| CSIRO Australia Telescope National Facility ATCA Users Guide |
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Observations on the compact array normally consist of a sequence of scans: a scan is a short period of observing where a single source is observed. A complete observation is made up of a number of scans. Observations typically alternate between program source scans and calibrator scans. Details of scans are kept in a schedule file.
The main observing task, CAOBS, reads the schedule file in order to determine what sources are to be observed, for how long, and in what order. The schedule file also defines the frequencies, integration averaging unit and correlator setup to use.
The ATCA web scheduler, located at http://www.narrabri.atnf.csiro.au/observing/sched/cabb/, is the tool to use to create ATCA schedule files in the CABB era. Schedules made with the old SCHED program are no longer understood by the CABB version of CAOBS and thus cannot be used. However, it is possible for the web scheduler to read in old schedules and save them for use with the new system. Please also note that an active ATNF username and password is required to gain access to the web scheduler page.
You should prepare at least two schedule files:
For observations of many sources, use the ATMOS program in MIRIAD to solve the “travelling salesman” problem and optimise the order in which the sources are observed.
With synthesis instruments, it is possible for system errors to lead to artefacts at the centre of the field. For experiments that are intolerant to such artefacts (such as detection experiments), displace the source position a few synthesised beamwidths from the field centre. Note that ATCA software never checks to see how close the telescope is pointing to the sun!
COORD is a useful program that calculates the altitude, azimuth, and rising and setting times of sources. Currently, this is most easily accessed via the Parkes website, http://www.parkes.atnf.csiro.au/cgi-bin/utilities/coord.cgi (though be sure to change from the default values when using the ATCA, i.e., change the site to Australia Telescope Compact Array, and change the elevation limit to 12 degrees for cm observing).
A quick estimate of how long a particular source will be observable for on the ATCA can be made with the table below. It shows, for a source at a particular declination, the LST range before/past zenith it is above 20^\circ elevation. For example, for a source with coordinates RA=08:25:26.869, Dec=-50:10:38.4 (the coordinates for 0823-500), its rise time will be 02:03 LST (take 06:22 from 08:25), and its set time will be 14:47 LST (add 06:22 to 08:25).
| Declination | LST range |
| -80 | always above horizon |
| -70 | 07:54 |
| -60 | 06:56 |
| -50 | 06:22 |
| -40 | 05:57 |
| -30 | 05:35 |
| -20 | 05:14 |
| -10 | 04:53 |
| +0 | 04:29 |
| +10 | 04:01 |
| +20 | 03:26 |
| +30 | 02:34 |
| +40 | 00:24 |
Figure 2.1: Wrap limits for the Compact Array and Mopra antenna.
The wrap limits for any antenna in the ATCA are CCW=152.5^\circ and CW=327.5^\circ as shown in Figure wraplimits.
The CABB web scheduler, located at http://www.narrabri.atnf.csiro.au/observing/sched/cabb/ is used to prepare schedule files. The web scheduler checks the schedule for completeness and, where possible, checks choices such as observing frequency for hardware compatibility. Drive times, and source azimuths and elevations are computed automatically.
Three steps are involved in using the web scheduler:
When constructing a schedule, consider:
The web scheduler can produce a list to help answer these questions.
It is a good idea to study these listings very
carefully before observing commences. The scheduler considers drive times:
all coordinate entries are automatically followed by a calculation of the
azimuth and elevation of the source, as well as the drive time from
the previous source, using the entered value for LST. When entering
a schedule file, the integration times are specified
for each source (and calibrator source). Note that,
except in the case of mosaic schedules and DWELL scantypes,
these integration times
include drive times, so the specified time must be somewhat longer
than the amount of integration on source required. This is
particularly important for secondary calibrators, when the drive time
(up to a minute or so) may be comparable to the integration time
(about two to three minutes). Alternatively, the sctype field can
be set to DWELL to ensure a source or calibrator
is tracked for the specified scan duration.
