AIPS HELP file for VBAPP in 31DEC26
As of Sun Sep 13 2:33:35 2026
VBAPP: Task which appends data sets with different frequencies
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 #
IN2NAME Second UV file name (name)
IN2CLASS Second UV file name (class)
IN2SEQ 0.0 9999.0 Second UV file name (seq. #)
IN2DISK 0.0 9.0 Second UV file disk unit #
IN3NAME Third UV file name (name)
IN3CLASS Third UV file name (class)
IN3SEQ 0.0 9999.0 Third UV file name (seq. #)
IN3DISK 0.0 9.0 Third UV file disk unit #
IN4NAME Fourth UV file name (name)
IN4CLASS Fourth UV file name (class)
IN4SEQ 0.0 9999.0 Fourth UV file name (seq. #)
IN4DISK 0.0 9.0 Fourth UV file disk unit #
OUTNAME Output UV file name (name)
OUTCLASS Output UV file name (class)
OUTSEQ -1.0 9999.0 Output UV file name (seq. #)
OUTDISK 0.0 9.0 Output UV file disk unit #.
FQCENTER >= 0 -> center frequency axis
HELP SECTION
VBAPP
Task: This task is meant to deal with data sets that do not work well
with VBGLU. That task assumes that the data sets have mostly
but not entirely the same sample times. This task assumes that
the data sets all have different times. It figures out an IF
frequency set that supports all of the data (like VBGLU). But
then it does not match up times. Instead it copies the first
data set filling the output IFs that match the IFs of the input.
Any other IFs are flagged in those records. It repeats this
process with data sets 2, 3, and 4 filling appropriate IFs and
flagging the others. This will make a very large uv data set,
but, since IMAGR can only handle one input file, you may require
this to image various observations together.
Do note that the subarray information is not changed in this
task. The antenna files of the first input data appear in the
output data set. Since these may be inappropriate for the
other data sets, you must not do things that depend on knowing
where the antennas are. That most explicitly includes UVFIX,
but may complicate plotting against hour angle, parallactic
angle, elevation, et al. For a relatively compact array such
as the VLA the plot isues may not matter but for VLBI they do.
(Remember UVRANGE applies to the header frequency and so has a
very different behavior with different IFs of wide-band data.)
The task will also glue together all table extension files,
combining all version 1 extensions of a given type into output
version 1, all version 2 extensions (if any) into output
version 2, and so forth. The user should be careful to make
sure that this is a meaningful operation - e.g. all CL table
versions 4 contain the same level of calibration. The one
exception is that the highest flag table versions found in
the input files will be combined into one output flag table.
Certain table types may not be handled by this task. FO
(frequency offset) and BS (baseline solution) tables are
not processed properly. The former is used by CVEL and PCVEL,
after SPLIT; the latter is used by BLAPP and BLING. FITS IDI
tables that are translated by FITLD are not handled. Any EOP
correction should be applied to the input files before VBAPP
unless the CT tables are all identical.
It is best if the random parameters are all the same in the same
order. The task will attempt to fix this if they are not. The
first input file should have the maximum number of parameters.
Note that you cannot over-write a pre-existing data set.
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.
IN2NAME....Second file name.
IN2CLASS...Second file class
IN2SEQ.....Second file sequence number.
IN2DISK....Second file sequence number.
IN3NAME....Third file name.
IN3CLASS...Third file class
IN3SEQ.....Third file sequence number.
IN3DISK....Third file sequence number.
IN4NAME....Fourth file name.
IN4CLASS...Fourth file class
IN4SEQ.....Fourth file sequence number.
IN4DISK....Fourth file sequence number.
OUTNAME....Output UV file name (name). Standard defaults.
OUTCLASS...Output UV file name (class). Standard defaults.
OUTSEQ.....Output UV file name (seq. #). 0 => highest unique
OUTDISK....Disk drive # of output UV file. 0 => highest with
space for the file.
SUBARRAY...Subarray to do. 0 -> all.
FQCENTER...> 0 => Change frequency axis reference pixel to
Nchan / 2 + 1
else => do not change reference pixel
EXPLAIN SECTION
VBAPP Glues VLBA data together IF by IF.
PURPOSE
NOTE: VBAPP will only work on single FREQID datasets.
It may now be used to combine data sets that would be different
FREQIDs if combined in other ways.
VBAPP Glues together data that were correlated in multiple passes on
the VLBA correlator, multiple pass is not used in its historical
sense of different baselines/pass but in the sense of different
IFs/pass. This is necessary because the VLBA correlator can only deal
with 8 IFs at a time, but when MkIII Mode A,B or C are correlated the
data have to be split up in frequency space.
VBAPP reads the file headers and FQ tables and constructs a composite
header/FQ table. It reads all input files constructing a time-ordered
list of output records. When times and baselines match the output
data will contain the input data from all input files. For times when
the files do not match, some output IFs will be marked as flagged
while the other output IFs will contain the appropriate data. No data
will be lost in this way, although some records may not contain data
for all IFs. The task is more efficient if the data are in strict
time-baseline order, but any order can be processed.
Several assumptions are implicit in this process:
(1) The source/antenna numbers are identical - use task MATCH to
fix this if needed.
(2) There are the same number of spectral channels per IF
(3) There is a sigle FREQID
VBAPP then copies those tables that have no IF dependency. It then
takes the tables that do have arrays that depend on IF and performs an
analogous operation to that done on the uv-data. The tables from the
first file are taken to be the master representations and the others
are merged with them. The tables that are bonded in this way are:
AT IM CL SN MC TY PC BP BL SU GC CQ
If there is a significant mismatch between the tables a warning
message is issued.
CT Table
One table that is treated differently than those above is the CT table.
The CT table is the CALC table and contains the earth orientation
parameters (EOPs) that were used by the correlator. It is possible that
the EOPs used were not the most accurate because it takes a week or two
after the observation date to get the most accurate EOPs (see VLBA
Test Memo 69). Inaccurate EOPs can substantially negatively effect
phase referencing experiments. Therefor it is recommended that they are
corrected with CLCOR (OPCODE=EOPS; see EXPLAIN CLCOR). CLCOR uses the
CT table to calculate the difference between the old EOPs and the new
EOPS. Therefor, it is VITAL that the CT table be correct. The CT table
has no IF dependence but it is possible in the two correlation that the
EOPs were different. So VBAPP compares the CT tables and if they are
identical and if they are not it refuses to copy them and issues a
message.
If VBAPP refuses to copy the CT table to the output file and you want
to fix the EOPs for your experiment you can follow these steps, BEFORE
running VBAPP:
1) run CLCOR (OPCODE='EOPS') on both files.
2) run SPLAT to apply those corrections (because VBAPP only copies the
first CL table).
3) run VBAPP on the SPLATed files