AIPS NRAO AIPS HELP file for ACSCL in 31DEC18



As of Sat Jan 20 0:19:42 2018


ACSCL: Correct cross correlations using auto correlations

INPUTS

INNAME                             Input UV file name (name)
INCLASS                            Input UV file name (class)
INSEQ             0.0     9999.0   Input UV file name (seq. #)
INDISK            0.0        9.0   Input UV file disk unit #
FLAGVER          -1.0              Flag table version to apply
                                   0 -> highest
TIMERANG                           Timerange:
                                    1-4 = start day,hr,min,sec
                                    5-8 = end   day,hr,min,sec
SOLINT                             Solution interval (min)
                                   > 0 => ignore scans
                                   < 0 => SOLINT=ABS(SOLINT)
                                   and average is terminated
                                   at the end of scans
                                   0 => -2 min; see help
SOURCES                            Source list
QUAL            -10.0              Source qualifier -1=>all
CALCODE                            Calibrator code '    '=>all
SELBAND                            Bandwidth to select (kHz)
SELFREQ                            Frequency to select (MHz)
FREQID                             Freq. ID to select.
SUBARRAY          0.0              Subarray, 0 -> all
DOCALIB           1.0      101.0   > 0 calibrate data & weights
                                   > 99 do NOT calibrate weights
GAINUSE                            CL (or SN) table to apply
DOPOL            -1.0       10.0   If >0 correct polarization.
PDVER                              PD table to apply (DOPOL>0)
BLVER                              BL table to apply.
DOBAND            1.0       10.0   If >0 apply bandpass cal.
                                   Method used depends on value
                                   of DOBAND (see HELP file).
BPVER                              Bandpass table version
SMOOTH                             Smoothing function. See
                                   HELP SMOOTH for details.
ICHANSEL                           Array of start and stop chn
                                   numbers, plus a channel
                                   increment and IF to be used
                                   for channel selection in the
                                   averaging. See HELP ICHANSEL.
                                   Default = center 75 percent of band.

HELP SECTION

ACSCL
Type: Task
Use:  ACCOR corrects amplitudes in cross-correlation spectra due to
      errors in sampler thresholds using measurements of
      auto-correlation spectra.  Some correlators, e.g. JIVE, apply
      this correction before writing out the data.  For them, ACCOR
      should not be needed.  On the other hand, ACCOR should also be
      benign since the auto-correlations should end up scaled to 1.0
      and so cause essentially no correction to be made by ACCOR.  For
      the VLBA correlators - both the older hardware correlator and
      new software DiFX correlator - this correction is not done prior
      to reading the data into AIPS.  For such data, ACCOR is
      required.

      The scaling of autocorrelation data may drift away from 1.0
      later in the processing, for example, because of the choice of
      normalization in BPASS.  ACSCL is designed to correct this drift
      by applying all calibrations and making sure that the
      autocorrelation data average 1.0.  Numerous "calibration"
      adverbs are offered, although DOCALIB and DOBAND are forced to
      true.  Otherwise, this task is ACCOR in disguise.

      Note that this task should be used before APCAL since that will
      make a CL table designed to scale the autocorrelations to the
      SEFD.  Actually GAINUSE may point at a CL table prior to that
      affected by APCAL even when APCAL has already been used.  The
      added scaling may then be applied to the highest CL table
      version with CLCAL.
Adverbs:
  INNAME.....Input UV file name (name).      Standard defaults.
  INCLASS....Input UV file name (class).     Standard defaults.
  INSEQ......Input UV file name (seq. #).    0 => highest.
  INDISK.....Disk drive # of input UV file.  0 => any.
  FLAGVER....Specifies the version of the flagging table to be applied.
             0  => highest numbered table.
             <0 => no flagging to be applied.
  TIMERANG...Time range of the data to be used. In order: Start day,
             hour, min. sec, End day, hour, min. sec. Days relative to
             reference date.
             TIMERANGE = 0  => All available data are used
  SOLINT.....The solution interval (minutes). The mean values of
             spectra are averaged through the SOLUTION intervals
             following the start time in TIMERANGE. If SOLINT is more
             than the range of times in TIMERANG, the time of
             averaging is equal to the range of times in TIMERANG.