The schedule file submitted to CAOBS (the online Compact Array control
task) need have no specific start time. Rather, the schedule file asks
CAOBS to start the first scan as soon as possible. The sequence of
scans will be exactly as specified in the schedule file. However, when
using the web scheduler, the calculation of azimuths, elevations and drive times
is possible only if the starting sidereal time is known (at least
approximately). The sidereal time for the first scan in the schedule
can be set in the web scheduler by giving the time in the Time field,
and setting TimeCode to LST. Alternatively, the starting UT
may be specified by setting TimeCode to UT, and by setting
the Date to the appropriate start date by clicking on it and using
the calendar that appears. Note that when viewing any scan other than the
first in the schedule, the fields Time, TimeCode and
Date all become read-only.
The environment flag can be used to instruct the online software to remember certain settings, such as attenuators, and to recall them whenever a scan with the same environment flag is encountered. However, this is generally not required, as the online software will remember user settings, and recall them when the same conditions are encountered again. There are at least 128 slots for user settings, so for a normal schedule, the environment flag will probably not need to be used.
In this section a simple schedule file is constructed. It will contain three sources: a program source and two calibration sources. The following observing sequence will be specified:
These sources will be observed at the standard C/X band frequencies of 5500/8800 MHz. To create this schedule file, perform the following steps:
| Navigate to http://www.narrabri.atnf.csiro.au/observing/sched/cabb/ in a web browser | The first screen is displayed. |
Change the Source field to test | |
Change the RA field to 05:21:35.5 and the Dec
field to -21:24:27.1 | The delimiter for coordinates must
be a colon, and the default Epoch of J2000 should only be
changed if necessary. |
Change ScanLength to 00:30:00 | |
Change ScanType to Dwell | This ensures that 30 minutes are spent on source, regardless of slewing times. |
Ensure Pointing is set to Global | Offset pointing is usually not required for low-frequency observations. |
Leave the Averaging, Environment, PointingOffset1
and PointingOffset2 fields with their default values | |
Change the Observer field to Jansky, the Project
code to C123 and the Config field to null | These fields should be set in the first source - they will propagate to the other sources as they are added |
Change the Time field to 02:00:00, and the
TimeCode field to LST | This indicates that the schedule will be started at an LST of 2h |
Click the Freq1 Setup bar | This brings up the frequency selection fields |
Enter 5500 in the Continuum frequency box, and
keep the Chn. BW selection box to 1 MHz. | The velocity resolution should be calculated to be 54 km/s, and the velocity range will be 111631 km/s |
Click the Freq2 Setup bar | This brings up the frequency selection fields |
Enter 8800 in the Continuum frequency box, and
keep the Chn. BW selection box to 1 MHz. | The velocity resolution should be calculated to be 34 km/s, and the velocity range will be 69769 km/s. |
Click the Scan Parameters bar | This brings up the scan details |
Click the Search Cal button | This presents a list of nearby calibrators to the right of the scan details, sorted by distance to the source. The closest strong (> 1 Jy) source to the program source is shown to be 0511-220. |
Click on the 0511-220 button | The details for this source are displayed from the calibrator database below the source list. This source is a suitable phase calibrator as it is strong and has only a small defect. |
Click on the 0511-220 button | Clicking on the button twice (without clicking another source button between clicks) inserts this source into the schedule above the currently selected source. |
Select source 1, and change the ScanLength field
to 00:05:00 | |
To copy this source to the end of the schedule, click on the
Edit menu and select Copy, then select the test
source, click on Edit and select Paste. | The schedule should now have three sources (in order): 0511-220, test, 0511-220. The first source needs to be changed to 0537-441. |
| Select source 1 | |
Change the Source field to 0537-441 and press TAB | The scheduler will automatically put the source coordinates for
this known calibrator in the RA and Dec fields |
Click on the File menu and select Save As | The Schedule file selector window will appear |
The schedule will be called c123_test.sch, so enter this
into the Filter box to ensure that there is not already a
schedule with this name | If there is another schedule with the desired name, consider changing it |
Enter c123_test.sch into the Schedule box and
press the Save button | The file selection window will disappear, and the name of the schedule should now appear at the top of the window. |
To produce a listing of the schedule, in order to check that the
schedule proceeds as expected, click on the Listing tab.
This section describes the actions of all the buttons found in the web scheduler, as well as the purpose of all the scheduler fields. It is current as of 12 May 2009.
