ATNF VLBI Wiki

Setup the DAS and cdisko and vnc session as per the general observing notes.

Copy the sched input file from the atnf ftp area for this experiment, e.g. v001a_At.com. This needs to be converted into a vlobs file. Normally these should be copied onto xbones at ~atcaobs/sched_vlbi. atcasched is used to do the conversion. E.g.

> atcasched > @v001a_At.com > write > exit

Antenna control is from the normal caobs program. Usually the experiment schedule has a 1 hour scan on 1934-638 or 0823-500 inserted before the start of the experiment proper. This time can be used to phase up the array. However it is recommended to take the experiment schedule and manually edit in atcasched to create a ò??relativeò?? cal schedule with a strong source to run CACAL at during the setup time.

When on source and the samplers have settled down, and if the cross spectra are interference free (at 20 cm wavelengths), perform cacal. Once you are happy with the cal solution, run the proper schedule.

Catie controls how many of the six dishes are added together before feeding the signal into the DAS and what polarisation is used.

ò??ò??ò??CATIE IS PART OF CAOBSò??ò??ò??

caobs has to be told to connect to the tied array box. This needs to be done every time caobs is restarted. Do this in caobs by typing

catie connect

This will show the catie config screen. To get back to this screen type

show catie

to show the normal caobs window type

show status

The required catie commands are:

catie function xx-yy xx+yy xx-yy xx+yy (RCP first) catie function xx+yy xx-yy xx+yy xx-yy (LCP first) catie gain 2 2 2 2 catie antennas 12345 12345 (we normally only tie 1 to 5)

function determines whether the signal is converted into Rcp/Lcp or Lcp/Rcp. This depends on cacal inserting a 90 deg phase shift. Recording linears is also possible.

At a the unix prompt on xbones run:

cacal

and use the option for interactive mode:

cacal i (for interactive mode) c (for calibrate array) 3 (reference antenna, probably the default) 3 (cycles to average, also the default) a (to calibrate all) n (to not determine leakage terms) n (do not apply 90 degree XY phase offset for circular) n (not calibrate continuously) y (to continue as many times as asked)

When this has finished, and the results are satisfactory, run cacal again. Run it again in interactive mode:

cacal i (for interactive mode) c (for calibrate array) 5 (reference antenna, probably the default) 3 (cycles to average, also the default) p (to calibrate phase) n (to not determine leakage terms) y (apply 90 degree XY phase offset for circular) n (not calibrate continously) y (to continue as many times as asked)

Comments should be entered into wiki as necessary. For each experiment please record

ò??tied array 12345 RCPò?? (tells which antennas are tied and polarisation)

ò??reference antenna ca03 on station w100 (15)ò??

                                    (reference antenna location as described from
                                     /gen page in CAMON and on the control
                                     room white board)

The CACOR gui displayed on nemesis runs the correlator and also produces a log file which is relevant to VLBI. To initiate generation of this log file type the following commands into the command box at the bottom of the gui

tda C12345 C12345 (for example, to specify circular

                    polarisation and that 1 to 5 are tied)

tdt 6 (to generate a log file entry every 6

                    correlator cycles, usually 1 minute)

Also, it is important to make sure that the correlator cycles on a 10 second tick. For example you should see messages going past in the correlator window like:

DELBAT : 20:38:40.000 0: 1296 1376 1312 0 -976 -28864 | 1296 1376 13 OLDCYCLE: 20:38:30.000 NA=6 NF=2 ON SRC: 000000

If the correlator cycle is not on a 10 second tick, the setup is not optimal for VLBI. Some data is lost but it is not a disaster. If possible try to reset the correlator. Alternatively, change the 5 second offset in the ATCA schedule. For example, if the correlator is cycling on the 6 second tick, change the 5 second offset to an 8 second offset. Here is John Reynolds' explanation:

During the observations the phases will wander with time. If the antennas are tied then this causes the combined signal to be degraded. The phase wander can be monitored in VIS by specifying

sel 6cc 6dd (select second frequency, both polarisations,

            and baselines to antenna 6)

If the spread in the phases seen in VIS gets worse than 30 degrees then perhaps some antennas should be removed from the tied array. A better solution is to insert occasional calibrator sources into the schedule so that a CACAL can be performed from time to time to recalibrate the phases and gains. To do this, wait for the calibrator scan and run cacal at a xbones prompt

cacal i c 6 3 p (to calibrate phases) y (apply 90 degree XY phase offset for circular) n y

Run the script atcalog.sh as vlbi on a Marsfield unix system (e.g. hydra or atlas).