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VLA Raster Page
This page will only be displayed if the "Offset Mode" item on the Source
page is set to "Mosaic" or "Holography". These modes allow rasters on the
sky which avoids the approximately 20 seconds setup time for a new scan.
- Mosaic: observe a set of positions along a line. The Mosaic mode
is used to observe a series of positions along a straight line on the
RA/Dec grid. As with Fast Switching this avoids the twenty second
start-up penalty associated with individual source cards, allowing very
short scans. The times per position, the position angle of the line, and
the number and separation of the samples along that line, are specified
on this page. The source names in the resulting data file will be derived
in an obvious way from the source positions. The different positions along
the line will therefore be stored correctly as different sources in AIPS.
- Holography: staff or special use only; used to measure the antenna
surfaces and beam shapes. The options are much like those for the Mosaic
mode, but using Az/El rather than RA/Dec coordinates. All pointings
are given the same name, that given on the Source page.
IF for Beam
Which VLA IF to use to compute the VLA antenna beam size. The offsets and
spacing between sample points for the grid are specified in units of the
full-width at half-maximum of the primary beam, which is proportional to
the wavelength. Since the different IFs could conceivably be observing
at different wavelengths (e.g., LP mode) you are given the option of
specifying which IF should be used in calculating the wavelength from
which the beam size is derived.
Azimuth and Elevation
These two items give the Azimuth and Elevation offsets, expressed in
beam size units, of the beginning of the beam raster in Holography mode.
It is ignored in Mosaic mode. In Mosaic mode the starting position of
the raster is that specified on the Source page.
Position Angle
The function of this value differs between Mosaic and Holography mode.
Mosaic
Position angle of raster line, measured in degrees, where positive RA is
+x and positive Dec is +y. PA=0 --> step in increasing RA, Dec constant;
PA=90 --> step in increasing Dec, RA constant.
Holography
Position angle of raster line, measured in degrees, where positive
azimuth is +x and positive elevation is +y. PA=0 --> step in increasing
azimuth, elevation constant; PA=90 --> step in increasing elevation,
azimuth constant.
Time per sample
Time to spend on each position, in seconds. Includes slew time between
raster positions, except for the first pointing.
If this is short remember to set an even shorter integration time on the
Source page -- otherwise all your data may be flagged as "not on source"!
Number of Samples
Number of samples in this raster line; maximum= 255. The length of the
raster line is the number of samples times the sample spacing.
Spacing
Sample spacings of the raster in units of the inverse of the FWHM of
the primary beam: 2 --> 2 samples per beam (Nyquist sampling).