I've been working on bandpass calibrating some WSRT data. Usually this is a pretty simple process, but for some reason I can't get it to work out here. When I calibrate the data I'm getting large closure errors that seem to be baseline and polarization dependent. Before bandpass calibration, I went through the data and flagged anything that looked like it had an amplitude that was off. After flagging I thought the data looked pretty good in SPFLG. Changing the reference antenna or the calibration source doesn't fix the problem. What had partially fixed the problem for the LL feed at least was going from the default SOLTYPE to SOLTYPE='L1R', which uses a more robust method of fitting the bandpass that is less sensitive to variations in the input data.
Any ideas? I've put my inputs to BPASS and some sample output below.
Update (2/4/08): It looks like using bpassprm(5) = -1 instead of bpassprm(5) = 0 works. bpassprm(5) = -1 is only supposed to be used in cases of decent phase stability. It averages over time and then averages the ichansel channels rather than averaging the ichansel channels on a record-by-record base as does bpassprm(5)=0. Therefore, bpassprm(5)=-1 gives me a bit more signal enabling better bandpass calibration. The phase stability of my observations is pretty good, so I think I'm safe using this parameter.]AIPS 1: BPASS Task to generate a "Bandpass" (BP) table.
AIPS 1: Adverbs Values Comments
AIPS 1: ----------------------------------------------------------------
AIPS 1: USERID 0 User number
AIPS 1: INNAME 'POLANG' Input UV file name (name)
AIPS 1: INCLASS '13CM' Input UV file name (class)
AIPS 1: INSEQ 3 Input UV file name (seq. #)
AIPS 1: INDISK 2 Input UV file disk unit #
AIPS 1:
AIPS 1: Data Selection
AIPS 1: CALSOUR '3C286' Bandpass calibrator sources.
AIPS 1: *rest ' '
AIPS 1: QUAL -1 Calibrator qualifier -1=>all
AIPS 1: CALCODE ' ' Calibrator code ' '=>all
AIPS 1: UVRANG 0 0 UV range to select
AIPS 1: TIMERANG *all 0 Time range to select
AIPS 1: SELBAND -1 Bandwidth to select (kHz)
AIPS 1: SELFREQ -1 Frequency to select (MHz)
AIPS 1: FREQID 1 Freq. ID to select.
AIPS 1: BIF 0 Lowest IF number 0=>all
AIPS 1: EIF 0 Highest IF number 0=>all
AIPS 1: SUBARRAY 0 Subarray, 0=>all
AIPS 1: ANTENNAS *all 0 Antennas to select
AIPS 1:
AIPS 1: CLEAN map (optional)
AIPS 1: IN2NAME ' ' Cleaned map name (name)
AIPS 1: IN2CLASS ' ' Cleaned map name (class)
AIPS 1: IN2SEQ 0 Cleaned map name (seq. #)
AIPS 1: IN2DISK 0 Cleaned map disk unit #
AIPS 1: INVERS -1 CC file version #.
AIPS 1: NCOMP *all 0 # comps to use for model.
AIPS 1: 1 value per field
AIPS 1: FLUX 0 Lowest CC component used.
AIPS 1: NMAPS 0 No. Clean map files
AIPS 1: CMETHOD ' ' Modeling method:
AIPS 1: 'DFT','GRID',' '
AIPS 1: SMODEL *all 0 Source model, 1=flux,2=x,3=y
AIPS 1: See HELP SMODEL for details.
AIPS 1:
AIPS 1: Control options
AIPS 1: DOCALIB 1 > 0 calibrate data & weights
AIPS 1: > 99 do NOT calibrate weights
AIPS 1: GAINUSE 2 CL table to apply (SN table
AIPS 1: to apply to single-source)
AIPS 1: DOPOL -1 If >0 correct polarization.
AIPS 1: BLVER -1 BL table to apply.
AIPS 1: FLAGVER 1 Flag table version
AIPS 1: DOBAND -1 If >0 apply bandpass cal.
AIPS 1: Method used depends on value
AIPS 1: of DOBAND (see HELP file).
AIPS 1: BPVER 0 Bandpass table version
AIPS 1: SOLINT 0 Solution interval (mins)
AIPS 1: -1 => do whole time range
AIPS 1: SOLTYPE 'L1R' Soln type,' ','L1','GCON',
AIPS 1: REFANT 5 Reference antenna
AIPS 1: OUTVERS 0 Output BP table version
AIPS 1: 0 => a new table to be
AIPS 1: generated.
