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Basic Information on imhol
Task: imhol
Purpose: Compute amplitude and phase images from real and imaginary
Categories: image analysis
IMHOL computes amplitude and phase images from real and
imaginary holographic images. The amplitude image can be debiased,
and the phase image is computed as atan2(imaginary/real).
Key: in
Two values; the real and imaginary images, respectively. The
imaginary image can be made using the INVERT task with options=imaginary
If the (u,v) coordinates for the holography data are in arcsec units
then the images have units of nanosec (The inverse of the usual
situation for astronomical imaging).
Alternatively, the real and imaginary images can be obtained from
images of the beam pattern using the MIRIAD task FFT.
The default units for the axes are in wavelengths.
Wild card expansion is supported.
Key: mag
Up to two values; the output intensity image and
optionally, its associated error image (which will be constant).
Default is no output images.
Key: phase
Up to two values; the output position angle image and optionally,
its associated error image (which will not be constant). These
will be in degress, radians or microns (see OPTIONS).
Default is no output images.
Key: sigma
The mean standard deviation of the noise in the images.
Required when debiasing or blanking; try to make it as accurate
a value as possible. The default is 0.
Key: sncut
This is the S/N ratio below which the output images
are blanked (see also options=zero below). It is STRONGLY
recommended that SNCUT of at least 2 is used.
The default is 0.
Key: pacut
The output images are blanked if the error in the phase
image (degrees, radians or mircrons depending on OPTIONS) is greater
than this value. This is active even if you don't output
the PA image. Note that there is no equivalent for the output
error of the POLI image because the error is constant and
equal to SIGMA. Keyword SNCUT essentially takes care of this.
The default is no position angle error based blanking.
Key: options
Task enrichment options. Minimum match is active.
bias If computing polarized intensity, do NOT remove the Ricean
bias in the image. By default, the bias is removed to first
order with P = sqrt(P_obs**2 - sigma**2) You should have
a very good reason for using this option. See VLA memo
no. 161 by Patrick Leahy for more details of bias removal.
zero When the output pixel is clipped, by setting CLIP(1),
setting OPTIONS=ZERO will cause the output polarized
intensity image (not the position angle image) pixel to
be set to 0.0 rather than being masked out. This is
very important if you are interested in doing statistics
on a region of low polarized intensity S/N ratio. If
you leave the region masked rather than zeroed, you will
bias the statistics in that region -- zero is a better
estimate of the pixel than just excluding it from the
statistics (provided the clip level is sufficiently small).
Residual bias in the statistical results from the area then
depend upon how well the bias remover works and at what
level clipping was performed. See VLA memo no. 161
by Patrick Leahy.
radians Output the phase image in radians instead of degrees.
microns Output the phase image as equivalent surface error in
microns.
relax Only warn about image axis descriptor mismatches
instead of giving a fatal error
bmfit Fit focus and pointing offsets to aperture E-field maps.