SourceThe name of the source. If the name of a known calibrator is entered
into this field, the scheduler will automatically fill the RA,
Dec, Epoch and CalCode fields. The information
in this field will be used by the correlator as the name of the source
in the output FITS files. If observing a mosaic, this field should be
set to the name of the mosaic file describing the mosaic pointings,
without the .mos extension.
RA & DecThese fields define where on the sky you want the array to point,
ie. the phase centre. You must specify RA in hours, minutes and
seconds separated by colons (:), and you must specify Dec in degrees,
minutes and seconds separated by colons (:). The RA and Dec will be
in the system defined by the Epoch field, thus they can also
be used to represent Azimuth and Elevation.
EpochThis defines the coordinate system used for the RA and
Dec fields. Three epochs are recognised:
CalCodeA flag used to specify if a certain source is used as a calibrator. It
should either be C for a calibrator, or blank for a non-calibrator.
For baseline solutions, the CalCode is set to B, a feature
that should not be used by normal observers. When a calibrator is selected
from the Search Cal screens, or matched from input to the
Source field, this code will automatically be set to C.
The CalCode field is rest to blank after a new scan is added
to avoid inadvertantly calling a target source scan a calibrator scan.
The ASSISTANCE program makes use of the CalCode field to
determine the level of on-line checking that is done. If a calibrator
scan is indicated, the checks (for stable amplitude, small delays,
closure phase, etc.) are more stringent. If a calibrator scan is not
labelled with CalCode=C then these checks will not be performed.
On the other hand, most target sources have structure and
insufficient flux, so
ASSISTANCE would sound alarms continuously if a source like that
was marked as a calibrator scan.
ScanLengthThis indicates the length of time to spend on the
current scan. It is specified in time format and should be specified
as HH:MM:SS, where HH,MM,SS is the number of hours, minutes and seconds
to spend on the source respectively.
Unless the ScanType is set to Dwell, the scan time includes the
time to drive to the source. The scan must be at least four integration cycles long.
ScanTypeThis field specifies special scan characteristics:
Normal: the scan will go for the length of time specified
by ScanType, inclusive of all slewing/setup time to get on source.
Dwell: the scan will go for the length of time specified
by ScanType, in addition to any slewing/setup time incurred while
moving to the source.
Mosaic: this must be selected to indicate that this scan is a
mosaic scan, and that the Source field should be interpreted as
the name of a mosaic file.
Point: indicates that this scan should be executed as a
pointing pattern, which can be used to determine offsets for each antenna.
This may be used for global pointing solutions as performed by local staff,
or for reference pointing during observations.
Paddle: indicates that the paddle should be inserted during
3mm observations; this scan type will have no effect during observations
at other bands.
Closefile: placing a scan with this scan type in a schedule
causes the correlator to close the current file when it reaches this
scan. No other scan parameters are interpreted when this scan type is
specified. This is useful for keeping file sizes small, or for separating
files based on the number of times the schedule has looped. The correlator
will automatically start a new file when the next scan begins.
Note that for any of the scan types, if the time taken to get on source
is longer than the time specified in ScanLength, the scan will
be skipped by the online system. For that reason, it is advisable to set
the ScanLength of scans such as Point and Paddle
to be much longer than the scan will actually take.
PointingThis field specifies the pointing mode of the antenna. Options are:
Global: the default pointing mode. It uses a global, all-sky
pointing model determined at each reconfiguration.
Offset: use the offsets determined on a nearby calibrator
to improve the pointing locally (near the calibrator) and for a limited
time (offsets are time variable).
Offpnt: use the PointingOffset1 and
PointingOffset2 values to shift the pointing away from the
specified RA & Dec.
Refpnt: determine new local pointing offsets by executing
a pointing pattern on a calibrator. Using this Pointing mode
causes the telescope to be put into global pointing mode before the new
offsets are determined. This option only makes sense when
combined with a ScanType of Point, and this should only
be done while observing a calibrator.
Update: determine new local pointing offsets by executing
a pointing pattern on a calibrator. Using this Pointing mode
causes the newly determined offsets to be added to the previously
determined pointing solution. This option only makes sense when
combined with a ScanType of Point, and this should only
be done while observing a calibrator.
AveragingThis parameter specifies the number of cycles to average together when writing the data out to an RPFITS file on disk. Averaging will restart on source and frequency changes.