AIPS 1: SMOOTH *all 0 Smoothing function.
AIPS 1: BE VERY CAREFUL HERE.
AIPS 1: ANTWT *all 0 Ant. wts (0 => 1.)
AIPS 1: WEIGHTIT 0 Modify data weights function
AIPS 1: MINAMPER 10 Amplitude closure error
AIPS 1: regarded as excessive in %
AIPS 1: MINPHSER 10 Phase closure error regarded
AIPS 1: as excessive in degrees
AIPS 1: BPASSPRM 0 2 Control information:
AIPS 1: 0 0 1: if > 0 use only the
AIPS 1: 0 10 autocorrelation data.
AIPS 1: 10 1 2: print level - see help
AIPS 1: 1 3 3: If > 0 do not divide data
AIPS 1: 0 by source model
AIPS 1: 4: If > 0 store phases only
AIPS 1: in the BP table.
AIPS 1: 5: Divide by 'channel 0'
AIPS 1: **** SEE HELP - NEW ****
AIPS 1: 6: amp closure error limit -
AIPS 1: print channels averaging
AIPS 1: over this if (2) > 0
AIPS 1: 7: phase closure error limit
AIPS 1: print channels averaging
AIPS 1: over this if (2) > 0
AIPS 1: 8: > 0 => scalar average
AIPS 1: 9: > 0 => interpolate over
AIPS 1: flagged channels if poss.
AIPS 1: 10:1 => normalize amplitudes
AIPS 1: using all channels
AIPS 1: 2 => normalize amplitudes
AIPS 1: using ICHANSEL channels
AIPS 1: 3 => normalize amplitudes
AIPS 1: and zero average phase
AIPS 1: using ICHANSEL channels
AIPS 1: 4 => normalize amplitudes
AIPS 1: and zero average phase
AIPS 1: using all channels
AIPS 1: 0 => no deliberate norm.
AIPS 1: 11: > 0 solution weights are
AIPS 1: independent of channel
AIPS 1: = -1 weights scaled
AIPS 1: by amplitude**2
AIPS 1: < -1.5 weights scaled by
AIPS 1: 1 / amplitude**2
AIPS 1: ICHANSEL 5 50 Array of start and stop chan
AIPS 1: 1 1 numbers, plus a channel
AIPS 1: 5 50 increment and IF to be used
AIPS 1: 1 2 to select channels to sum to
AIPS 1: 5 50 find a 'channel 0'. If all
AIPS 1: 1 3 0, range set to inner 75% of
AIPS 1: 5 50 observing band.
AIPS 1: 1 4 5 50
AIPS 1: 1 5 5 50
AIPS 1: 1 6 5 50
AIPS 1: 1 7 5 50
AIPS 1: 1 8 *rest 0
AIPS 1: SPECINDX 0 Spectral index to correct
AIPS 1: 'Channel 0' uv-data
AIPS 1: IN3NAME ' ' Channel 0 uv name (name)
AIPS 1: must be '' to suppress option
AIPS 1: IN3CLASS ' ' Channel 0 uv name (class)
AIPS 1: must be '' to suppress option
AIPS 1: IN3SEQ 0 Channel 0 uv name (seq. #)
AIPS 1: IN3DISK 0 Channel 0 uv disk unit #
AIPS 1: BADDISK *all 0 Disks to avoid for scratch
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 1 Rpol
localh> BPASS1: 1- 2 25% 0d 1- 10 123% 0d 1- 13 44% 1d
localh> BPASS1: 1- 14 32% 1d 2- 4 34% 1d 2- 6 73% 178d
localh> BPASS1: 2- 7 25% 2d 2- 8 133% 0d 2- 9 238% 180d
localh> BPASS1: 2- 10 917% 178d 2- 11 662% 2d 2- 12 298% 178d
localh> BPASS1: 2- 13 80% 0d 2- 14 118% 1d 3- 8 53% 0d
localh> BPASS1: 3- 10 13% 1d 3- 13 20% 0d 3- 14 27% 0d
localh> BPASS1: 4- 6 74% 0d 4- 8 20% 2d 4- 9 63% 1d
localh> BPASS1: 4- 10 120% 178d 4- 11 31% 0d 4- 12 52% 0d
localh> BPASS1: 4- 13 35% 1d 4- 14 52% 1d 5- 8 63% 1d
localh> BPASS1: 5- 13 19% 0d 5- 14 28% 0d 6- 7 28% 1d
localh> BPASS1: 6- 9 111% 1d 6- 10 906% 0d 6- 11 84% 1d
localh> BPASS1: 6- 12 126% 1d 6- 14 24% 2d 7- 8 243% 2d
localh> BPASS1: 7- 9 26% 0d 7- 10 167% 0d 8- 11 140% 176d
