AIPS HELP file for APCAL in 31DEC21
As of Tue Oct 26 21:49:55 2021
APCAL: Task to generate an amplitude calibration SN table
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 #
ANTENNAS Antennas to calibrate
SUBARRAY 0.0 9999.0 Subarray 0 -> all
STOKES Stokes type ('R','L',' ')
BIF 0.0 4096.0 Start IF number
EIF End IF number
FREQID Freq. ID to calibrate
SOURCES Sources to calibrate
TIMERANG Time range to calibrate
TYVER TY table version number.
GCVER GC table version number.
SNVER Output SN table version.
0 -> highest unique
appends to existing file
OPCODE Opacity correction opcode:
' ' -> no opacity corr.
'LESQ' -> fit for opacity
'PLOT' plot input fit
APARM Calibration parameters:
(1) -> B factor (def. 1.0)
(2) -> ZA limit (deg)
(3) >=0 use spillover
(4) > 0 use all FQ in
(5) Limit to sec(z) plotted
(6) > 0 => normalize Tsys
by average Tsys
(7) Use IF AP(7) for
SOLINT Solution interval (min)
0 -> use all data
INVERS WX table version number.
0 -> use text file
CALIN Text file with weather info.
Must be ' ' if using WX
TRECVR Receiver temperatures (K);
in (R,L) pairs.
TAU0 Zenith opacities (per ant.)
DOFIT Fit type (per ant.)
-1 -> no opacity correction
0 -> apply TRECVR only
1 -> solve for Trec using
fitting method spec.
in OPCODE; then apply
PRTLEV Print level
DOTV TV selection (<0 -> PL file)
LTYPE Plot labeling
GRCHAN 0.0 TV Graphics channel to use
Task: This task takes as input a system temperature (TY) table and a
gain curve GC table, as produced by ANTAB or loaded by FITLD,
and generates a solution (SN) table containing amplitude gain
calibration information. A correction can be made for
atmospheric opacity as well. The data can be used to solve for
the opacity or a pre-defined receiver temperature can be
NOTE: run MERGECAL before running this task using calibration
transfer data loaded using FITLD. If you do not APCAL may
fail producing an intelligible error message.
Plots are made showing system temperature versus zenith angle
and apparent zenith opacity as a function of time.
The task assumes that all included IFs have the same receiver
temperature and opacity. If you wish to solve them separately,
you may set BIF, EIF, and, optionally, SNVER. If you end up
writing multiple SN tables, you must combine them when running
CLCAL (adverbs SNVER and INVERS). Note, the behavior of this
task has changed (Jan 8, 2020) when SNVER specifies an existing
NOTE that source fluxes are used in the computation of opacity
so for precision work, they should probably be entered in the
SU table with SETJY. These fluxes are the full flux in the
antenna beam, not the VLBI correlated flux and are used to
adjust the atmospheric temperature by the antenna temperature
induced by the source. In most cases, this is negligible.
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.
ANTENNAS...Antenna numbers to calibrate. A prefix of a minus sign
on any one or more of the antennas in the list, means
that all antennas in the list are not calibrated and all
antennas not in the list are calibrated.
SUBARRAY...Subarray number to calibrate (0 -> all, 1 at a time but
all to the same SN table)
STOKES.....Stokes type to calibrate ('R' -> RCP, 'L' -> LCP,
' ' -> both RCP and LCP)
BIF........Start IF number to calibrate
EIF........End IF number to calibrate
FREQID.....Freq. ID to calibrate. If there are multiple FREQIDs in
the FQ table, then you can do only one at a time since
the code requires the frequencies of each IF. If there
is only one, you may set FREQID=-1 to allow all FREQIDs
in the TY and GC table to be used. That may not be a
good idea, depending on how far apart the FREQID
SOURCES....Sources to calibrate. A prefix of a minus sign indicates
that all sources except the given source are to be
TIMERANG...Time range to calibrate, in format: start_day,
start_hour, start_min, start_sec, end_day, end_hour,
end_min, end_sec, where the day numbers are relative to
the reference date in the catalog header.
TYVER......System temperature TY table version number to use
(0 -> highest)
GCVER......Gain table GC table version number to use
(0 -> highest)
SNVER......Output solution SN table version number
(0 -> new table with highest unique version
number; else ***append*** to the specified SN table)
OPCODE.....Opacity correction opcode:
' ' -> no opacity solution or correction.
'PLOT' -> no fit, plot model specified in TRECVR, TAU0
'GRID' -> If DOFIT(ant) > 0 then perform an opacity
solution using a simple grid search from zero
to twice the specified receiver temperature
(TRECVR) and zenith opacity (TAU0). If TRECVR
and/or TAU0 are not set then reasonable
defaults will be used.
'GRDR' -> If DOFIT(ant) > 0 same as 'GRID' except it adds
a search down-weighting system temperature
outliers. This takes about 10 times longer
'OPAC' -> If DOFIT(ant) > 0 then perform an opacity
solution using a simplex minimization taking
the values of receiver temperature (TRECVR)
and zenith opacity (TAU0) as starting points.