             If SOLINT > 0, the averaging does not pay attention to
             scan boundaries and several scans inside SOLINT may be
             averaged together.  The averaging in this case may
             include more than one source ID or/and frequency ID.
             The source ID and frequency ID written to the output SN
             table is the same for the all rows of the SN table and
             equal 0.  Use CALSOUR = '' when running CLCAL to convert
             this SN table to the relevant CL table.

             If SOLINT < 0, ABS(SOLINT) is used as the average
             interval and averaging is terminating at the end of the
             scan.  The correct source IDs and frequency IDs appear in
             the output SN table for the each averaged interval.
             CALSOUR can be used in CLCAL to select calibrators in
             this case.  Because of the gaps between scans, small
             intervals can appear at scan ends.  Intervals less than
             0.1*MIN(SOLINT, scan length) are not recorded to the
             output SN table.

             0 => -2 min
  SOURCES....Source list.  The task loops over all sources specified.
             '*' = all; a "-" before a source name means all except ANY
             source named.
  QUAL.......Only sources with a source qualifier number in the SU table
             matching QUAL will be used if QUAL is not -1.
  CALCODE....Sources may be selected on the basis of the calibrator code
             given in the SU table.
                  '    ' => any calibrator code selected
                  '*   ' => any non blank code (cal. only)
                  '-CAL' => blank codes only (no calibrators)
                  anything else = calibrator code to select.
             NB: The CALCODE test is applied in addition to the other
             tests, i.e. SOURCES and QUAL, in the selection of sources
             to process.
  SELBAND....Bandwidth of data to be selected. If more than one IF is
             present SELBAND is the width of the first IF required.
             Units = kHz. For data which contain multiple
             bandwidths/frequencies the task will insist that some form
             of selection be made by frequency or bandwidth.
  SELFREQ....Frequency of data to be selected. If more than one IF is
             present SELFREQ is the frequency of the first IF required.
             Units = MHz.
  FREQID.....Frequency identifier to select (you may determine which is
             applicable from the OPTYPE='SCAN' listing produced by
             LISTR). If either SELBAND or SELFREQ are set, their values
             override that of FREQID. However, setting SELBAND and
             SELFREQ may result in an ambiguity.  In that case, the task
             will request that you use FREQID.
  SUBARRAY...Desired subarray - 0 => loop over all subarrays.
  DOCALIB....If true (>0), calibrate the data using information in the
             specified Cal (CL) table for multi-source or SN table for
             single-source data.  Also calibrate the weights unless
             DOCALIB > 99 (use this for old non-physical weights).
  GAINUSE....version number of the CL table to apply to multi-source
             files or the SN table for single-source files.
             0 => highest.
  DOPOL......If > 0 then correct data for instrumental polarization as
             represented in the AN or PD table.  This correction is
             only useful if PCAL has been run or feed polarization
             parameters have been otherwise obtained.  See HELP DOPOL
             for available correction modes: 1 is normal, 2 and 3 are
             for VLBI.  1-3 use a PD table if available; 6, 7, 8 are
             the same but use the AN (continuum solution) even if a PD
             table is present.
  PDVER......PD table to apply if PCAL was run with SPECTRAL true and
             0 < DOPOL < 6.  <= 0 => highest.
  BLVER......Version number of the baseline based calibration
             (BL) table to appply. <0 => apply no BL table,
             0 => highest.
  DOBAND.....> 0 is not an error if there is no BP table, but this
                 task is really designed to be used after there is a
                 bandpas correction.
             If true (>0) then correct the data for the shape of the
             antenna bandpasses using the BP table specified by BPVER.
             The correction has five modes:
             (a) if DOBAND=1 all entries for an antenna in the table
             are averaged together before correcting the data.
             (b) if DOBAND=2 the entry nearest in time (including
             solution weights) is used to correct the data.
             (c) if DOBAND=3 the table entries are interpolated in
             time (using solution weights) and the data are then
             corrected.
             (d) if DOBAND=4 the entry nearest in time (ignoring
             solution weights) is used to correct the data.
             (e) if DOBAND=5 the table entries are interpolated in
             time (ignoring solution weights) and the data are then
             corrected.
  BPVER......Specifies the version of the BP table to be applied
             0 => highest numbered table.
             <0 => no bandpass correction to be applied.
  SMOOTH.....Specifies the type of spectral smoothing to be applied to
             a uv database . The default is not to apply any smoothing.
             The elements of SMOOTH are as follows:
             SMOOTH(1) = type of smoothing to apply: 0 => no smoothing
               To smooth before applying bandpass calibration
                 1 => Hanning, 2 => Gaussian, 3 => Boxcar, 4 => Sinc
               To smooth after applying bandpass calibration
                 5 => Hanning, 6 => Gaussian, 7 => Boxcar, 8 => Sinc
             SMOOTH(2) = the "diameter" of the function, i.e. width
               between first nulls of Hanning triangle and sinc
               function, FWHM of Gaussian, width of Boxcar. Defaults
               (if < 0.1) are 4, 2, 2 and 3 channels for SMOOTH(1) =
               1 - 4 and 5 - 8, resp.
             SMOOTH(3) = the diameter over which the convolving
               function has value - in channels.  Defaults: 1,3,1,4
               times SMOOTH(2) used when input SMOOTH(3) < net
               SMOOTH(2).
  ICHANSEL...Array of start and stop channels plus a channel increment
             and IF, used to select the channels to be averaged.  For
             instance, if you wished to exclude channels 1 - 10 and
             121 - 128 because of bandpass effects, and channels 56 -
             80 of IF 1 but not IF 2 because of interference, then you
             would set ICHANSEL = 11,55,1,1, 81,121,1,1, 11,121,1,2.
             If you only wished to use every other channel from the
             second IF then you would set ICHANSEL = 11,55,1,1,
             81,121,1,1, 11,121,2,2.  Up to 20 groups of start, stop
             and increment channel numbers plus IF numbers can be
             specified. The default (ICHANSEL = 0) is to average the
             center 75 percent of the band, i.e.
             ICHANSEL(1,1) = (# channels)/8 + 1
                       For example: # channels=16 => ICHANSEL(1,1)=3
             ICHANSEL(2,1) = (# channels + 1)*7/8
                       For example: # channels=16 => ICHANSEL(2,1)=14
             ICHANSEL(3,1) = 1
             ICHANSEL(4,1) = 0 (meaning all IFs).
             If ICHANSEL describes averaging explicitly for some IFs,
             but skips other IFs, then the center 75 percent of the band is
             averaged for the skipped IFs.  For example:
             ICHANSEL=2,6,1,2 => The channels 2-6 will be averaged for
             IF=2 and the center 75 percent of the band will be averaged for
             the rest of the IFs.

