msuvbin(vis, field='', spw='', taql='', outvis='', phasecenter='', nx=1000, ny=1000, cell='1arcsec', ncorr=1, nchan=1, fstart='1GHz', fstep='1kHz', wproject=False, memfrac=0.5)¶
grid the visibility data onto a defined uniform grid (in the form of an ms); multiple MS's can be done onto the same grid
WARNING: This task is currently experimental.
msuvbin is a uv gridding task. It is primarily designed to be used for large volumes of data from multiple observing epochs that need to be imaged together in order to obtain a final image product for the target source or field of interest. One way of proceeding with such large multi-epoch data is to image each epoch separately and average the images afterward. Instead, msuvbin averages the visibilities on a common uv grid, then the resulting data product can be imaged using the task clean or tclean. Averaging such uv data into a common uv grid first has several conveniences and advantages, such as easily doing the proper weighted average. More details on this task can be found in the EVLA Memo 198, which explains the current implementation in msuvbin and its limitations. In particular, note the issue/limitation of creating a uv grid with wprojection and then using Cotton-Schwab major cycles to image it; see the EVLA Memo 198 for more details.
Name of input visibility file
Field name list; note that this position will define the phase center of the output uv grid
Spectral window selection
Name of output grid
Phase center of the grid, to be used when the phase center of the selected field is not the desired output phase center. Example: phasecenter=’J2000 18h03m04 -20d00m45.1’
Number of pixels along the x axis of the grid. Default: 1000
Number of pixels along the y axis of the grid. Default: 1000
Cellsize of the grid (given in sky units). Default: ‘1arcsec’
Number of correlation/polarization plane in uv grid (allowed 1, 2, 4). For example, if the input data set has the correlations RR and LL, and ncorr =1, then the output uv grid will be written as Stokes I. If ncorr= 2, then the output grid will have both the RR and LL correlations. Default: 1
Number of spectral channels in the output uv grid. Default: 1
Frequency of the first channel. Default: ‘1GHz’ (the user needs to give a useful input here)
Width of spectral channel. Default: ‘1kHz’
Do wprojection correction while gridding. Default: False
Controls how much of computer’s memory is available for gridding. Default=0.5
Let’s assume we have multi-epoch observations on a particular field of interest with measurement sets vis_1.ms, vis_2.ms, … vis_n.ms. The task msuvbin needs to be executed n times, one for each input data set with all the other parameters that define the output data set intact. For instance, for the 1st execution of msuvbin, one may set the following parameters:
vis = 'vis_1.ms' field = '0' spw = '' taql = '' outvis = "uvgrid.ms' phasecenter = '' nx = 2048 ny = 2048 cell = '2.0arcsec' ncorr = 2 nchan = 320 fstart = "1025.00MHz" fstep = "62.5kHz" wproject = False memfrac = 0.9
Here we note the following:
Field ‘0’ was selected from the input data, and its position will define the phase center of the output uv grid. If another position is desired for the phase center, then the parameter phasecenter needs to be specified.
nx and ny define the number of the pixels along the x and y axes of the grid, respectively. The size of each pixel is defined by the parameter cell. These would be the same values that one would use in the task clean/tclean for imsize and cell to image the output uv grid, and therefore need to be set by taking into account the image that one will eventually be making.
ncorr defines the number of the correlations in the output uv grid. If the input data set has the correlations RR and LL, and ncorr is set to 1, then the output uv grid will be written as Stokes I. If ncorr is set to 2, then the output grid will have both the RR and LL correlations.
nchan determines the number of channels in the output uv grid with a frequency width per channel set by the parameter fstep. The lowest frequency of the output data is set by the parameter fstart. Note that msuvbin will perform on the fly Doppler correction; the resulting grid will be in the LSRK frame. The fstart value is the starting frequency in the LSRK frame. The above example will produce a uv grid with 320 channels starting at 1025 MHz in LSRK, with each channel having a width of 62.5 kHz.
memfrac may be used to set how much memory the task should use. In the above example 90% of the available memory will be utilized by the task.
After gridding the 1st data set, the task msuvbin will need to be executed on the other data sets one at a time by changing the vis parameter only (i.e., vis=’vis_2.ms’, then vis=’vis_3.ms’, etc…) and keeping the other parameters intact. The task msuvbin will perform the proper averaging when gridding the data sets on the same uv grid. The volume of the output data set stays the same regardless of how many measurement sets are added onto the same grid.
No additional development details
- Parameter Details
Detailed descriptions of each function parameter
vis (string)- Name of input visibility file (MS)
field (string='')- Field selection of input ms
spw (string='')- Spw selection
taql (string='')- TaQl string for data selection
outvis (string='')- name of output uvgrid
phasecenter (string='')- phase center of uv grid
nx (int=1000)- Number of pixels of grid along the x-axis
ny (int=1000)- Number of pixels of grid along the y-axis
cell (string='1arcsec')- pixel cell size defined in sky dimension
ncorr (int=1)- number of correlations to store in grid
nchan (int=1)- Number of spectral channels in grid
fstart (string='1GHz')- Frequency of first spectral channel
fstep (string='1kHz')- spectral channel width
wproject (bool=False)- Do wprojection correction while gridding
memfrac (double=0.5)- Limit how much of memory to use