The performance of wvrgcal The influence of self-cal

Transcription

1 The performance of wvrgcal The influence of self-cal Marcel Clemens, Astrophysics Group, Cavendish Laboratory, University of Cambridge, UK 18/9/2011 Summary Here I have done a simple test on the influence of s in the reduction of datasets to which self-calibration is applied. I have used the 4 quasar experiments in band 6 ( uid A002 X X4cd.ms taken on 2011, June 13 th ) and band 9 ( uid A002 X2979fa X9f.ms taken on 2011, August 31 st ). In the initial calibration (filed 0) was used as a calibrator for band 6 and (field 3) for band 9. The reduction of these is described in my previous 2 reports. I ran the calibration and imaging script twice for each dataset, once with s and once with corrections. The script included 2 passes of self-calibration, the first correcting only the phases and the second correcting amplitude and phase. The self-cal part of the script is given in the appendix. In order to simulate a dataset with weak sources I limited both self-cal passes to have a solution interval (solint) of 10 minutes. (For bright sources one could of course reduce this very significantly and obtain better images). The difference between the two reductions is therefore that, in the case of the s, the complex gains are corrected on timescales shorter than 10 minutes via WVRGCAL, whereas for s the gains are corrected only on 10 minute timescales. The table summarises the results of these reductions and the figures compare the resulting maps for the two cases. Only for the band 9 data can any difference be discerned in the maps. Conclusions In band 6 the S/N improvements obtained by doing the wvr corrections were: , , , The improvements are modest (2-32%). At band 9 the improvements were: , 25 48, , These reflect falls in the rms values. These are big improvements (40-92%) and suggest that s are important even at band 9 where the pwv has to be very low. This is especially true for fainter sources. In fact, for the four sources in this dataset, the improvement provided by WVRGCAL was inversely proportional to the source brightness. Though the conclusion for band 9 here is based only on a single dataset, and so should be taken cautiously, it appears that for real observations, where self-cal would often be used, and S/N will often be low, s are extremely important.

2 Table 1: Stokes I image properties after self-cal with and without s. Max and min refer to the whole image, rms refers to a large background area not including the source, omin is the minimum within the background area. Flux, a, b and PA refer to the Gaussian fit to the source by imfit. wvrgcal max min rms omin max/rms imfit Gaussian parameters flux a b PA Band no wvr ± ± ± wvr ± ± ± no wvr e ± ± ± wvr e ± ± ± no wvr e ± ± ± wvr e ± ± ± no wvr e e ± 6e ± ± wvr e e ± 6e ± ± Band no wvr ± ± ± wvr ± ± ± no wvr ± ± ± wvr ± ± ± no wvr ± ± ± wvr ± ± ± no wvr ± ± ± wvr ± ± ±

3 field 0 field 0 field 1 field 1 field 2 field 2 field 3 field 3 Figure 1: Band 9.

4 Appendix Self-calibration script with image statistics. #Previous clean was run with the default calready=true. Required for subsequent self-cal. print ---- SELF-CAL ON BAND 9 DATASET WITH WVR CORRECTIONS ---- root = wvr1 #Root name to use for all cal tables and png files. split2 = X9f wvr1 split cont.ms #Name for new ms after final split that merges channels. print ---- FIRST PASS OF PHASE ONLY SELF-CALIBRATION ---- default(gaincal) caltable = self 1.pcal #Output table gaintype = T #Average polarisations to increase S/N. calmode = p #Phases only for now. solint = 10min #Might want inf. combine = #Separate solutions for each SPW. refant = DV06 minblperant = 4 minsnr = 2 gaincal() print ---- MAKING PLOTS OF PHASE VS TIME IN THE CAL TABLE ---- #Look at the resulting table: default(plotcal) caltable = self 1.pcal xaxis = time yaxis = phase spw = antenna = iteration = antenna subplot = 441 plotrange = [0,0,-120,120] figfile = self 1 phase.png plotcal() print ---- APPLYING PHASE ONLY CAL TABLE ---- default(applycal) gaintable = self 1.pcal applycal() print ---- MAKING PHASE ONLY SELF-CAL IMAGES ---- default(clean) calready = T imagermode = csclean cell = 0.1arcsec imsize = 512 niter = 200 stokes = IQUV weighting = briggs robust = 0.0 spw = mode = mfs mask = [245, 245, 267, 267]