EXPLAIN SECTION

ACSCL: Task to correct amplitudes distorted by errors of samplers.
Documentor:  L.R. Kogan
RELATED PROGRAMS: FITLD, ACCOR

                          PURPOSE

Calibration of a cross correlation spectrum supposes knowing parameters
of samplers (digitizers in particular). Unfortunately these parameters
are not identical to optimal ones and their real values are not known
with enough accuracy. As a result the cross correlation spectrum
amplitudes are measured with errors. These errors can be estimated
having measured the mean value of auto correlation spectrum of a given
interferometer's antennas. The theory of this correction is given by
L. Kogan in VLBA SCIENTIFIC MEMO #9, 1995.

The task estimates the mean values of the auto correlation spectra of
all antennas of a given VLBI network and records them in a new SN table.
The following application of the correction is provided by standard
procedure of applying the SN table to an CL table (CLCAL) and following
application this table with DOCALIB = 1 in relevant tasks.

Some correlators apply this correction to the data before writing them
out - notably the EVN JIVE correlator.  The VLBA hardware and software
(DiFX) correlators do
NOT apply this correction to the data.  Therefore, ACSCL is required
for the VLBA correlators and any others that do not apply the
correction.  ACSCL should be benign (do nothing) for those correlators
that do apply the correction prior to reading the data into AIPS.
Note that you can always run ACSCL and look at the SN table produced
with PRTAB or SNPLT to see if it was benign or not.