EnvironmentSetting this field requests CAOBS to record the attenuator and
sampler settings assocated with each different field value, and recall
those settings whenever a scan is encountered with the same value.
Each frequency setting (modulo 1 MHz) allows for up to 128 such values.
Valid Environment values are from 0 to 127.
PointingOffset1 & PointingOffset2Use these fields and the Offpnt Pointing type to
make the telescope point away from the specified RA & Dec.
PointingOffset1 is the offset in seconds to move in RA,
and PointingOffset2 is the offset in arcseconds to move in
Dec.
ObserverThis is an ASCII string up to 12 characters long that is used to identify who made the observations. It is not critical to any on-line system and is merely a label for reference.
ProjectThis is the project identification number (nnn) allocated to
observing projects, and should be specified here as Cnnn. The
Project field value is used by CAOBS to name the extension
of the output RPFITS files. It is important that it is set correctly
as it is used for record keeping and by the ATNF Online Archive.
The default C999 code is used for system testing and test files;
such files will not be considered as valuable when it comes to deleting
data, and will not appear in the archive.
ConfigThe value of this field will (eventually) be used to configure the
correlator. Currently, there is only one correlator configuration, and
this field should be left blank, or given the default value of null.
TimeThe value of this field can only be set for source 1 in the schedule, and is used as the start time of the schedule. Note that this does not mean that CAOBS will start the schedule only at this time. Instead, this start time is used by the scheduler when computing whether each source is above the horizon, and when calculating drive times.
TimeCodeThe selection made here sets the type of time desired for the
Time field above. It can be either local sidereal time (LST) or
coordinated universal time (UTC). This field can only set for source
1 in the schedule.
DateThe observing date can be set here to allow the scheduler to correctly determine the source positions and drive times. Of course, the date does not matter when setting LST.
TVChanWhen switching between different observing frequencies, the usable range
of correlator channels will also change, due to RFI or bandpass shape.
The correlator uses only a certain subset of channels – called the TV
channels – to determine system temperatures, and if the TV channels are
not kept constant for each individual frequency config then you may
encounter amplitude problems later in the reduction process. The TV
channels setting also affects what is displayed by VIS. This parameter
allows you to set the TV channels on a per scan basis, which makes
frequency switching a far more automated process. This parameter should
be either null to allow the current TV channel setting to be
left as-is, or a set of four numbers, separated by commas, indicating (in
order) the first and last TV channel for IF 1, and the first and last TV
channel for IF 2.
Continuum (MHz)Enter the central frequency (in MHz) of the 2 GHz wideband IF here. There is
one of these fields in each of the Freq1 and Freq2
tabs, one for each available CABB wideband IF.
Chn. BW (MHz)Select how wide (in MHz) each wideband channel will be for each IF.
The channel width will also set the minimum bandwidth of each zoom band
derived from this IF. There is one of these fields in each of the
Freq1 and Freq2 tabs, one for each available CABB wideband
IF.
LineEnter a frequency (in MHz) that will appear within the zoom band. This
will then fix the wideband channel that must be used. This frequency
must be within 1 GHz of the frequency specified in the
Continuum (MHz) field. There is currently a maximum of 4 such bands
that can be specified in the scheduler for each IF.
ChannelEnter the wideband channel number to use for the zoom band. This will then restrict the frequencies available in the zoom band.
WidthThis is the number of wideband channels to use for this zoom band.
Note that a maximum of 4 wideband channels can be used for all zoom
bands, so some of the Width boxes will be disabled if more than
1 wideband channel is assigned to any particular zoom band.
The following buttons are available on the main scheduler page.
New ScanThis button duplicates the currently selected scan and inserts it
as the next scan in the schedule, and moving all subsequent scans
down by one. The duplicated scan will however have its CalCode
reset.
DeleteThis button deletes the currently selected scan, and moves all subsequent scans up by one.
Search CalThis button initiates a search through the ATCA calibrator database for calibrators near the position of the currently selected scan. The results of this search are displayed to the right of the scan parameters, and are sorted by distance to the selected scan’s position. Each calibrator returned by this search can be viewed in detail by clicking the button next to the calibrator’s name, and it can be inserted at the currently selected position in the schedule by clicking on the button twice in succession.
