AIPS HELP file for ICHANSEL in 31DEC24

HELP SECTION

```ICHANSEL
Type: Adverb  (REAL, 4 elements x 20 rows)
Use:  Data selection adverb used to specify a list of start and stop
channel numbers plus an increment AND IF NUMBER to be used in
some way, such as to be averaged to form a pseudo-continuum
channel.  Up to 20 sets of channel numbers/IFs can be specified.
The first two elements specify the start and stop channel
numbers, the third specifies the increment within that range,
i.e.if CHANSEL = 10,20,2 that means take every other channel in
the range 10 - 20, so that channels 10,12,14,16,18,20 would be
averaged together.  The fourth element specifies the IF to which
the previous three apply.
Null Value:  0
A completely null value means use all channels in some tasks:
ICHANSEL(1,i) <= 0  =>  1
ICHANSEL(2,1) <= 0  =>  Nchan
ICHANSEL(2,i) <= 0  =>  (i > 1) -> no more channel sets
ICHANSEL(3,i) <= 0  =>  1
ICHANSEL(4,i) <= 0  =>  all IFs
BPASS, CPASS, AVSPC, REWAY and many other tasks use the center
75 percent of the channels for any IF for which no ICHANSEL was
specified.  Other tasks have different defaults including being
unwilling to set defaults.
Examples:
ICHANSEL = 9,56         includes all channels 9-56 in all IFs
ICHANSEL = 12,53,1,1, 62,116,1,1, 12,116,1,2
includes all channels from 12-53 and
62-116 in IF 1, and all channels from
12-116 in IF 2.  If there are more
IFs, default actions are taken.
ICHANSEL = 17,112,1,0, 23,45,1,1, 87,112,1,1
includes all channels from 17-112 in
all IFs except IF 1 which includes
channels 23-45 and 87-112.  NOTE
WELL: the all IF instruction must
precede any instruction that
partially overrides it.
In most tasks ICHANSEL selects the channels to be averaged.
ACSCL......Corrects cross amplitudes using auto correlation
measurements.
AVSPC......Averages uv-data in the frequency domain.
BASRM......Task to remove a spectral baseline from total power
spectra.
BLCAL......Compute closure offset corrections.
BLCHN......Compute closure offset corrections on a channel-by-channel
basis.  ICHANSEL selects the "good" channels to be fit.
BPASS......Computes spectral bandpass correction table.  ICHANSEL
selects channels to be averaged in the normalization.
BPWAY......Determines channel-dependent relative weights.  ICHANSEL
selects the channels used to find a normalized rms.
CALIB......Determines antenna calibration: complex gain.
CPASS......Computes polynomial spectral bandpass correction table.
ICHANSEL selects the channels to average for a "channel 0".
DEFLG......Edits data based on decorrelation over channels and time.
FIXAL......Least squares fit special EVLA aliasing function and
remove.  ICHANSEL slects channels to be fit.
FLGIT......Flags data based on the rms of the spectrum.
PCAL.......Determines instrumental polarization for UV data.
PHSRF......Perform phase-referencing within a spectral line database.
REWAY......Computes weights based in rms in spectra.
RFLAG......Flags data set based on time and freq rms in fringe
visibilities.  ICHANSEL selects channels to use in the
spectral median window flagging operation.
RLCAL......Determines instrumental right-left phase versus time (a
self-cal).
SPLAT......Applies calibration and splits or assemble selected
sources.
SPLIT......Converts multi-source to single-source UV files w
calibration.
UVHOL......Prints holography data from a UV data base with
calibration.
UVLIN......Fits and removes continuum visibility spectrum, also can
flag.  ICHANSEL selects channels to fit.
UVLSD......Least squares fit to channels and divides the uv data.
ICHANSEL selects channels to fit.
UVLSF......Least squares fit to channels and subtracts from uv data.
ICHANSEL selects channels to fit.
UVMLN......Edits data based on the rms of the spectrum.  ICHANSEL
selects channels to fit.
Procedures:
FXALIAS....Least squares fit special EVLA aliasing function and
remove.  ICHANSEL selects the channels to be fit.
IMSCAL.....Large image self-cal with IM2CC and OOCAL.  ICHANSEL
selects the channels to be averaged.
OOCAL......Determines antenna complex gain with frequency-dependent
models.   ICHANSEL selects the channels to be averaged.
VLBAAMP....Applies a-priori amplitude corrections to VLBA data.
ICHANSEL selects the channels to be used in ACSCL.
```

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