localh> BPASS1: 8- 12 19% 178d 8- 13 32% 2d 8- 14 51% 2d
localh> BPASS1: 9- 10 801% 0d 9- 11 72% 0d 9- 12 107% 1d
localh> BPASS1: 9- 14 18% 2d 10- 11 586% 1d 10- 12 864% 0d
localh> BPASS1: 10- 13 205% 0d 10- 14 60% 176d 11- 12 64% 1d
localh> BPASS1: 12- 13 11% 0d 12- 14 28% 0d
localh> BPASS1: Closure error statistics: IF 7 correlator 1
localh> BPASS1: Channel Mean amp & amp**2 Mean phase & phase**2 Excess
localh> BPASS1: 1 52.84 104.74 16.72 53.59 53
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 2 Rpol
localh> BPASS1: 1- 13 13% 0d 2- 8 19% 2d 2- 13 13% 7d
localh> BPASS1: 2- 14 21% 5d 4- 8 11% 1d 4- 13 11% 2d
localh> BPASS1: 4- 14 16% 0d 5- 14 10% 1d 6- 12 12% 0d
localh> BPASS1: 7- 10 11% 1d 8- 12 1% 11d 9- 12 12% 1d
localh> BPASS1: 10- 12 12% 3d 10- 14 13% 1d
localh> BPASS1: 2 4.90 6.60 2.13 3.30 14
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 3 Rpol
localh> BPASS1: 6- 10 13% 0d 6- 14 11% 1d 9- 10 13% 1d
localh> BPASS1: 3 2.66 4.15 0.58 0.78 3
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 4 Rpol
localh> BPASS1: 6- 10 11% 0d 6- 14 11% 1d 9- 14 10% 2d
localh> BPASS1: 10- 13 12% 1d
localh> BPASS1: 4 2.66 4.08 0.57 0.76 4
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 5 Rpol
localh> BPASS1: 6- 10 11% 0d 6- 14 10% 1d 10- 13 12% 1d
localh> BPASS1: 5 2.60 3.99 0.53 0.69 3
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 6 Rpol
localh> BPASS1: 6- 7 10% 0d 6- 10 11% 0d 6- 14 12% 1d
localh> BPASS1: 9- 14 11% 2d 10- 13 12% 1d
localh> BPASS1: 6 2.65 4.10 0.55 0.73 5
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 7 Rpol
localh> BPASS1: 6- 7 10% 1d 6- 14 12% 1d 10- 13 12% 1d
localh> BPASS1: 7 2.58 3.92 0.51 0.69 3
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 8 Rpol
localh> BPASS1: 10- 13 10% 1d
localh> BPASS1: 8 2.20 3.30 0.55 0.73 1
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 9 Rpol
localh> BPASS1: 6- 7 11% 1d 10- 13 11% 1d
localh> BPASS1: 9 2.24 3.37 0.51 0.68 2
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 35 Rpol
localh> BPASS1: 6- 14 11% 1d
localh> BPASS1: 35 2.27 3.37 0.56 0.75 1
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 36 Rpol
localh> BPASS1: 6- 14 11% 1d
localh> BPASS1: 36 2.23 3.38 0.55 0.74 1
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 37 Rpol
localh> BPASS1: 6- 14 12% 1d
localh> BPASS1: 37 2.28 3.46 0.52 0.71 1
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 38 Rpol
localh> BPASS1: 6- 7 11% 0d 6- 14 12% 1d
localh> BPASS1: 38 2.32 3.58 0.54 0.73 2
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 39 Rpol
localh> BPASS1: 6- 7 10% 1d 6- 14 11% 1d
localh> BPASS1: 39 2.31 3.52 0.54 0.73 2
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 40 Rpol
localh> BPASS1: 6- 7 11% 0d 6- 14 11% 1d
localh> BPASS1: 40 2.28 3.49 0.54 0.73 2
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 41 Rpol
localh> BPASS1: 6- 7 11% 1d 6- 14 11% 1d 9- 10 10% 0d
localh> BPASS1: 10- 13 12% 1d
localh> BPASS1: 41 2.47 3.78 0.52 0.70 4
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 42 Rpol