                          COMMENTS

This method of correction is especially effective in the case of TWO
bits (FOUR levels) digitizers. Uncorrected amplitudes of cross
correlation spectra can differ from real ones in this case up to 25 percent.
The correction can minimize the errors to a bit of percent.
The behavior of the samplers' errors is unknown. In many cases it is
stable for hours for a given sampler. But cases of minute changing can
not be excluded. The default value of SOLINT is 60 minutes. But smaller
value 5-10 minutes can be recommended as well.
The correction quality can be checked by POSSM (for example) with
DOCALIB=1 and GAINUSE equaled to the CL table number gotten from
ACSCL's SN table.
The mean values of all auto correlation spectra would be close to unit.
  The task corrects other small errors of spectrum amplitudes caused by
other errors besides samplers errors.
  The task requires TB order of data. If it is not the task UVSRT has
to be applied to the data early.

                   COMMENTS FOR ACSCL BY CRAIG WALKER

The data from the correlator are correlation coefficients - the ratio of
the correlated power to the noise power.  The apriori calibration
process can be considered to have two major steps.  The first is to
adjust the data for all of the effects introduced by the various bits of
hardware in the system, mainly the analog and digital filters.   The
second step is to multiply by the SEFD - the ratio of Ts/gain which
converts power to flux density.  That second step might also include
some corrections for propagation effects like opacity.

The autocorrelations are a measure of the total power in the system.
For the second step to work properly, the net effect of the various
corrections for instrumental effects should produce autocorrelations
that are unity across the portion of the bandpass of interest.  Then,
after multiplying by the SEFD, the autocorrelation amplitudes will be
the SEFD, which is the desired situation.  The same scaling of cross
correlations will produce the calibrated flux density.

One of the first calibration steps for VLBI data is to run ACCOR.
That has the effect of making the average of the autocorrelations
unity, but  it does not correct for spectral shape.

With BPASS in its current form, one can get unity calibration results
for the autocorrelations by normalizing by power over the full band
using autocorrelation based bandpasses.  Note that one might actually
not want to normalize at all to force the results to unity, but it
doesn't really matter because ACCOR has already forced the average to
unity.  The problem with this method is that the autocorrelations do
not contain information about the phase bandpass, and one really wants
to calibrate the phases along with the amplitude.

If one uses strong-source cross correlation data to derive the
bandpasses, complete with phases, and normalize on power across the
full band, you get close.  But there are aliasing effects in the edge
channels that cause the bandpass shape in the autocorrelations to be
somewhat different, in those channels, from the cross correlation
bandpass.  The autocorrelations are higher in the edge channels
because the aliased signals don't correlate.  This effect is stronger
for the PFB, with soft edge filters, than the DDC or the legacy system
with harder edged filters.  The result is that, when the
cross-correlation derived bandpass is applied to the autocorrelations,
the values over the central channels that you want to keep are
slightly suppressed.  In my test data, the average value is 0.977 for
the PFB and 0.993 for the DDC. One can use CLCOR with the power gain
option (new) to correct that and get good calibration.  But it would
be nice to not have to do the CLCOR correction.

In principle, one could run BPASS with autocorrelation based
bandpasses, apply that, and make an incremental cross-correlation
bandpass and it should work.  But AIPS is not set up to do that
conveniently.  One would have to make a new file with the calibration
applied after the first BPASS.

What would be most convenient, I believe, is for BPASS to make a cross
correlation based bandpass, apply it to a the autocorrelations, and
determine the average result over the selected channels, probably not
including the edge channels.  Then the normalization would be to cause
those autocorrelation channels to come out 1.00 on average (and
individually to within the noise).  This would introduce the
complexity of having to read the crosscorrelations, determine the
bandpass shapes, read the autocorrelations, apply the bandpass,
determine the average in the selected channels, then apply to the
bandpass the normalization required to make that average unity.  The
program would thus have to deal with both cross and autocorrelations,
which it probably does not do now.  But the end results, I think, is a
proper calibration to do in preparation for the scaling by the SEFD to
convert correlation coefficients to Jy.




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