5 interactive = F #Change if you want to interact. for field in [ 0, 1, 2, 3 ]: field = field imagename = root+ 1pcal f +field clean() #SECOND PASS of phase only self-calibration (not used here): #Don t default it. # #caltable = self 2.pcal #Output table #solint = 2min #gaincal() print ---- AMP AND PHASE SELF-CAL ---- tget(gaincal) caltable = self ap.cal gaintable = self 1.pcal #If you have just done one iteration of phase self-cal. calmode = ap solint = 10min gaincal() print ---- MAKING PLOTS OF PHASE VS TIME IN AMP & PHASE CAL TABLE ---- default(plotcal) caltable= self ap.cal xaxis = time yaxis = phase spw = iteration = antenna plotrange = [0,0,-60,60] #This range will change for different datasets. subplot=441 figfile= self ap phase.png plotcal() print ---- MAKING PLOTS OF AMP VS TIME IN AMP & PHASE CAL TABLE ---- tget(plotcal) yaxis = amp plotrange = [] figfile = self ap amp.png plotcal() print ---- APPLYING AMP & PHASE CAL TABLES TO DATA ---- default(applycal) gaintable = [ self 1.pcal, self ap.cal ] calwt = F applycal() print ---- PLOTTING AMP VS TIME FOR CORRECTED DATA ---- default(plotms) xaxis = time yaxis = amp avgchannel = #Already continuum. ydatacolumn = corrected coloraxis = spw plotfile = selfcal time.png plotms()

6 print ---- PLOTTING AMP VS UV-DIST FOR CORRECTED DATA ---- xaxis = uvdist plotfile = selfcal uvdist.png plotms() #Could flag plenty of high baselines. Don t bother flagging. print ---- MAKING FINAL AMP & PHASE SELF-CAL IMAGES ---- tget(clean) niter = 200 interactive = F #Change if you want to interact. for field in [ 0, 1, 2, 3 ]: field = field imagename = root+ apcal f +field clean() print ---- IMAGES MADE, NOW COMPUTING STATS ---- flds = [ 0, 1, 2, 3 ] for field in flds: imname = root+ apcal f +field+.image obj = imhead(imname, mode= get, hdkey= object ) st = I gstat I = imstat(imname, stokes=st) bgstat I = imstat(imname, stokes=st, box= 20,320,490,490 ) st = Q gstat Q = imstat(imname, stokes=st) bgstat Q = imstat(imname, stokes=st, box= 20,320,490,490 ) st = U gstat U = imstat(imname, stokes=st) bgstat U = imstat(imname, stokes=st, box= 20,320,490,490 ) st = V gstat V = imstat(imname, stokes=st) bgstat V = imstat(imname, stokes=st, box= 20,320,490,490 ) print str(obj[ value ])+ \n + ST MAX MIN RMS OMIN MAX/RMS\nI +str(gstat I[ max ][0])+ + str(gstat I[ min ][0])+ +str(bgstat I[ rms ][0])+ +str(bgstat I[ min ][0])+ +str(gstat I[ max ][0]/bgstat I[ rms ][0]) + \n + Q +str(gstat Q[ max ][0])+ +str(gstat Q[ min ][0])+ +str(bgstat Q[ rms ][0])+ +str(bgstat Q[ min ][0])+ +str(gstat Q[ max ][0]/bgstat Q[ rms ][0]) + \n + U +str(gstat U[ max ][0])+ +str(gstat U[ min ][0])+ +str(bgstat U[ rms ][0])+ +str(bgstat U[ min ][0])+ +str(gstat U[ max ][0]/bgstat U[ rms ][0]) + \n + V +str(gstat V[ max ][0])+ +str(gstat V[ min ][0])+ +str(bgstat V[ rms ][0])+ +str(bgstat V[ min ][0])+ +str(gstat V[ max ][0]/bgstat V[ rms ][0]) print ---- EXPORTING FITS (I,Q,U,V) IMAGES ---- default(exportfits) for field in flds: imname = root+ apcal f +field+.image exportfits(imagename=imname, fitsimage=imname+.fits ) print ---- RUNNING IMFIT ON STOKES I SELF-CAL IMAGES ---- default(imfit) for field in flds: imname = root f +field.image print imname fit vals = imfit(imname, box = 243,243,269,269, stokes = I ) flx = fit vals[ results ][ component0 ][ flux ] shp = fit vals[ results ][ component0 ][ shape ] print Flux: + str(round(flx[ value ][0],4))+ +/ +str(round(flx[ error ][0],5)) + \n +

7 Major axis FWHM: + str(round(shp[ majoraxis ][ value ],3))+ +/ +str(round(shp[ majoraxiserror ][ value ],4)) + \n + Minor axis FWHM: +str(round(shp[ minoraxis ][ value ],3))+ +/ +str(round(shp[ minoraxiserror ][ value ],4)) + \n + PA: + str(round(shp[ positionangle ][ value ],1))