The performance of wvrgcal The 4 quasar, band 6 experiment, uid A002 X X4cd.ms

Transcription

1 The performance of wvrgcal The 4 quasar, band 6 experiment, uid A002 X X4cd.ms Marcel Clemens, Astrophysics Group, Cavendish Laboratory, University of Cambridge, UK 18/9/2011 Summary This document reports on an analysis of the performance of the wvrgcal program and focuses on the effects of performing wvr correction when wvr data is not available for some antennas. This is done simply by using the wvrflag option in the call to wvrgcal. In this case the program uses wvr data from neighbouring antennas to derive a correction via interpolation. A test is also made on the effect of not using the tie option to wvrgcal. The results are quantified directly with the resulting images via both image statistics and Gaussian fits to the source using imfit. The dataset used was the quasar experiment uid A002 X X4cd.ms taken on 2011, June 13 th in band 6 (230 GHz). There were 14 antennas in the array but 5 are flagged so that the final maps contain data from only 9 (see fig. 1). Note that antenna DV01 was probably unecessarily flagged (see below). The brightest source, , was used as a calibrator for the other 3 (phase, amplitude and bandpass). There is no absolute flux calibrator in the dataset so fluxes are relative to that has an assumed flux of 1 Jy. The calibration process therefore lacks the setjy and fluxscale steps that would normally be used. Antenna DV02 was used as the reference antenna. I referred to both Ed Fomalont s script on a similar dataset and T. Sawada s report on the same dataset, but also to the casa guides on the NRAO web pages. Data reduction The script used to calibrate, image and quantify these data is given in the appendix. Here is a summary of the procedure. Antenna position corrections were applied using gencal using the corrections reported in T. Sawada s report (I did not re-derive these). s were then optionally applied. The antenna position corrections and tables were then applied ( applycal ) and the 4 science spws were split out. A priori flagging commands were then applied to the split dataset (autocorrelations, shadowing, quack and end spectral channels). A little further flagging was also applied in which the XX correlation for antenna DV01 was flagged. This effectively flags tha antenna becasue gaincal requires both polarisations in order to find a solution at present. This could be improved by flagging only spw 2 in XX. This is unlikely to change any of the conclusions reached here. A bandpass calibration table was then made using after correcting its phase variations as a function of time. Gaincal was then run on in a similar to the casa guide on TWHydra: ie. first calculate phases only for each integration time, then for each scan. Then solve also for amplitudes applying the integration time table on-the-fly for the calibrator and the scan time table on-the-fly for the other sources. Applycal was then run, to apply the bandpass, phase and amplitude calibration tables and the dataset split merging all spws and channels to create a continuum dataset for imaging. Stokes I,Q,U and V, pixel images were made with 0.25 arcsec pixels. Weighting was Briggs with robust=0, and 100 iterations of clean were used with an 18 pixel square clean box centred on the source. For every version of the script uniformly weighted maps were also made (robust = -2) in order to give more weight to long baselines. The script was run several times with different calls to wvrgcal, and also without wvrgcal, as a benchmark. The wvrgcal calls were as follows: wvrgcal not called. No flagged antennas: wvrgcal --ms uid A002 X X4cd.ms --output uid A002 X X4cd.W --toffset -1 --segsource --statsource tie , , ,