Pick SourceClick this button to load in a locally-hosted file containing sources that will be used in the schedule. After clicking on this button, a new frame will appear next to this button allowing the selection of a locally-hosted file with the format:
source_name hh:mm:ss.s dd:mm:ss.s ...
This file may have any number of sources, with one source listed per
line, along with its RA (in hh:mm:ss.s format), and Dec
(in dd:mm:ss.s format), separated by spaces. Click the Submit
button to load in this file. After the file is loaded, a Source
Selection frame will appear to the right of the Scan Parameters,
and each source will be listed, with buttons for each source in the
file. Clicking on the button labelled with the source name will insert
that source into the schedule at the currently selected scan number,
and move all subsequent scans down by one. Clicking the AddAll
button will add all the scans to the schedule, in order, at the
currently selected scan number, and will move all subsequent scans
down by the number of sources in the Source Selection frame. The
sources that are added will have the same scan parameters and frequencies
that the currently selected source has.
SchedSelecting this tab will show the schedule editing functions.
ListingSelecting this tab will generate a time-ordered summary of scans from the current schedule. It is most useful for checking the validity and correctness of the schedule.
The scheduler has three drop-down menus, which are described below.
FileThe File menu has the following items.
OpenOpens a pre-existing schedule from the server.
SaveSaves the schedule with the name that it already has. If the schedule does not currently have a name, this action will not work. This action saves the schedule to the server, ready for loading in CAOBS.
SaveAsAllows the schedule to be saved with a different name than it currently has. If the schedule does not currently have a name, this action is the only way to save it to the server.
CloseCloses the current schedule and resets the interface back to an empty schedule with 1 generic scan and no schedule name.
EditThe Edit menu has the following items.
CutRemoves the currently selected scan from its position in the schedule,
but keeps it in memory so that it can be inserted elsewhere. Multiple
scans cannot be selected, and therefore cannot be Cut at the
same time.
CopyCopies the currently selected scan into memory so that it can be
inserted elsewhere. Multiple scans cannot be selected, and therefor
cannot be Copy-ed at the same time.
PasteCopies the scan information held in memory from a prior Cut
or Copy action back into the schedule, as a scan after the
currently selected scan. This moves all subsequent scans down by one.
It is not possible to Paste a scan into position 1 in the
schedule.
ToolsThe Tools menu has the following items.
Recalc freq/veloGoes through the schedule and recomputes all frequency and velocity information present. This is useful to execute before examining the listing, to ensure that all the zoom mode information is correct.
Global changeAllows a change in one scan to be duplicated in all other scans. The
scan to change should be selected first, followed by the
Global change menu item. While the global change mode is
activated, it is not possible to change the selected scan. Make as
many changes to the scan as is necessary, in any of the
Scan Parameters, Freq1 and Freq2 tabs, and then
select either the Global Apply button to confirm the changes
and duplicate them in the other scans, or the Global Cancel
button to keep the changes but do not duplicate them.
The third
button, Global Apply Matching is used to duplicate the changes
only to other scans that initially matched the scan being changed. An
example of this is for a schedule that has multiple frequencies in
a single IF. Consider a schedule that has scans with IF1 frequencies of
5500, 5700 and 8600 MHz. If a scan with an IF1 frequency of 8600 MHz
is changed to 8700 MHz, and the Global Apply button is used, then
all scans in the schedule will be changed to have an IF1 frequency of
8700 MHz. However, if the Global Apply Matching button is used,
then only scans that have an IF1 frequency of 8600 MHz, like the selected
scan, will be changed.
It is also important to note that if nothing changes, then using the
Global Apply button will not do anything. For example, if a scan
is changed individually from Project=C123 to Project=C124,
and this change needs to propagate to all scans, then simply deleting
the entry in this scan and replacing it with the same thing while in
Global change mode will not work, as it will still appear as though
the entry has not changed. Instead, move to another scan where the
Project code has not changed before entering Global change
mode.
Load local catalogThis menu item is used in conjunction with the Pick Source
button. If the Pick Source button is used before a local catalog
is loaded, then a frame allowing a catalog is shown. However, after this
catalog is loaded, subsequent selections of the Pick Source
button will display only the Source Selection frame. To show the
catalog loading frame again, so that a different catalog can be loaded,
use the Load local catalog menu item.