Table of Contents

Scheduling LBA experiments

When an LBA experiment is scheduled, the PI is sent a letter giving them the details of the observation, information on preparing a schedule, and contact details for correlator personnel. Questions related to the correlator and correlation of LBA observations should be sent to Cormac Reynolds at Cormac [dot] Reynolds [at] csiro [dot] au.

All LBA experiments should be scheduled using the “VLBA” Sched program. You can either download and install it on your own Unix workstation/laptop or use the Linux system at the ATNF.

Unless you have specific frequency requirements (e.g. spectral line experiments, specific antenna limitations, known RFI issues) please always use the standard setups. Any changes from the standard setups increase the risk of your experiment failing. Sched comes with a comprehensive manual. Please read it carefully before you start scheduling. A set of examples are distributed with SCHED ($SCHED/examples/lba*).

Antenna names

Antenna names in SCHED specify not just the physical antenna, but also the back end being used to record the data where there are multiple options. You must use the correct name for your particular experimental setup. Current names for LBA and associated stations (with notes in brackets) are:

ASKAP
ATCA
CEDUNA (LBA DAS)
CDDBBC (Ceduna with Mark5 recorder)
HART (26m Hartebeesthoek, Mark5)
HART15M (15m Hartebeesthoek, Mark5)
HOBART (Hobart 26m with LBA DAS)
HOB_MK5 (Hobart 26m with MarkIV rack and Mark5 recorder)
HOB_DBBC (Hobart 26m with DBBC and Mark5 recorder)
KATHERIN
MOPRA
PARKES (LBA DAS)
PARK_MK5 (Parkes with Mark5 recorder)
WARK12M (12m Warkworth, Mark5 recorder)
WARK30M (30m Warkworth, Mark5 recorder)
YARRAGAD
DSS43LBA (Tid 70m with LBA DAS)
DSS43 (Tid 70m with Mark5 recorder)
DSS34LBA (Tid 34m with LBA DAS)
DSS34 (Tid 34m with DVP)
DSS35LBA (Tid 34m with LBA DAS)
DSS35 (Tid 34m with DVP)
DSS36LBA (Tid 34m with LBA DAS)
DSS36 (Tid 34m with DVP)

Parkes observations will normally use the LBA DAS, unless there are special requirements.

Since April 2017 Hobart and Ceduna should normally be scheduled as HOB_DBBC and CDDBBC to use the DBBC, unless otherwise instructed.

Tidbinbilla now has DSS36. This telescope is very similar to DSS34 (and DSS35) and should be scheduled as DSS34LBA if your version of the SCHED catalogues do not yet contain DSS36.

DAS Setups

LBA observatories use the ATNF DAS, which is a digital filter bank. The ATNF observatories (Parkes, ATCA, Mopra) each have 2 DAS installed, all other LBA observatories have only a single DAS. The DAS have two independent “IFPs” which sample the analogue signal and produce up to two digitised channels each, which are then recorded. The two IFPs are generally connected to the two polarisations of the receiver but can be set up to record two different frequencies of the same polarisation.

Normally the output of each IFP is either a single channel of bandwidth 2, 4, 8 ,16, 32 or 64 MHz or two channels adjacent in frequency (ie upper and lower sideband) with a bandwidth of 2-16 MHz. Generally the two adjacent channels are only used with 16 MHz bandwidth, except in special situations.

LBA specific scheduling issues

LBADR

Please use the latest version of NRAO SCHED to prepare your schedule files. SCHED changes regularly to reflect changes in the capabilities of the telescopes and correlator. Older versions of SCHED will not necessarily produce a correct .vex file for a current observation.

SCHED is installed on linux machines at ATNF (hydra and draco) or can be obtained from ftp://ftp.aoc.nrao.edu/pub/sched/ . Example LBA .key files are distributed with the latest version of SCHED ($SCHED/examples/lba*.key). Please prepare your schedule using one of these. The lba* setup files distributed with SCHED should cover all standard continuum modes. If your schedule requires a non-standard mode or frequency (e.g., for spectral line observations) then you will need to consult with correlator personnel before submitting your schedule.

Standard configs are described at http://www.atnf.csiro.au/vlbi/observing/lba_configs.html .

Dual or Single Polarisation

The observer sometimes needs to decide between observing dual polarisations or single polarisation with twice the bandwidth. If possible you should always choose to observe with dual polarisation, even if you are not interested in polarimetry (it removes one common mode of failure, i.e. observing the wrong polarisation). You should only observe single polarisation if some of the telescopes only have a single polarisation available or if the wider frequency coverage is essential to your scientific goals.

Frequency setup

The ATCA and Mopra LO chain generally restricts the frequency selection to 1 MHz steps. For continuum experiments this should not cause any difficulty (particularly if you use the standard setups!). For spectral line experiments, letting Sched select the Doppler frequency will be a problem. Use the DOPINCR keyword to force Sched to round the sky frequency to the nearest MHz. E.g.:

DOPINCR = 1000

However, the mixed backends on the LBA, with their often mismatched filters, means that SCHED's automated Doppler setting often does not work for LBA experiments. Its use is deprecated.

Fringe Finders

Normally the first full hour of your observation is reserved for telescope setup and fringe finding. Please schedule the first hour of your experiment on a strong fringe finder, including Mark5 transfer instructions if necessary. A list of standard fringe finders is available. This plot shows the optimal fringe finder to use at any given Parkes LST for a typical array. (It is not usually possible to fringe test Parkes and Hart at the same time, so here is a similar plot for fringe finding Hart with only a subset of the LBA).

