Pulsar Observing

by Stephan W. Witz last modified Mar 15, 2017 by Heidi Medlin

The VLA can be used for several kinds of pulsar observing: phase-binning using the WIDAR correlator, using the phased-array for single-beam pulsar processing in either search or fold modes, or simply standard imaging mode with fast integrations. Any of these types of pulsar observing are considered Resident Shared Risk Observing (RSRO), and participants in this program are expected to work closely with NRAO staff. Of these, phased-array and fast-dump observing are significantly more mature than phase binning. For any questions not addressed here regarding the capabilities of these observing modes, please contact the NRAO Helpdesk.

Phased-array pulsar processing

The "Y" Ultimate Pulsar Processing Instrument (YUPPI) is a suite of software that runs in the correlator backend (CBE) computer cluster and can process a single-beam phased/summed-array data stream for pulsar observations in real time, into either folded profiles or search mode (filterbank) output. Coherent dedispersion can be optionally applied in either mode.

In this mode, the voltage data streams from each antenna are divided into a number of frequency subbands within the correlator, then summed and requantized before being output to the cluster for pulsar processing. The limitation on bandwidth comes primarily from the available network connections between the correlator and cluster. In all cases, a maximum of 64 subbands total can be processed. Depending on the number of bits chosen, this results in the following total bandwidth constraints:

Table 3.18.1: Pulsar Observing Bandwidth Constraints

Subband bandwidth

Subband quantization

Max total bandwidth

Samplers

32 MHz 8 bits 2048 MHz 8-bit
64 MHz 4 bits 4096 MHz 3-bit
128 MHz 2 bits 8192 MHz 3-bit
<32 MHz 8 bits 64*BWsub 8-bit

As described in the VLA Frequency Bands and Tunability section, the 8-bit samplers provide two independently tunable 1 GHz IFs, while the 3-bit samplers provide four tunable 2 GHz IFs. 

The pulsar-specific processing is done in real time using the DSPSR software package and, in principle, any processing option supported by DSPSR can be used, although this will be constrained by the real-time computing power available in the cluster. In general, each subband can be divided into an arbitrary (2n) number of channels; 1 (summed), 2 or 4 detected polarization products can be output; and coherent dedispersion can be enabled or not.

Fold mode

In fold mode, the data are averaged modulo a known pulsar ephemeris (provided via a standard TEMPO/TEMPO2 "par file") into pulse profiles. The data can also be folded at a constant topocentric period, for example at 10 Hz to detect the injected noise cal signal. Fold integration times as short as 1 second have been tested. Up to 16384 profile bins can be used. The data are recorded in PSRFITS format using the standard 16-bit data encoding. This means the final output data rate is given by:

Data rate = 2 bytes × Nsubband × Nchannel × Nbin × Npoln / Tint

If the desired data rate exceeds ~25 MB/s, additional testing ahead of time may be required.

Search mode

In search mode, the data are simply detected and averaged over a specified amount of time before being output to disk, resulting in a filterbank data array (power vs time and frequency). Coherent dedispersion at a known DM can optionally be enabled for this.  Data can be recorded using 2, 4, 8, 16 or 32 bits, resulting in a final data rate of:

Data rate = (Nbit/8) bytes × Nsubband × Nchannel × Npoln / Tint

The maximum sustained output rate in this mode should be kept less than ~400 MB/s.

Gated or binned visibilities

The WIDAR correlator has the capability to internally integrate (fold) visibilities into 1 or more pulse phase bins. Constraints and trade-offs on number of bins, bin width, pulse period, bandwidth, and integration time are currently not well quantified. This mode is likely to require significant development work to become usable.

Fast-dump visibilities

While not specifically a pulsar mode, standard visibility data can be dumped as fast as 5 ms, which may be sufficient for imaging of slow pulsars.  See the Time Resolution and Data Rates section for more details.