View source for Notes on EVLA OSRO continuum data sets

[[Category:EVLA]][[Category:Imaging]][[Category:Deconvolution]][[Category:Continuum]]
Calibrating and imaging EVLA continuum data really isn't much different from the strategies we used with the VLA--so check out [[Calibrating_a_VLA_5_GHz_continuum_survey | this tutorial]]. The one big difference is that we've increased the number of channels from one (in VLA continuum mode) to 128 (in OSRO mode 1). That means you'll have to do a [[Calibrating_an_EVLA_OSRO_HI_data_set#Calibrate_the_Bandpasses | bandpass calibration]], just as you would for a spectral line data set. You'll also have to tell [[clean]] to do multi-frequency synthesis imaging. Easy enough---the defaults in [[clean]] are set up for this:
<pre>
#  clean :: Invert and deconvolve images with selected algorithm
vis                 =         ''        #  Name of input visibility file
imagename           =       ['']        #  Pre-name of output images
outlierfile         =         ''        #  Text file with image names, sizes, centers for outliers
field               =         ''        #  Field Name or id
spw                 =         ''        #  Spectral windows e.g. '0~3', '' is all
selectdata          =      False        #  Other data selection parameters
mode                =      'mfs'        #  Spectral gridding type (mfs, channel, velocity, frequency)
     nterms         =          1        #  Number of terms used to model the sky frequency dependence
     reffreq        =         ''        #  Reference frequency for MFS (relevant only if nterms > 1)

gridmode            =         ''        #  Gridding kernel for FFT-based transforms, default='' None
niter               =        500        #  Maximum number of iterations
gain                =        0.1        #  Loop gain for cleaning
threshold           =   '0.0mJy'        #  Flux level to stop cleaning, must include units: '1.0mJy'
psfmode             =    'clark'        #  Method of PSF calculation to use during minor cycles
imagermode          =         ''        #  Options: 'csclean' or 'mosaic', '', uses psfmode
multiscale          =         []        #  Deconvolution scales (pixels); [] = standard clean
interactive         =      False        #  Use interactive clean (with GUI viewer)
mask                =         []        #  Cleanbox(es), mask image(s), and/or mask region(s)
imsize              = [256, 256]        #  x and y image size in pixels. Single value: same for both
cell                = ['1.0arcsec']     #  x & y cell size(s). Default unit arcsec.
phasecenter         =         ''        #  Image center: direction or field index
restfreq            =         ''        #  Rest frequency to assign to image (see help)
stokes              =        'I'        #  Stokes params to image (eg I,IV, QU,IQUV)
weighting           =  'natural'        #  Weighting of uv (natural, uniform, briggs, ...)
uvtaper             =      False        #  Apply additional uv tapering of visibilities
modelimage          =         ''        #  Name of model image(s) to initialize cleaning
restoringbeam       =       ['']        #  Output Gaussian restoring beam for CLEAN image
pbcor               =      False        #  Output primary beam-corrected image
minpb               =        0.2        #  Minimum PB level to use
calready            =       True        #  True required for self-calibration
async               =      False        #  If true the taskname must be started using clean(...)
</pre>
Setting '''mode=''' 'mfs' will use all channels in your data set to make an image. If you'd only like to use a subset of channels for the image, set '''spw'''. 

The '''nterms''' and '''reffreq''' sub-parameters under '''mode=''' 'mfs' improve synthesis across wide fractional bandwidths. If '''nterms'''> 1, [[clean]] will fit a polynomial of order '''nterms''' to model flux density as a function of frequency for each source in the imaging field (using an algorithm developed by U. Rau). For example, imagine a data set with two spectral windows: one at 1300 MHz and one at 1800 MHz. The difference between these two frequencies is a significant fraction of the frequencies themselves, therefore implying that the sources' flux densities, the synthesized beam, and the primary beam will all vary significantly between the two spectral windows. In this case, imaging can be greatly improved by setting '''nterms='''2 (or perhaps even higher), so that clean will fit a line to each source's spectrum. The image that is produced by [[clean]] will represent the sky at frequency '''reffreq'''---set this to be somewhere in the middle of the two observed frequencies (right now, [[clean]] seems to run more stably if '''reffreq''' is at the low end, closer to 1300 MHz in our example). 
<div style="background-color: #dddddd;">
'''Warning:''' The implementation of multi-frequency synthesis imaging with '''nterms'''> 1 is still rather preliminary in [[clean]], so use caution. It does not seem to work with interactive imaging at the moment, so be sure to set '''interactive='''False.
</div>


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--[[User:Lchomiuk|Laura Chomiuk]] 21:29, 18 March 2010 (UTC)