In addition, you should always observe a few strong compact sources during your experiment, probably every three hours (e.g. 3 times for a 6 hour experiment). The fringe finders help in the correlation process for adjusting the clock offsets and checking the correlation process is working properly. They also are good candidates for use with the real-time fringe testing software so any observational problems can be fixed immediately (see next point). Choose one which is strong at your observing frequency and has minimal slew time to your target sources. Note that if you know your target source is quite strong, you may not need to observe a separate fringe finder, but you should note this in your schedule file (using the 'note' keyword at the top of the schedule and the 'intent' keyword at the relevant scan - see below). Please always add a note in the “NOTE” section which sources are your fringe finders. The VLBA calibrator list has a list of VLBI sources, but note that it does not have good coverage for Southern declinations. Leonid Petrov's catalog http://astrogeo.org/calib/search.html is typically better for LBA observations.

Please use SCHED's 'INTENT' keyword to identify your fringe finder scans. E.g.:

dur = 2:00 source = '1921-293' intent = 'FIND_FRINGE' /
dur = 2:00 source = 'target' intent = 'OBSERVE_TARGET' /

Note that you must reset the intent to some other value (e.g. OBSERVE_TARGET) on the following scan or the first intent will be inherited by all ensuing scans!

Real-time Fringe Testing

Real-time fringe testing can be run on all experiments. Periodically (generally when a strong continuum source is observed) a second or so of data is sent over the network links and correlated on the DIFX software correlator. The fringe plot is made available on the web. This allow observatories to be confident they have their observing setup correct, or correct any problems. Scheduling a few strong sources throughout your experiment (and adding an INTENT='FIND_FRINGE' in the schedule) will increase the chance of success for your experiment.

If you are using any stations with Mark5 recorders (AuScope, Hartebeesthoek, NewZealand), fringe test scans must allow a 1 minute gap after the fringe finder to allow some data to be copied off the Mark5 following these instructions (please request assistance if required).

Phase Calibrator Sources

For calibration purposes it is often beneficial to observe a calibrator in the vicinity of your target source (e.g. when phase referencing). A good source for finding these sources is the LBA calibrator survey:

http://astrogeo.org/lcs/cat/

There is also a search form:

http://astrogeo.org/vlbi/search.html

Another good source of calibrator sources for the LBA is the VLBA calibrator database:

http://www.vlba.nrao.edu/astro/calib/

Scan Gaps

Sched allows you to insert a “gap” before a scan to allow for telescope slew time, etc. For schedules using only the LBA DAS it is recommended not to insert manual gaps. If you need to allow extra time for a long slew, increase the length of the scan instead. At the beginning of the scan section of your schedule it is recommended you use:

gap=0

If there are Mark5 stations participating in your schedule (e.g. Hartebeesthoek, Warkworth, Katherine, Yarragadee - on special occasions Hobart and Parkes may also use Mark5 recorders), then you must include regular gaps of at least 20 seconds in your schedule to allow the Mark5 to insert breaks in the recording. SCHED will warn you if you have insufficient gaps, but you should make them as frequent as you can without compromising your ability to get good quality phase referencing. The required frequency of the gaps depends on the data rate, but every 15 minutes is a good guide.

Dwell (do note use!)

Sched has a scan type called “Dwell”. This inserts a variable gap before the start of each scan equal to the expected slew time of the slowest telescope. Don't use this option. If you are worried about long slew times, use the Sched summary option and look at the slew times and the time on source. Adjust scan times as appropriate (e.g. make scan times longer to accommodate long slew times). This has the advantage over Dwell in that you start getting data from the faster antennas as soon as they are on source. This also avoids compromising phase referencing observations with long gaps inserted to avoid predicted long slews at a single telescope (e.g. cable wraps which in any case may not be correctly predicted).

Scan times

Please ensure all scans lengths are rounded to the nearest 5 seconds. The ATCA observing systems run on a 5 second cycle. This is another reason not to use “dwell” observing.

Scan Overhead

There is a 3 “cycle” overhead at ATCA. This means that at the ATCA there is a always a minimum 15 sec of invalid data between scans (this includes frequency changes, two scans observing the same source at the same frequency etc). This is not additional to slew time (ie the invalid time between scans is either 15sec or the slew time, whichever is largest).

If you try and schedule very short scans (e.g. 20 GHz phase referencing) you run the risk of losing a lot of observing time.

Sched Summary

Sched always produces a summary file (experimentname.sum). To make this more useful you should always use the sumitem. Please use the following as a minimum selection of summary items (more are available):

sumitem=el1, el2, slew, dwell

ATCA Issues

Please keep in mind the following for any observation using the ATCA:

http://www.narrabri.atnf.csiro.au/calibrators/

In order to identify a scan in your schedule to be used for re-phasing, please use SCHED's 'INTENT' keyword to mark the relevant scans as 'AUTOPHASE_DETERMINE'. E.g.:

dur = 2:00 source = 'strong-cal' intent = 'AUTOPHASE_DETERMINE' /
dur = 2:00 source = 'weak-target' intent = 'OBSERVE_TARGET' /

Note that you must reset the intent to some other value (e.g. OBSERVE_TARGET) on the following scan or the previous intent will be inherited by all ensuing scans!

Wiki

The VLBI observers for the session use a wiki to keep track of the experiment setup (frequency, recorder modes, etc). The initial wiki page for each experiment is automatically generated from the experiment schedule. Please check the wiki page for your experiment to ensure that the setup is correct. Any specific observing notes must be added to the wiki (although it is recommended to email the observers as well). Please remember that if you add too much information to the wiki it may not get read! Keep your notes concise and to the point.

Uploading Schedules

Your schedule files (.key, .vex, .sum plus any non-standard configuration files) should be uploaded to the ftp site following the instructions in the PI letter.