Pulsar Gate Observers Guide

VLBA Pulsar Gate Observing and Correlating Requirements

Sources to be gated should be marked by setting CALCODE = 'G'. The CALCODES may be set by including CALCODE = 'G' in the SCHED command file.
Pulsar positions should be adjusted for proper motion and the updated positions included in the SCHED files.
The observers must communicate the gate on and off phases directly to the contact person and the data analysts. There is currently no way to transmit gate phaes through the SCHED program.
The observers are responsible for supplying the pulsar phase ephemerides that set the gate phase during correlation. These are in the form of polyco.dat files that are generated by the TEMPO program. Gated correlation is essentially run blind. The polyco.dat files must be of sufficient accuracy to predict the pulse phase to at least one percent of the pulse period. The default gate operation assumes that the pulse is centered on 0.0 phase (0.0 - 1.0 phase across a pulse period). Up-to-date pulsar timing observations will probably be necessary for most pulsars.

Notes from Alan Roy Concerning Phase Referenced Observations.

1. Calibrator selection

For calibrator source lists, try:

The VLBA calibrator survey: http://magnolia.nrao.edu/vlba_calib/index.html

USNO Radio Reference Frame Image Database: http://maia.usno.navy.mil/rorf/rrfid.html

For best results, source strength should be more than a few hundred mJy. Ed Fomalont had success with a 20 mJy calibrator that was within the same primary beam as his programme source, obviating the need for driving off source for calibration. (This will probably require two correlation passes, however.)

2. Switching angle

You want a calibrator within 6d, and preferably within 2d. Test observations at 15 GHz switching between pairs of strong sources shows that the residual rms phase after phase referencing remains fairly constant out to 6d then rapidly gets worse.

3. Switching time

Recommended maximum switching time is frequency dependent. Using the requirement that the rms phase between calibrators be < lambda/4, which should allow phase connection in 95% of cases, the following switching times are derived by Beasley & Conway 1995 ("VLBI and the VLBA", ASP Conf Ser, Vol 82, p327)). The switch cycle times are given for typical and poor seeing conditions.

Cycle Time (minutes)
4.8 GHz	7.6	3.7
8.4 GHz	3.8	1.9
15 GHz	2.0	1.0
22 GHz	1.2	0.6
43 GHz	0.6	0.3

4. Phase check source

You should include periodic looks at a second phase referencing source. Phase transfer fails sufficiently frequently, leaving large residual phase errors, that you really want a test source included that's strong enough to see, so that when you reduce the data and get a non-detection, you can see whether it's due to phase referencing problems, and to get a measure of the decorrelation expected on your programme source.

5. Expected performance

Tests at 15 GHz with a 6-min cycle time and switching angles between 1.3 and 9.9d yielded phase referenced images with dynamic ranges of 13 to 33 (25th & 75th percentiles). These test have yet to be completed and extended to other frequencies.

VLBA Correlator Pulsar Gate Specifications

Gate resolution (time) : 1024 points over the pulse period
Gate resolution (spectral) : Separate gates for each spectral channel (<= 2048)
Gate tracks pulse dispersion delay across baseband channel bands and between separated baseband channels
All correlator FFT sizes and spectral averaging are supported
The correlator full polarization mode is supported
Gate phase updates at 16 microsecond intervals
Gate on and off phases are user selectable (at the polyco.dat ref frequency)
The number of gate on FFT cycles (gate weights) are written into the correlator output FITS file. There are separate gate weights each spectral channel.

Correlator Mode Restrictions with the Pulsar Gate

One pulsar subarray only, no limit non-gating subarrays
No digital filter
One accumulation time interval
New gate mask upload requires 10 secs (speedup x1)

Useful Links to Pulsar Webs Sites

Email comments to: John Benson