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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 differnt 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 (Holography
mode only)
These two items give the Azimuth and Elevation offsets, expressed in
beam size units, of the beginning of the beam raster.
Positive is increasing azimuth or elevation.
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.
Times 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).