as shown below:

Overview of the Parkes observing system.

Further characterisation of the reciever can be found in the following:

3.2.4. H-OH

The H-OH receiver has an optional quarter-wave plate which can be inserted in the circular waveguide between the feedhorn and the OMT to achieve circular polarisation on the sky. The quarter-wave plate is inserted before the cal injection so in this case the cal signal resembles a 100% circularly-polarised signal on the sky but the cal signal alone cannot be used to model the precise properties of the quarter-wave plate.

3.2.5. Galileo

The GALILEO receiver has cal injection into circular waveguide but uses a circularly-polarised OMT (cal signal resembles 100% linear on sky).

3.2.6. C/X-BAND

The C-band and X-band receivers in the AT Multi-band receivers also have quarter-wave plates ahead of the cal injection.

3.2.7. MARS

The MARS (8.4 GHz; X–band) receiver has a built-in (non-removable) waveguide circular polariser also with cal injection between the polariser and OMT.

3.2.8. KU-BAND

The LNA response is impacted by poor return loss effects (from the OMT and possible feed combination) below 12.6 GHz. All the test data measured in the lab starts from 12.5GHz (and typically goes to between 15 and 18GHz).

3.2.9. 13MM

A K-band receiver covering 16-26 GHz was delivered and commissioned in September 2008 and July 2009. The receiver has wider frequency coverage than the older K-band receiver and appears to have the anticipated ~threefold advantage in Tsys at 22 GHz over the older package. The receiver can be installed with either of two feeds: a narrow-band feed and quarter-wave plate providing dual orthogonal circular polarisation over the frequency range 21.0 to 22.3GHz, or the standard feed providing dual orthogonal linear polarization over the 16 to 26GHz range. The package has two independent conversion systems allowing simultaneous operation at any two arbitrarily-spaced frequencies within the band limits. The 13MM receiver also has an optional quarter-wave plate used with the narrow-band VLBI feed covering the 22 GHz water transition. As with the H-OH receiver, the cal injection occurs after the polariser (between the polariser and the OMT). More information is available here .

3.3. Conversion System

The Parkes Conversion System (PCS) is summarised as follows:

  • It is possible to observe simultaneously two widely separated spectral line features within a receiver passband. Alternatively, in the case of a dual band receiver (eg. The S-X receiver covering 2.2-2.5 GHz and 8.1-8.7 GHz), spectral line or broadband noise observations may be made simultaneously for each of the bands.
  • Dual polarisation is available for each of the observing frequencies, necessitating a total of four conversion channels. However, as the modules are paired, only two independent Local Oscillator (LO) systems are needed.
  • The input bands are 300-750 MHz (UHF-band), 1.2-1.8 GHz (L-band), 2.2-3.6 GHz (S-band), and 4.5-6.1 GHz (C-band). Observations outside these bands, for example at K-band (22 GHz) are accommodated using an extra conversion on the receiver package or using LOs in the focus cabin and/or upstairs control room.
  • Wherever possible signals generated by the local oscillator system should not fall within any signal or intermediate frequency (IF) bands to reduce the incidence of internally generated interference. Unfortunately, due to the very wide S-band (2.2-3.6 GHz), one of the LO frequencies may fall inside the band for some observing frequencies.
  • Frequency switching may be used for observations of a single spectral line. For C-band inputs, frequency switching is available for two spectral lines simultaneously.
  • In order to ensure the conversion system is capable of supporting simultaneous use of DFB4, BPSR, APSR and HIPSR, a number of buffered outputs for each output bandwidth have been provided. Each of the 4 channels has 4 of 64 MHz, 3 of 128 MHz, 3 of 256 MHz, and 2 of 900 MHz bandwidth (BW) outputs available. One complete set of outputs for each channel (64, 128, 256, and 900 MHz BW) have been provided at the front of the conversion rack. The remaining system outputs are cabled to bulkhead connectors in the rear of the rack for permanent connection to the DAS and an RF Switch Matrix. The latter operates the standard connections from the conversion system to the several correlators/backend units. It is operated by software and in most cases the connection Conversion System output to backend is automatically instated by the observation control software (TCS: Telescope Control System).

An in-depth discussion of the PCS (including block diagrams) is available here.

3.4. Signal Path

An overall outline of the Parkes observing system is shown below.

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alt:Overview of the Parkes observing system signal path.
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Single-beam spectral-line observations have back-end options using 4, 8 or 64 MHz bandpass capabilities of the 2-bit Multibeam correlator, or patching in an ATCA-style bandpass filter to provide 16 or 32 MHz bandpass capability. Wider bandwidths (>64 MHz) are available using the 8-bit Digital Filterbank (DFB4), but it is also possible to achieve smaller bandpasses with DFB4 (ie., 8, 16, 32 MHz). For Pulsar observations, it is possible to switch simultaneously record data on several back ends at once.

3.5. Backends

A number of backend units are available:

  • DFB4: spectral line, pulsar, continuum and polarimetry, for one IF dual polarization observations
  • BPSR: multi beam digital backend for pulsar observations (up to 13 IFs dual polarization).
  • HIPSR: a reconfigurable digital backend for the Parkes Multibeam receiver.
  • CASPSR: a GPU-based backend for pulsar observaitons. Capable of phase-coherent dispersion removal for 2 IF, dual-polsarisation signals with a maximum bandwidth of 400 MHz
  • DAS: baseband analog to digital sampling system (VLBI only)
  • MK-V: combines a set of tuneable samplers and a disk recorder for VLBI (VLBI only)

Please check the Parkes Correlator Guide. for information on capabilities or email ATNF-Parkes-Remobs[at]csiro.au to ascertain requirements.