If TRECVR and/or TAU0 are not set then
reasonable defaults will be used.
'OPCR' -> If DOFIT(ant) > 0 same as 'OPAC' except it adds
a search down-weighting system temperature
outliers. This takes about 20 times longer
'LESQ' -> If DOFIT(ant) > 0 then perform an opacity
solution using a least squares method. LESQ
does not use TRECVR or TAU0.
If DOFIT <= 0 then the receiver temperatures specified in
TRECVR are used WITHOUT ALTERATION. If OPCODE is set, for
best results set either CALIN or INVERS for the weather
data. Otherwise a peak ground temperature of 20 degrees
Celsius will be used.
(1) B factor (def. 1.0)
Opacity parameters (use only if OPCODE <> ' ', and
(2) Zenith angle limit (deg) (def. 75). This
sets the range of Zenith angles that are
used in the fits. Therefor a relatively large
number (like 75, the default) is desirable.
(3) >=0 then use nominal spillover correction
= 2 the use the 7mm correction for all
frequencies (as done before October 2014)
< 0 then no spillover correction
(4) > 0 then use all FQ in opacity solution
opac. soln. (NOT RECOMMENDED)
<=0 then solve for each FQ ID separately
(5) Plot sec(z) no higher than APARM(5);
< = 0 => very large
(6) > 0 => Modify the recorded Tsys(IF,t) on the
assumption that the Tcal's are not qquite correct.
For each antenna, this operation does
alpha(IF) = average over time of
Tsys(IF,t) / average over IF of Tsys(IF,t)
We then use as Tsys(IF,t) = TsysIn(IF,t)/alpha(IF)
(7) If APARM(6) > 0, use APARM(7) as the IF to define the
correct Tsys(t) rather than the average over all IFs.
APARM(7) < 1 or > Nif or Tsys(APARM(7),t) blanked
causes the average to be used.
SOLINT.....Solution interval for opacity solution (min)
0 -> use all data
INVERS.....WX table version number, to be used instead of
CALIN. CALIN MUST BE BLANK, OR ELSE INVERS
IS IGNORED AND INVERS MUST NOT BE ZERO.
CALIN......Text file with weather information (in format
'LOGICAL:FILE.NAME'). Leave blank if using the
TRECVR.....Receiver temperatures (K) for each antenna in (R, L)
pairs (i.e.,. Ant 1 RCP, Ant 1 LCP, Ant 2 RCP, Ant 2
LCP,..). If all 0 and OPCODE is set to 'OPAC', 'OPCR',
'GRID', or 'GRDR' ('LESQ' does not use this input)
then receiver temperatures are estimated from the TY
table. For most cases this default works well.
If DOFIT <= 0 then the receiver temperature is
used with NO FITTING.
TAU0.......Zenith opacities (per antenna; no polarization
dependence). If all 0 and OPCODE is set to 'OPAC',
'OPCR', 'GRID', or 'GRDR' ('LESQ' does not use this
input) then the zenith opacity is estimated depending
on the observing frequency.
DOFIT......Opacity correction type (per ant.; no polarization
dependence). If ANTENNAS is set to include the list of
antennas (none < 0), then DOFIT refers only to those set
in ANTENNAS, i.e., DOFIT(i) refers to the antennas
number in ANTENNAS(i). Otherwise DOFIT(i) refers to
-1 -> do not do opacity correction
0 & OPCODE <> ' ' -> APPLY TRECVR ONLY
1 & OPCODE <> ' ' -> solve for receiver temp.
using method specified in OPCODE; then apply
PRTLEV.....Print level (1 -> echo weather file as read)
DOTV.......TV device selection (1 -> TV; <0 -> PL file)
LTYPE......Labelling type, see HELP LTYPE for details:
1 = border, 2 = no ticks, 3 or 7 = standard, 4 or 8 =
relative to ref. pixel, 5 or 9 = relative to subimage
(BLC, TRC) center, 6 or 10 = pixels. 7-10 all labels
other than tick numbers and axis type are omitted.
Less than 0 is the same except that the plot file
version number and create time are omitted.
Add n * 100 to alter the metric scaling.
GRCHAN.....Graphics channel to use: 0 -> more than one.
APCAL: Task to generate an amplitude calibration solution (SN) table.
Documenter: A. J. Kemball, Amy Mioduszewski
Related programs: ANTAB, CLCAL, FITLD, SNPLT, SNSMO, PRTAB, VLOG
APCAL reads the system/antenna temperatures in the TY table and
the gain information in the GC table and generates an amplitude
calibration solution (SN) table. A correction for opacity can
optionally be made. The opacity is the attenuation caused by the
atmosphere. The solution table can subsequently be smoothed or
edited using SNSMO, and is applied using CLCAL. This allows the
selection of primary flux density calibrators. At present TY
and GC tables for VLBA stations are attached to the data. For
experiments before April 1998 the TY and GC tables can be filled
by task ANTAB which reads an external calibration text file.
