Mosaic

The ATCA offers an observing mode which allows you to move pointing centres as frequently as every integration cycle. The facility is usually used to map a wide field by mosaicing several smaller fields. Mosaicing mode may also be useful for observations of large numbers of nearby sources, as observing overheads are reduced. Other applications include holography and source surveys using one dimensional cuts. In interferometry adjacent pointings are not independent and so we can get fundamentally better images by processing the different pointings together. Necessary information about the pointing grid pattern to use for proper Nyquist sampling and the time to be spent on each position to optimise tangential $uv$ coverage can be found in Chapter 21 of the Miriad Users Guide.

For close-packed mosaicing (pointing centres every half-beamwidth), drive times are determined by the acceleration limit of 800°/min$^2$. Half the drive time is spent accelerating and half decelerating. For drive times much further than 108' in azimuth or 27' in elevation, the drive times are dominated by the slew rate limits of 38°/min in elevation and 19°/min in azimuth. For example, a 16' drive for a 20cm mosaic will take, depending on the elevation and azimuth of the source, approximately 2.2 seconds. Data taken during the off-source period is blanked in the correlator using a predicted drive time. As drive times can be a significant overhead, care should be taken to not move the array more than necessary.

Mosaic observations use a standard schedule file and an additional file, called the mosaic file, which lists the field centre positions. You can specify up to 500 field centres. The schedule file is like a usual ATCASCHED file except that each scan which uses mosaic mode must:

• have sctype = MOSAIC
• have a source name ($\leq~12$ characters) (i.e., source = sourcenam) which is the same as the name of the mosaic file (sourcenam.mos).
• specify the mosaic file reference position in the RA and DEC fields
• specify the minimum time to be spent in mosaic mode (using the scan field). The scan time must be greater than three integration cycles. The actual time, if the start n/m command is used, is m times the total time specified in the mosaic file.
• the schedule file must be in the $ATCA_SCHED directory and the mosaic file must be in mosaic on XBONES.

The mosaic file, sourcenam.mos, must consist of one line per pointing centre in the mosaic pattern. Lines with the # character in the first column 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 +1hr 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.

The Compact Array observes the pointing centres listed in the mosaic file repeatedly until the time specified in the Scan field of the schedule file expires. The sequence will always be executed an integral number of times. So, regardless of the scan length, the final mosaic cycle will be completed. The observer may find it useful to use the /obs page of CAMON to follow the observations.

When observing in mosaic mode, you need to ensure that the HOLD INCREMENT is set to 512ms. Check this on the CAMON /status page. If it is not, use the program CAIN to set the HOLD INCREMENT time to 512ms before your observations. This will cause the system to exclude off-source data from the start of the first integration cycle for each pointing.

Invoke mosaic mode in CAOBS with the command: CAOBS$>$start n,k/m where n is the scan number, k is the starting position in the mosaic file and m is the number of times to repeat the whole process.

As an example, the ATCASCHED fields and mosaic file are shown for a four pointing centre pattern on a 0.25° grid. Note that the RA offsets are coordinate increments not sky distances. If the CA's integration time has been set to 15 seconds, then the four pointing centres will be observed every 120 seconds and the pattern repeated 15 times during the scan.

SCHED fields:	Source:	mymap	Observer:	Smith	Project_id: C555
	RA:	05:21:33
	Dec:	-65:56:41
	Epoch:	J2000
	Scan length:	00:30:00
	Envflg:	0
	Sctype:	MOSAIC
	Config file:	null

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