Antenna positions might be not accurately measured while the array is reconfigured. The uvw coordinates calculated in on-line software might be subject to significant errors which will degrade the image quality and must be corrected for.
The uvw coordinates can be fixed after the position offsets are determined or the updated antenna position file is provided.
In Miriad the task uvedit now supports SMA users to recalculate the uvw coordinates. The usages of uvedit are provided for the SMA users in the following two cases:
1. An original antenna file and antenna offsets
From the SMA log, users might be able to find the offsets from baseline-fitting program which gives the antenna position offsets usually in unit of the observing wavelength. The offsets need to be converted to in unit of millimeter to use uvedit. The original antenna file (antennas) porvided in the raw SMA data file need to be copied over.
Here is an example of SMA antenna file (050616_04:31:25):
1 4.439216e+00 -6.387564e+01 -2.183787e+01 2 -5.702343e+00 -1.898588e+01 1.561008e+01 3 -5.078151e-01 -2.515378e+01 1.279670e+00 4 5.227606e+00 -2.007746e+01 -1.484450e+01 5 -1.791900e+01 -5.955825e+01 3.006906e+01 6 0.000000e+00 0.000000e+00 0.000000e+00 7 -1.656510e+01 -2.702557e+01 3.076923e+01 8 -6.401991e+00 -6.800136e+01 3.637059e+00The first column is the antenna identification number. The next three columns are the geocentric coordinates (xyz) in meter.
Task: uvedit vis = uvtest_rx0.lsb %input vis file. source = 3c279 %the name of the source to apply the corrections to. apfile = antennas %the original antenna file. smaoffset= 1,0,1.5,0,4,2.2,0,0 %position offsets for the specific antennas; here are the case of: The antenna 1 has offsets of 0 mm in x, 1.5 mm in y, and 0 mm in z; the antenna 4 has offsets 2.2 mm in x, 0 mm in y and 0 mm in z. out = uvtst_rx0.lsb.uvw %output file with the corrected uvw. options = sma %options for sma specifications of geocentric coordinates system and the format of antenna file.
2. An updated antenna file
If an updated antenna file with corrected antenna position coordinates is provided, the usage of uvedit is straightforward:
uvedit vis=uvtest_rx0.lsb source=3c279 apfile=antennas \ out=uvcor_rx0.lsb.uvw options=sma