localh> BPASS1: 6- 10 11% 0d 6- 14 10% 1d 7- 14 13% 1d
localh> BPASS1: 10- 13 12% 1d
localh> BPASS1: 42 2.79 4.24 0.54 0.73 4
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 43 Rpol
localh> BPASS1: 6- 7 10% 1d 7- 10 15% 0d 7- 14 13% 1d
localh> BPASS1: 9- 10 11% 1d 10- 13 10% 1d
localh> BPASS1: 43 2.85 4.38 0.55 0.75 5
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 44 Rpol
localh> BPASS1: 6- 14 10% 1d 7- 14 13% 1d 9- 10 10% 1d
localh> BPASS1: 10- 13 10% 1d
localh> BPASS1: 44 2.57 3.94 0.56 0.76 4
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 45 Rpol
localh> BPASS1: 6- 7 11% 1d 6- 14 11% 1d 7- 14 12% 1d
localh> BPASS1: 9- 10 10% 0d 9- 14 10% 2d 10- 13 10% 1d
localh> BPASS1: 45 2.57 3.97 0.54 0.74 6
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 46 Rpol
localh> BPASS1: 6- 7 10% 1d 6- 14 11% 1d 7- 14 12% 1d
localh> LOGFILE FOR USER 333 GETTING LARGE: USE PRTMSG AND CLRMSG NOW!
localh> BPASS1: 46 2.52 3.87 0.56 0.77 3
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 47 Rpol
localh> BPASS1: 6- 14 11% 1d 7- 14 13% 1d 9- 10 10% 0d
localh> BPASS1: 47 2.67 4.04 0.58 0.76 3
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 48 Rpol
localh> BPASS1: 6- 14 11% 1d 10- 13 11% 1d
localh> BPASS1: 48 2.58 3.86 0.57 0.76 2
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 49 Rpol
localh> BPASS1: 6- 14 10% 1d 10- 13 12% 1d
localh> BPASS1: 49 2.59 3.87 0.55 0.72 2
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 50 Rpol
localh> BPASS1: 6- 14 11% 1d 9- 10 10% 0d 10- 13 12% 1d
localh> BPASS1: 50 2.59 3.88 0.56 0.73 3
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 51 Rpol
localh> BPASS1: 9- 10 10% 0d
localh> BPASS1: 51 2.27 3.41 0.52 0.70 1
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 52 Rpol
localh> BPASS1: 9- 10 10% 0d 9- 14 10% 2d
localh> BPASS1: 52 2.07 3.11 0.52 0.69 2
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 62 Rpol
localh> BPASS1: 8- 11 11% 0d 8- 14 12% 1d 9- 14 10% 0d
localh> BPASS1: 10- 13 14% 2d
localh> BPASS1: 62 2.98 4.31 0.88 1.11 4
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 63 Rpol
localh> BPASS1: 1- 12 11% 3d 2- 4 15% 4d 2- 12 10% 8d
localh> BPASS1: 3- 7 13% 1d 4- 7 7% 12d 4- 8 19% 10d
localh> BPASS1: 4- 11 14% 5d 5- 11 10% 3d 5- 12 11% 1d
localh> BPASS1: 7- 11 11% 7d 7- 12 16% 4d 8- 12 16% 3d
localh> BPASS1: 8- 13 14% 4d 8- 14 18% 2d 10- 13 13% 5d
localh> BPASS1: 10- 14 13% 6d 11- 13 12% 3d 11- 14 12% 1d
localh> BPASS1: 63 6.02 7.44 3.66 4.36 18
localh> BPASS1: Closure errors at 3/ 2 47 50. IF/Chn no. 7 64 Rpol
localh> BPASS1: 1- 7 23% 2d 1- 8 35% 12d 1- 11 2% 12d
localh> BPASS1: 1- 12 17% 8d 1- 13 2% 11d 1- 14 2% 15d
localh> BPASS1: 2- 3 14% 5d 2- 4 63% 8d 2- 6 16% 12d
localh> BPASS1: 2- 7 203% 27d 2- 8 156% 14d 2- 10 15% 7d
localh> BPASS1: 2- 11 4% 55d 2- 12 107% 48d 2- 13 34% 45d
localh> BPASS1: 2- 14 3% 56d 3- 7 28% 4d 3- 8 41% 12d
localh> BPASS1: 3- 11 6% 12d 3- 12 9% 13d 3- 14 4% 13d
localh> BPASS1: 4- 7 73% 35d 4- 8 68% 28d 4- 10 5% 10d
localh> BPASS1: 4- 11 53% 23d 4- 12 5% 46d 4- 13 64% 2d