2 An antenna with close neighbours (DV06) flagged: wvrgcal --ms uid A002 X X4cd.ms --output uid A002 X X4cd.W --toffset -1 --segsource --statsource tie , wvrflag DV06 A rather isolated antenna (DV08) flagged: wvrgcal --ms uid A002 X X4cd.ms --output uid A002 X X4cd.W --toffset -1 --segsource --statsource tie , wvrflag DV08 3 flagged antennas (DV06, DV08, DV10): wvrgcal --ms uid A002 X X4cd.ms --output uid A002 X X4cd.W --toffset -1 --segsource --statsource tie , wvrflag DV06 --wvrflag DV08 --wvrflag DV10 A very central antenna (DV10) flagged: wvrgcal --ms uid A002 X X4cd.ms --output uid A002 X2 --toffset -1 --segsource --statsource tie , , , wvrflag DV10 No flagged antennas but omittng the tie option: wvrgcal --ms uid A002 X X4cd.ms --output uid A002 X X4cd.W --toffset -1 --segsource --statsource Figures 2-8 show the results of running the script with these wvrgcal calls. For each source the colour scales and contours are identical. Analysis Imstat was used to quantify the images. The background area was a large rectangle covering a large part of the image above the source (see appendix). Imfit was used to fit Gaussians to the sources in the Stokes I images. Conclusions from this dataset The results discussed here are summarised in table 1. Applying the water vapour radiometer corrections when all antennas have wvr data is a big improvement over not applying the corrections. S/N values increase from 372, 260 and 248 to 535, 489 and 304, for fields 1,2 and 3 respectively. Image background rms was reduced by 30, 45 and 17%. The size of Gaussians fitted to the images is also always reduced (very slightly) indicating that there are fewer phase errors. The flagging of a single antenna during wvr corrections and the use of interpolation from nearby antennas to derive corrections for the flagged antenna, works extremely well if the flagged antenna has several neighbours (DV06 or DV10). It doesn t seem to be important whether this antenna is near the centre of the array or not (though the dataset is rather limited in this sense). The S/N, rms, and fitted Gaussians are virtually identical to the no flagged antenna case. If the flagged antenna has fewer, nearby neighbours (DV08) the results are still good, but some deterioration is seen over the no flagging case. Rms values are 519, 430 and 302 for fields 1,2 and 3 respectively. For 3 flagged antennas the situation changes. The S/N values are similar to the no wvr correction case. However, the minima in the images are significantly lower, and the resulting images show the effects very clearly. In this case wvrgcal degrades the resulting maps. Perhaps surprisingly, not using the tie option to wvrgcal does not seem to effect the results. At least for this dataset the images were equivalent to tie -ing all 4 sources (no antennas were flagged for this comparison). Uniformly weighting the data did not appear to change any of the trends seen between the various runs.

3 Table 1: Stokes I image properties for various applications of wvrgcal. The first column describes how wvrgcal was run: none = no wvr corrections, norm = no flagged antennas, antenna names refer to the antenna which was flagged via wvrflag. 3ant =DV06, DV08, DV10 flagged, and tie = tie option not used. 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 none ± ± ± norm ± ± ± DV ± ± ± DV ± ± ± ant ± ± ± DV ± ± ± tie ± ± ± none ± ± ± norm ± ± ± DV ± ± ± DV ± ± ± ant ± ± ± DV ± ± ± tie ± ± ± none ± ± ± norm ± ± ± DV ± ± ± DV ± ± ± ant ± ± ± DV ± ± ± tie ± ± ± none ± ± ± norm e ± ± ± DV e ± ± ± DV e ± ± ± ant ± ± ± DV e ± ± ± tie e ± ± ±

4 Figure 1: Antenna positions. The flagged antennas, DV05, DV09, DV11 and DV13 are marked with black dots. Note that the flagged antennas still had WVR data that could be used for interpolation.

5 No Figure 2: Field 0, , used as the phase calibrator. Now wvr corrections. Colour scale (-0.003,0.1), contour level No Figure 3: Field 1, Now wvr corrections. Colour scale (-0.005,0.05), contour level

6 no flagged antennas DV06 flagged DV08 flagged DV06, DV08, DV10 flagged DV10 flagged no flagged antennas - no tie Figure 4: Field 1, with wvr corrections.

7 No Figure 5: Field 2, Now wvr corrections. Colour scale (-0.005,0.05), contour level

8 no flagged antennas DV06 flagged DV08 flagged DV06, DV08, DV10 flagged DV10 flagged no flagged antennas - no tie Figure 6: Field 2, with wvr corrections.

9 No Figure 7: Field 3, Now wvr corrections. Colour scale (-0.002,0.01), contour level

10 no flagged antennas DV06 flagged DV08 flagged DV06, DV08, DV10 flagged DV10 flagged no flagged antennas - no tie Figure 8: Field 3, with wvr corrections.

