Basic Information on passfit

Task: passfit
Purpose: Fit polynomials to a passband calibration set
Categories: calibration

PASSFIT is a MIRIAD task that fits polynomials to the passband 
calibration data.
The data and fits are displayed as amplitudes and phases with
all spectral windows for a baseline displayed at one time.
The amplitudes appear as red dots (open dots for flagged data);
the phases appear as yellow pluses (open triangles for flagged data).
The amplitudes are autoscaled; the phases range from -180 to 180 
degrees from bottom-to-top of screen. Bad data may to flagged by deleting
the corresponding amplitude point with the cursor. If you do edit data
on a baseline, the passband is automatically refit and shown again when
you attempt to exit that baseline. The fits are shown as solid colored lines.

Commands in cursor mode are:

   ?   This help (also redraws screen)
   q   quit (immediate quit without save)
   e   exit (go to next baseline or exit with save)
   d   delete point nearest cursor (amplitude ONLY)
   a   add back point nearest cursor
   1   numbers 1,2,3... zooms to that window
   x   Toggle zoom around in x
   y   Toggle zoom around in y
   z   Toggle zoom around in x and y
   u   unzooms in both x and y

Key: pcal
The passband calibrator visibility dataset, produced by passmake.
No default.

Key: drop
Number of channels to drop at the start and end of each window
before fitting. Fits will be extrapolated to cover these channels.
A different number of dropped channels for each window can be
given, up to a maximum of ``nspect/2'' (nspect is the total number of 
spectral windows - see UVLIST - we assume USB and LSB). For negative
numbers it's absolute value will be the fraction of the window size
to be dropped at each side.
Default: -8 (1/8th of window dropped from each side)
Note: For mode 4, and additional 1/8th is dropped from the center.

Key: order
The polynomial order for the amp and phase fit to each passband window.  
Default: 1,1 (1st order for amplitude and 1st order for phase)

Key: chave
Number of channels to average together before fitting polynomials
to amp and phase.  This is useful for poor signal-noise data to
reduce amplitude bias.  Data for individual channels are still displayed
in the plot; consequently, there may be an offset between the fits and
the points in the amplitude plots (amplitude bias). 
Default: 1

Key: split
A value of true results in mode 4 windows being split into 2 sub-windows
which are independently fit. False results in the standard fit of
one polynomial to each window. This is probably only useful for data
taken in 1993/94 with BIMA's old correllator.
Default: false.

Key: device
This keyword is used to specify the plotting device. It uses
the normal PGPLOT format of

  file/type

 or

  device/type

Usually there exists a default file name or device name.
The part after the slash gives the plotter type. Examples of the
type are:

  xs    X-windows output. This window is resizeable and
        persistent. Prepending a number allows you to use multiple
        windows, e.g. 1/xs, 2/xs etc.
  xd    X-windows output. A transient window that disappears
        when your task exists.
  ps    Postscript (landscape mode).
  vps   Postscript (portrait mode).
  cps   Color postscript (landscape mode).
  gif   gif files. Only supported with g77, not gfortran.
  vcps  Color postscript (portrait mode).
  tek   Tektronix 4010 or equivalent.
  xterm Xterm window (better for simple plots over slow X links).

A `?' as device name generates a complete list of the recognized
types (which will most likely vary from system to system).

Files created for hardcopy devices must be spooled to the printer
separately. The command needed for this is site and device specific.

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