APCAL takes as input the uv-file for which amplitude calibration
is to be generated and expects TY and GC tables. General selection
adverbs are provided (TIMERANG, SOURCES, BIF, EIF etc.) to select
those source and time ranges for which amplitude calibration is
required. In most cases default values are appropriate in which
case no selection is performed. The version numbers of the input TY
and GC tables are specified by TYVER and GCVER respectively. The
output SN table is specified using SNVER.
APCAL can operate in two distinct modes:
i) No opacity correction (OPCODE = ' '):
In this mode APCAL will generate amplitude calibration without a
correction for atmospheric opacity. All adverbs below OPCODE
excluding APARM(1), which specifies the b-factor, can be ignored
in this case.
ii) Opacity correction (OPCODE = 'GRID', 'GRDR', 'OPAC', 'OPCR'
In this mode APCAL will generate amplitude calibration including
a correction for atmospheric opacity. A local solution for opacity
can be solved for (DOFIT = 1) or nominal receiver temperatures can
be entered in TRECVR and used directly (DOFIT >= 1). Three
algorithms are presently implemented for the local solution
for opacity ('GRID', 'OPAC' or 'LESQ'). With "robust" versions of
'GRID' and 'OPAC' available as 'GRDR' and 'OPCR'. The robust
fits down-weight system temperature outliers, but take significantly
longer than their non-robust versions.
SOURCE FLUX DENSITIES
APCAL will look in the source (SU) table for source flux densities
only if they are required to generate the amplitude calibration gain
factors. This is the case when calibrating antennas for which Ta/Tsys
or Ta values have been entered (e.g.,. VLA) or, more generally, if an
opacity solution is being performed. The source flux densities are
entered in the SU table using task SETJY. An error will be reported by
APCAL if source flux densities are required but cannot be found in the
source table. For weak sources which are not going to be used as
primary flux density calibrators when applying the amplitude
calibration using CLCAL (as selected by adverb CALSOUR), unit flux
density should be entered. Separate frequency Id's must be calibrated
separately as the source table can store flux densities for only one
FREQID at a time.
This information is only required if the opacity solution is being
The ground weather data is provided as an WX table attached to the VLBA
data as of April 1998. Use this table by setting INVERS to the table
number and setting CALIN=''. IGNORE BELOW IF YOU ARE USING AND
ATTACHED WX TABLE.
WEATHER INFORMATION FROM AN EXTERNAL TEXT FILE
The ground weather data also available in the general VLBA calibration
text file but needs minor editing before it can be used by APCAL. Use
VLOG to get it in the correct format (recommended) or edit the file as
i) Write the weather data section to a separate file.
ii) Replace all asterisks (*) in column 1 by a comment marker (!)
iii) Comment out all data lines containing an asterisk after the
temperature data by inserting a comment marker (!) in col. 1.
iv) Add the keywords: "WEATHER station_name /" at the beginning
of each station section. Terminate each station section with
a line containing a single backslash "/"
A truncated example file is given below:
! For antenna(s): SC,HN,NL,FD,LA,PT,KP,OV,BR,MK
! For UT timerange: 1994DEC01/335 at 23:02:00 to 1994DEC02/336 at 11:00:00
----- Weather information for SC -----
! All values are instantaneous readings at the time indicated.
! A '*' following a value indicates it is not reliable.
! Temp Press DewPt Wind Spd/Dir Rain Gust
!UT Day-Time C mBar C m/s deg cm m/s
WEATHER SC /
336-08:02:08 27.5 1012.2 23.5 2.6 95.0 0.00 6.0
336-08:10:40 27.5 1012.4 23.6 2.0 138.0 0.00 6.3
336-08:19:12 27.4 1012.6 23.0 2.1 92.0 0.00 4.2
336-08:27:44 27.5 1012.7 23.2 2.2 103.0 0.00 6.0
336-08:36:16 27.5 1012.5 23.8 1.6 85.0 0.00 6.0
336-08:44:48 27.5 1012.4 23.9 1.9 70.0 0.00 5.3
336-08:53:20 27.4 1012.7 23.9 1.4 92.0 0.00 5.3
! 336-09:01:52 27.3* 1012.4 23.7* 3.3 78.0 0.00 4.9
! 336-09:10:24 27.3* 1012.6 23.7* 3.3 111.0 0.00 4.6
336-09:18:56 27.3 1012.9 23.4 1.2 113.0 0.00 5.3
336-09:27:28 27.4 1012.6 23.6 2.5 88.0 0.00 4.9
336-09:36:00 27.4 1012.6 23.5 2.4 91.0 0.00 4.9
336-09:44:32 27.5 1013.0 23.7 2.3 110.0 0.00 5.6
336-09:53:04 27.5 1013.1 23.4 1.9 105.0 0.00 5.6
336-10:01:36 27.6 1013.3 23.2 2.1 99.0 0.00 6.3
336-10:10:08 27.6 1013.3 23.7 2.9 132.0 0.00 5.6
! ----- Weather information for HN -----