11 Appendix Script for calibration, imaging and statistics. print Starting script for 4 quasar data set reduction... print Here we apply wvr corrections and flag a more isolated antenna (DV08). #Set up some name conventions specific to this version of the script. root = wvr1 #Root name to use for all cal tables and png files. split1 = X4cd wvr1.ms #Name for new ms after initial split of science data. split2 = X4cd wvr1 split cont.ms #Name for new ms after final split that merges channels. #Cal table names become root + etc. #T sys corrections: none. scriptmode = False #Antenna position corrections. print ---- ANTENNA POSITION CORRECTIONS ---- #gencal inputs. default(gencal) vis = uid A002 X X4cd.ms caltable = antpos fix caltype = antpos antenna = DV02,DV04,DV05,DV06,DV07,DV08,DV10,DV12,DV13,PM01,PM03 parameter = [ , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ] gencal() #WVR CORRECTIONS. INCLUDE WITH VARIOUS OPTIONS. print Executing the following wvrgcal command: print "os.system(../wvrgcal/bin/wvrgcal --ms uid A002 X X4cd.ms --output uid A002 X X4cd.W --toffset -1 --segsource --statsource tie , , , wvrflag DV08 )" os.system(../wvrgcal/bin/wvrgcal --ms uid A002 X X4cd.ms --output uid A002 X X4cd.W --toffset -1 --segsource --statsource tie , , , wvrflag DV08 ) print ---- APPLY CAL FOR ANTENNA POSITIONS (AND POSSIBLY WVR) ---- #Applycal default(applycal) vis = uid A002 X X4cd.ms #gaintable = antpos fix gaintable = [ antpos fix, uid A002 X X4cd.W ] applycal() print SPLIT OUT SCIENCE DATA ----

12 #Split off science spws. default(split) vis = uid A002 X X4cd.ms output datacolumn = corrected #We have applied the antpos fix table (+wvr table perhaps). spw = 1,3,5,7 split() print ---- SAVE ORIGINAL FLAG TABLE ---- #Save original flag table. default(flagmanager) #(Output name from split above) mode = save versionname = Original comment = Right after split. flagmanager() print ---- A PRIORI FLAGGING ---- #A priori flagging. Use flagmanager to back up. print...autocorrelations #Autocorrelations default(flagautocorr) flagautocorr() print...shadowing #Shadowing (looks like there is no shadowing) default(flagdata) mode = shadow flagbackup=f flagdata() print...quack #Quack (as little as possible. 1.5s beginning of scan) default(flagdata) flagbackup=f mode = quack quackinterval = 1.5 quackincrement = T flagdata() #First 5 channels always have high amplitudes. Last 2 a little low. #Flag end channels. (first 5 last 2) print...1st 5 channels and last 2 default(flagdata) flagbackup=f spw = [ *:0 5, *:62 63 ] flagdata() #Look at result with plotms... print ---- BACKUP FLAGS AFTER A PRIORI FLAGGING ----

13 #Backup flags. default(flagmanager) mode = save versionname = apriori comment = After, autocorr, shadow, quack and end channel flagging. flagmanager() print ---- FURTHER FLAGGING ---- #Other flagging. #According to previous study antennas DV05,DV09,DV11 and DV13 were flagged because 9,11 and 13 #didn t have position corrections and 5 had bad phases in the Y polarisation. #--> DV05 corr YY indeed has bad phases. #So flag, DV05, DV09, DV11, DV13. default(flagdata) flagbackup = F antenna = DV05, DV09, DV11, DV13 flagdata() print...backup flags after flagging DV05, DV09, DV11, DV13 #Backup flags. default(flagmanager) mode = save versionname = antennae comment = After flagging antennae 5, 9, 11, 13. flagmanager() print ---- MORE FLAGGING ---- #Further inspection: #All fields have a wide spectral dip in spw 1 centred on channel 38 (chans affected). #In spw 2 some high fluxes across the whole band. Always antenna DV01 correlation XX. #All fields. All times. default(flagdata) flagbackup = F antenna = DV01 correlation = XX flagdata() #Note: as gaincal REQUIRES both polarisations this last step effectively flags #antenna DV01! SHOULD HAVE HAD SPW = 2 at least. default(flagdata) flagbackup = F spw = 1:32 44 flagdata() #SETJY would go here. No flux cal for this dataset. #---> bandpass cal #Strongest source is (field 0). #Probably want to correct for phase changes during abservations. #Significant phase variation within scans so we want to correct for phase variations #with time. print ---- CORRECT PHASES FOR BANDPASS CALIBRATOR ----