localh> BPASS1: 4- 14 66% 20d 5- 6 10% 2d 5- 7 11% 17d
localh> BPASS1: 5- 8 2% 21d 5- 11 27% 4d 5- 12 30% 1d
localh> BPASS1: 5- 13 16% 9d 5- 14 30% 8d 6- 7 6% 19d
localh> BPASS1: 6- 8 27% 26d 6- 11 19% 9d 6- 12 34% 7d
localh> BPASS1: 6- 14 27% 6d 7- 8 178% 15d 7- 9 15% 16d
localh> BPASS1: 7- 10 12% 11d 7- 11 125% 45d 7- 12 257% 31d
localh> BPASS1: 7- 13 50% 47d 7- 14 88% 43d 8- 9 11% 22d
localh> BPASS1: 8- 10 22% 21d 8- 11 55% 55d 8- 12 164% 42d
localh> BPASS1: 8- 13 17% 56d 8- 14 9% 53d 9- 11 5% 12d
localh> BPASS1: 9- 12 27% 3d 9- 13 4% 13d 9- 14 3% 15d
localh> BPASS1: 10- 11 10% 14d 10- 12 20% 9d 10- 13 12% 9d
localh> BPASS1: 10- 14 20% 13d 11- 12 128% 20d 11- 13 72% 6d
localh> BPASS1: 11- 14 90% 1d 12- 13 96% 23d 12- 14 106% 16d
localh> BPASS1: 13- 14 47% 6d
localh> BPASS1: 64 27.85 45.82 15.40 21.64 67
localh> BPASS1: Antenna 1 IF 7 corr 1 had 12 excess closure errors
localh> BPASS1: Antenna 2 IF 7 corr 1 had 26 excess closure errors
localh> BPASS1: Antenna 3 IF 7 corr 1 had 11 excess closure errors
localh> BPASS1: Antenna 4 IF 7 corr 1 had 24 excess closure errors
localh> BPASS1: Antenna 5 IF 7 corr 1 had 13 excess closure errors
localh> BPASS1: Antenna 6 IF 7 corr 1 had 51 excess closure errors
localh> BPASS1: Antenna 7 IF 7 corr 1 had 40 excess closure errors
localh> BPASS1: Antenna 8 IF 7 corr 1 had 31 excess closure errors
localh> BPASS1: Antenna 9 IF 7 corr 1 had 29 excess closure errors
localh> BPASS1: Antenna 10 IF 7 corr 1 had 54 excess closure errors
localh> BPASS1: Antenna 11 IF 7 corr 1 had 26 excess closure errors
localh> BPASS1: Antenna 12 IF 7 corr 1 had 31 excess closure errors
localh> BPASS1: Antenna 13 IF 7 corr 1 had 40 excess closure errors
localh> BPASS1: Antenna 14 IF 7 corr 1 had 62 excess closure errors
Monday, February 4, 2008
Bandpass calibration trauma
Friday, January 4, 2008
Double Sources-- and not the AGN kind
I'm having this problem with some archival data from 1990. I have two nights of data on a galaxy, spaced a few nights apart from one another. I calibrate the nights separately, and image them separately. It rapidly becomes obvious that the images from the two nights are shifted relative to one another, but not in any simple way. The galaxy center, at the image center, is at the same position in both, but as you go out radially, the first night's sources are further out, radially. It's as if the image is stretched one night, relative to the other.
AIPS thinks that both images have the same pixel scale, so that means a source will have different R.A. and Dec (by about an arcminute, pretty bad!) between the two nights' images. If I DBCON the two nights, I get lots of double sources.
The observational setup claims to be exactly the same between the two nights-- same pointing center, same correlator set-up.
Anyone have any idea what might be causing this? I think I've had a similar problem with GMRT data before, and gave up temporarily and went on to a different project. I'm guessing it's a calibration issue?
