| CSIRO Australia Telescope National Facility ATCA Users Guide |
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Mosaic observations use a standard schedule file and an additional file,
called the mosaic file, which lists the positions of the mosaic
field centres. Up to 500 fields can be specified in a single file.
This mosaic file must consist of one line per pointing centre in the
mosaic pattern. Lines beginning with the # character
are ignored and can be used for comments.
The general mosaic file format is:
# Comments .... ignored by the software
#
d(RA) | d(DEC) | INT | $FIELD_1 |
d(RA) | d(DEC) | INT | $FIELD_2 |
d(RA) | d(DEC) | INT | $FIELD_3 |
d(RA) | d(DEC) | INT | $FIELD_4 |
The items in each line are: d(RA) is the offset Right Ascension
of the pointing centre in “degrees of polar rotation” from the
reference position. For example, d(RA)~=~+15.00 will move +1 hr in RA at
all declinations. d(DEC) is the offset declination of the
pointing centre in degrees from the reference position. eg
d(DEC)~=~+1.000 will move the pointing centre one degree north. INT
is the integral number of integration cycles to spend on each
pointing centre. The ‘$’ is a prefix to the field name, and indicates that scans
will follow in the order as they are listed in the mosaic file.
The name of the field, $FIELD_n, must be different for each
pointing centre and should be \leq 9 characters (naturally, $ can not be
used as a first character of the field name). Note: The UVSPLIT
task in MIRIAD needs the field names to be different by ‘_n’,
where n is an integer. This file should be named mosnam.mos,
where mosnam should be a short name that describes the mosaic.
To include a mosaic in a schedule file, insert a scan and set:
Source to the name of the mosaic file, excluding the
.mos extension, ie. mosnam
ScanType to Mosaic
RA and Dec to the reference position for the
mosaic, ie. the position that all the mosaic field centres are
offset from
ScanLength to the minimum time to be spent doing the mosaic,
and must be greater than three integration cycles
All other scan settings have their usual effect, including the
Pointing mode, and the observing frequencies.
The mosaic file must be saved in the /atca/mosaic directory
on XBONES so that CAOBS can locate it during the observations.
The mosaic file name should be completely in lowercase letters, even
if the corresponding source name in the schedule has uppercase letters,
as CAOBS will report that the mosaic file could not be found unless
it has an all-lowercase file name.
The mosaic scan will be observed repeatedly until the time specified
by ScanLength has elapsed, after which the mosaic scan will
continue until the last specified field centre is next observed. In this
way, the mosaic sequence will always be executed an integral number
of times, regardless of ScanLength.
The following example shows a mosaic file specifying a four pointing centre pattern on a 0.25^\circ grid. Note that the RA offsets are coordinate increments not sky distances.
| Mosaic file: mymap.mos | # mymap.mos by P. Smith |
| # A four pointing centre mosaic | |
| # Ref. Position = 05:21:33, -65:56:41 |
| 0.0000 | 0.000 | 2 | $mymap_1 | |
| 0.6133 | 0.000 | 2 | $mymap_2 | |
| 0.6133 | -0.250 | 2 | $mymap_3 | |
| 0.0000 | -0.250 | 2 | $mymap_4 |
The scan in the corresponding schedule file might have the parameters:
| Scheduler fields: | Source: | mymap | Observer: | Smith | Project: | C555 |
RA: | 05:21:33 | |||||
Dec: | -65:56:41 | |||||
Epoch: | J2000 | |||||
ScanLength: | 00:30:00 | |||||
ScanType: | Mosaic | |||||
Pointing: | Global |
With these settings, and with the default integration time of 10 seconds, the four pointing centres will be observed every 80 seconds, and the pattern will be repeated 23 times during the scan.