14 #Correct bandpass calibrator phases. #(We don t need a gaintable; antenna positions already applied in split). default(gaincal) caltable = root+ bpphase.gcal field = 0 refant = DV02 calmode = p #Phase only. spw = *:20 30 #Few channels so bandpass phase slopes don t decorrelate the signal. solint = int minsnr=2.0 minblperant=4 gaincal() print ---- PLOTCAL TO CHECK PHASE VS TIME FOR BANDPASS CAL (bpphase.x.png etc) ---- #Check ffile = root+ bpphase.x.png os.system( rm + ffile) default(plotcal) caltable = root+ bpphase.gcal xaxis = time yaxis = phase iteration = antenna plotrange = [0,0,-180,180] poln = X figfile = ffile subplot = 441 if scriptmode: showgui = True else: showgui = False plotcal() ffile = root+ bpphase.y.png os.system( rm + ffile) poln = Y figfile = ffile if scriptmode: showgui = True else: showgui = False plotcal() print ---- BANDPASS CALIBRATION ---- #Do bandpass calibration. default(bandpass) caltable = root+ bandpass.bcal #Output name. gaintable = root+ bpphase.gcal #Input table with phase corrections. Applied on the fly. field = 0 spw = combine = refant = DV02 solint= inf solnorm=t minblperant=4 fillgaps = 14 #(Interpolate across the flagged channels in spw 1)

15 bandpass() #Lots of errors due to flagged antennas. OK. #Check bandpass solutions. Do once for each spw. print ---- PLOTCAL FOR BANDPASS TABLE (bandpass.phasespw+spw+.png etc) ---- default(plotcal) if scriptmode: showgui = True else: showgui = False caltable = root+ bandpass.bcal xaxis = chan yaxis = phase iteration = antenna plotrange = [0,0,-180,180] subplot=441 for spw in [ 0, 1, 2, 3 ]: figname = root+ bandpass.phasespw +spw+.png os.system( rm + figname) spw = spw figfile = figname plotcal() yaxis = amp plotrange = [] for spw in [ 0, 1, 2, 3 ]: figname = root+ bandpass.ampspw +spw+.png os.system( rm + figname) spw = spw figfile = figname plotcal() #Uneven bandpass for antenna 12 (presumably YY). SPW 0 only. #Otherwise phases are smooth and near 0 and amplitudes are all near 1 and fairly smooth. print ---- GAIN CALIBRATION (FIELD 0, ) ---- print...phase for each integration time #GAIN CALIBRATION. Use field 0 as the phase calibrator too. default(gaincal) field = 0 gaintable = root+ bandpass.bcal refant = DV02 caltable = root+ intphase.gcal calmode = p solint = int #One phase solution per integration. minsnr=2.0 minblperant=4 gaincal() print...phase for each scan time caltable = root+ scanphase.gcal solint = inf #One phase solution per scan. gaincal() #Rest as above.

16 print...amp and phase for each scan time caltable = root+ amp.gcal gaintable=[root+ bandpass.bcal, root+ intphase.gcal ] #Apply more frequent phase solutions. solint = inf #Generate one solution per scan. calmode= ap gaincal() #Rest as above. print ---- PLOTS OF CALIBRATION TABLES ---- print...intphase X.png, intphase Y.png #Inspect...Solutions for each integration time. default(plotcal) if scriptmode: showgui = True else: showgui = False caltable = root+ intphase.gcal xaxis = time yaxis = phase field = spw = iteration = antenna subplot = 441 plotrange=[0,0,-180,180] poln= X figfile = root+ intphase X.png os.system( rm + figfile) plotcal() #Colours are different SPWs. All sources in each line (higher noise for weaker source). poln = Y figfile = root+ intphase Y.png os.system( rm + figfile) plotcal() print...scanphase X.png, scanphase Y.png #Solutions for each scan. caltable = root+ scanphase.gcal poln = X figfile = root+ scanphase X.png os.system( rm + figfile) plotcal() poln = Y figfile = root+ scanphase Y.png os.system( rm + figfile) plotcal() print...amp phase.png #Amplitude and phase solution table. caltable = root+ amp.gcal plotrange = [0,0,-1,1] figfile = root+ amp phase.png os.system( rm + figfile) plotcal() #Phases always within + 1 degree. print...amp X.png, amp Y.png yaxis = amp