Monday, December 3, 2007
Setting up New Data Disks
My latest organizational scheme for AIPS is to give each project a new AIPS data disk. I just spent some time adding a new data disk to my AIPS installation on my laptop. Here's what I did to get it. (BTW, I've got a MacBook, but that shouldn't matter for this.)
In $AIPS_ROOT, cd to DA00. Save a copy of the DADEVS.LIST file as DADEVS.LIST.old. Then edit the DADEVS.LIST. I just copied the first uncommented line (set up by AIPS) and incremented the data disk number by 1, i.e., LOCALHOST_2. Now save a copy of the NETSP file in the same directory to NETSP.old. Again, edit the file by copying the first uncommented line and incrementing the data disk number by 1. Finally, cd to $AIPS_ROOT/DATA, mkdir LOCALHOST_2 (or whatever your new data disk is named), cd to LOCALHOST_2, and create a file named SPACE (touch SPACE). Now when you start up AIPS you should have additional data disks available.
Most of this tip was taken from the AIPS Manager FAQ (see "How do I configure new AIPS data areas?").
Thursday, October 4, 2007
Your friend, the tilde
Ever had to enter a lot of numbers into an array in an AIPS script and run out of room (like in this post)? Trying to figure out the mysteries of plcolor without typing 5 million numbers? In your time of need, the handy tilde will help you out. See below the fold for more details.
The popsym help file (type help popsym to access) gives you this handy tidbit of information on the tilde
A(i) ~ 1,2,3 Store values in A(i),A(i+1)... (change only as many as on RHS)
Now here's an example,
filename = '10208722.UVF','10208724.UVF','10208726.UVF','10208727.UVF'
filename(5) ~ '10208733.UVF','10303423.UVF','10303424.UVF','10303461.UVF'
Tah-Dah!! That's all there really is to it. Use the tilde, Luke -- your fingers will thank you.
Friday, September 21, 2007
Beam Squint and Self Calibration (relevant even if you don't care about polarization!)
Here's another tip from Juan Uson about a self-cal trick to get the very best dynamic range in your images.
The VLA has beam squint in Stokes V. What this means is that away from the center of your field, the RR and LL polarizations separate a bit (I don't really understand the details of this). This leads to an image artifact-- if you have a bright source off the center of your field, it'll seem like you can't clean it very well. Even if you've done your very best job at self calibration and cleaned very deeply, there will still remain rings around the source (which of course drives up your noise).
One easy fix for this problem is to do a normal Phase self calibration, but when it's time for your A&P self calibration, average your RR and LL. I believe the relevant parameter for this in CALIB is APARM(3) > 1. This should give you a self cal that is not affected by the beam squint, and models your bright sources better!
Clean Components and Self Calibration
Well, I just talked to Juan Uson from NRAO-Charlottesville, and he delivered some useful information for us burgeoning self-cal experts.
He says that you should ALWAYS use all your clean components for self calibration. Don't cut at the first negative! He says that when they were first learning about self cal, they were very timid and therefore advised people to cut at the first negative, but now they know better. You want to use your very best clean component model for self cal, which includes all components!
Here are some reasons why--
a bright source that is centered on the edge between two pixels will actually take an infinite number of clean components to model, because basically the clean algorithm keeps exchanging flux back and forth between the surrounding pixels. so, to model such a source, there are lots of negative clean components required.
if a source has any complex structure at all, you can't really effectively model it without using some negative components.
I told him that occasionally, the word on the street is "Just self cal off of one bright source that you know the structure of, and leave the rest of the flux alone." When I tried to do that once, the self cal created fictional ghost sources in my data and it scared me! Juan was not surprised; he said its a terrible idea to self cal on only a select few sources. Basically what self cal does is divide your real fluxes by your model fluxes, and try to make the residuals look like noise. If there is still real flux in your residuals (because your clean model is based off only a select few sources), then it will try to turn that real flux into noise and crazy things will happen.
So, clean out all your flux, and use all your clean components!
Wednesday, September 19, 2007
Filling data that includes midnight
When you get your data from the archive, it's normally in several files. For me, each file is usually a day's worth of data, so I fill and calibrate each data file separately. However, if you have observations that go from say 20:00 on 17-Feb to 6:00 on 18-Feb, the archive creates a separate files for the observations on the 17th and for the observations on the 18th. You need to read in both files to calibrate that series of observations.
If you have a question about AIPS that CANNOT be answered by reading the