17 plotrange = [0,0,0.0,0.08] poln = X figfile = root+ amp X.png os.system( rm + figfile) plotcal() poln = Y figfile = root+ amp Y.png os.system( rm + figfile) plotcal() #Very steady amplitude values. Constant (but different) for each source. #All seem to bo OK. #BOOTSTRAPPING WOULD GO HERE. But we have no flux calibrator. #fluxscale not run. #APPLY CALIBRATION. print ---- APPLY CALIBRATION (applycal) ---- print...field 0 #Bandpass calibrator and phase reference source. default(applycal) field = 0 interp = [ nearest, nearest, nearest ] gaintable = [root+ bandpass.bcal, root+ intphase.gcal, root+ amp.gcal ] gainfield=[ 0, 0, 0 ] flagbackup = True applycal() print...field 1, 2, 3 field = 1,2,3 interp=[ nearest, linear, linear ] gaintable=[root+ bandpass.bcal, root+ scanphase.gcal, root+ amp.gcal ] gainfield=[ 0, 0, 0 ] applycal() #All look very regular and reasonable in terms of relative fluxes. #Look at residual amp/phase for sources. Copied from Ed Fomalont s script. #This is just a diagnostic step. print --- CALCULATE RESIDUAL PHASE ERRORS ---- default(gaincal) field = caltable = root+ gresid gaintable=[root+ bandpass.bcal, root+ scanphase.gcal, root+ amp.gcal ] solint = int refant = DV02 gaincal() print...generate plots of phase vs time for 3 baselines. resid phase spw0 X.png. default(plotcal) figfile = root+ resid phase spw0 X.png os.system( rm + figfile) caltable = root+ gresid xaxis = time yaxis = phase markersize = 2

18 plotrange=[0,0,-40,40] spw = 0 poln = X antenna= DV10 ;subplot=311 overplot = F; field = 0 ; plotcolor = red ; plotcal() # DV02 DV10 overplot = T; field = 1 ; plotcolor = green ; plotcal() # overplot = T; field = 2 ; plotcolor = blue ; plotcal() # overplot = T; field = 3 ; plotcolor = black ; plotcal() # antenna= DV07 ;subplot=312 overplot = F; field = 0 ; plotcolor = red ; plotcal() # DV02 DV07 overplot = T; field = 1 ; plotcolor = green ; plotcal() # overplot = T; field = 2 ; plotcolor = blue ; plotcal() # overplot = T; field = 3 ; plotcolor = black ; plotcal() # antenna= DV06 ;subplot=313 overplot = F; field = 0 ; plotcolor = red ; plotcal() # DV02 DV06 overplot = T; field = 1 ; plotcolor = green ; plotcal() # overplot = T; field = 2 ; plotcolor = blue ; plotcal() # overplot = T; field = 3 ; plotcolor = black ; plotcal() # #SPLIT print ---- SPLIT, MERGING CHANNELS ---> X4cd nowvr split cont.ms ---- default(split) outputvis = split2 datacolumn = corrected width = 64 split() #Image merging all SPWs and polarisations to get deeper image. print ---- CLEANING ---- default(clean) vis = split2 cell = 0.25arcsec imsize = 512 niter = 100 stokes = IQUV weighting = briggs robust = 0.0 spw = mode = mfs mask = [247, 247, 265, 265] for field in [ 0, 1, 2, 3 ]: field = field imagename = root+ f +field clean() print ---- IMAGES MADE, NOW COMPUTING STATS ---- #STATS: imnames = [root+ f0.image, root+ f1.image, root+ f2.image, root+ f3.image ] for imname in imnames: 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

19 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 ---- #Export final images as fits. default(exportfits) for imname in imnames: exportfits(imagename=imname, fitsimage=imname+.fits ) Script for imfit #Script to run imfit on each field (0-3) in the 4 quasar dataset. root = wvr1 #Root name for the images. flds = [ 0, 1, 2, 3 ] 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 + 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))