AIPS HELP file for XMOM in 31DEC24
As of Sat Apr 20 1:39:16 2024
XMOM: Finds the moments of each row of an image
INPUTS
INNAME Input image name (name)
INCLASS Input image name (class)
INSEQ 0.0 9999.0 Input image name (seq. #)
INDISK 0.0 9.0 Input image disk unit #
OUTNAME Output image name (name)
OUTSEQ -1.0 9999.0 Output image name (seq. #)
OUTDISK 0.0 9.0 Output image disk unit #.
BLC Bottom left corner of input
TRC Top right corner of input
FLUX Use only data > FLUX
ICUT Use only data > in abs value
than ICUT (> 0). Use only
data < in abs value than
ICUT (< 0.).
OPTYPE '': blank illegal velocities
'MAX' do max instead of moms
PBPARM Beam parameters:
(1) Cutoff: 0 -> no PB corr
(2) > 0 -> Use (3)-(7)
(3)-(7) Beam shape
BADDISK Disk to avoid for scratch
HELP SECTION
XMOM
Task: Fits one-dimensional moments to each row of an image. (Normally
XMOM will be used on images with frequency or velocity as the
first axis, but it will proceed with others as well.) The task
fits 4 moments to each row and writes 5 n-1 dimensional images
containing the moments and a count of the number of data samples
used. XMOM provides only very elementary blanking capabilities.
Normally, users will wish to do more elaborate blanking with
BLANK before running XMOM. The high order moments are evaluated
around the 1st moment (e.g. central velocity). Task MOMNT
performs a similar function after smoothing the input image in 3
dimensions. This does make the flux cutoff more meaningful, but
enormously more expensive. XMOM is faster and will run on all
AIPS systems.
Users should be aware that the image of the first moment is in
single-precision floating point. If the first axis is frequency,
there may be not be enough accuracy to represent the variation in
frequency about some very high central frequency. The task will
subtract the central value from the image of the first moment
whenever the difference in the axis values from one end to the
other is < 10**-3 of the central value. NOTE: THIS PRODUCES AN
INCORRECT FIRST MOMENT SINCE AIPS HEADERS NO LONGER SUPPORT THE
CONCEPT OF A BIAS AND SCALE.
XMOM offers the option of writing an image of the maximum value
found on each X axis and of the pixel coordinate at that
point. The moment images are not computed or written. All of
the cutoff and primary beam adverbs still apply in the 'MAX'
OPTYPE.
Adverbs:
INNAME.....Input image name (name). Standard defaults.
INCLASS....Input image name (class). Standard defaults.
INSEQ......Input image name (seq. #). 0 => highest.
INDISK.....Disk drive # of input image. 0 => any.
OUTNAME....Output image name (name). Standard defaults.
The OUTCLASSes for the moments are XMOMn, for n
= 0 through 3. The count image uses XMOMNC.
OUTSEQ.....Output image name (seq. #). 0 => highest unique.
OUTDISK....Disk drive # of output image. 0 => highest
number with sufficient space.
BLC........Bottom right corner in input image of desired
subimage. Default is entire image.
TRC........Top right corner in input image of desired
subimage. Default is entire image.
FLUX.......A flux cutoff in the same units as the input image (i.e.
Jy/beam). Data values below FLUX are ignored in the moment
computation. NOTE that 0.0 is not a null value. Instead,
it means ignore all negative brightnesses.
ICUT.......A flux cutoff in the same units as the input image (i.e.
Jy/beam). When ICUT > 0.0, data values less in absolute
value than ICUT are ignored. When ICUT < 0.0, data values
greater in absolute value than ICUT are ignored. NOTE that
ICUT and FLUX are both always used.
OPTYPE.....'' : blank illegal first moments. 'NBIV': do not blank
illegal first moments. First moments are illegal when they
fall outside the range of input channels. The blanking in
XMOM would cause such a pixel to be blanked in ALL moment
maps. Especially for zero moment maps, one may want to
keep such a pixel to avoid "ugly holes." Note that the use
of FLUX = 0 forces all first moments to be legal.
OPTYPE = 'MAX' means to write out an image containing the
maximum brightness found along the X axis and an image of
the X coordinate value at that pixel. The moment images
are not computed.
PBPARM.....Primary beam parameters: Adjust the cutoff levels to
account for the primary beam.
(1) Lowest beam value to believe: 0 -> do not do a
primary beam correction. The maximum correction is
a factor of 100.
(2) > 0 => Use beam parameters from PBPARM(3)-PBPARM(7)
Otherwise use default parameters for the VLA (or
ATCA where appropriate)
(3-7)..For all wavelengths, the beam is described by the
function:
1.0 + X*PBPARM(3)/(10**3) + X*X*PBPARM(4)/(10**7) +
X*X*X*PBPARM(5)/(10**10) + X*X*X*X*PBPARM(6)/(10**13)
X*X*X*X*X*PBPARM(7)/(10**16)
where X is (distance from the pointing position in arc
minutes times the frequency in GHz)**2.
See explain for details
BADDISK....Disk drives to avoid for scratch files.
EXPLAIN SECTION
Using only those pixels included above the cutoffs, XMOM computes
Moment 0: S0 = Sum (T(i))
Moment 1: S1 = Sum (T(i) * i) / S0
Moment 2: S2 = sqrt (Sum (T(i) * i * i) / S0 - S1 * S1)
Moment 3: S3 = Sum (T(i) * i * i * i) / S0
S3 = S3 - 3 * S1 * S2 + 2 * S1*S1*S1
S3 = S3 ^(1/3)
XMOM has the option of scaling the cutoff values on a
pixel-by-pixel basis to "correct" for the primary beam. Thus, as the
beam value goes down the cutoff value goes up. This allows XMOM to be
run on data cubes after the application of PBCOR. Since the primary
beam is a function of frequency, the spectral moments are affected by
the primary beam correction. Unfortunately this correction also
raises the noise, making the option to raise the cutoff useful.
XMOM corrects an image for the primary beam attenuation of
the antennas. The function used to model the primary beam for normal
VLA frequencies
F(x) = 1.0
+ parm(3) * 10E-3 * x
+ parm(4) * 10E-7 * x*x
+ parm(5) * 10E-10 * x*x*x
+ parm(6) * 10E-13 * x*x*x*x
+ parm(7) * 10E-16 * x*x*x*x*x
where x is proportional to the square of the distance from the
pointing position in units of [arcmin * freq (GHz)]**2, and F(x)
is the multiplicative factor to divide into the image intensity at the
distance parameter x. For other antennas, the user may read
in appropraite constants in PBPARM(3) through PBPARM(7). The
flag, PBPARM(2) must be set to a positive number to invoke this
option and PBPARM(3) must not be zero.
This correction scales with frequency and has a cutoff
beyond which the map values are set to an undefined pixel value GIVEN
in PBPARM(1). At the VLA frequencies the default cutoff is
1.485 GHz 29.8 arcmin
4.885 GHz 9.13 arcmin
15 GHz 2.95 arcmin
22.5 GHz 1.97 arcmin
and occurs at a primary beam sensitivity of 2.3 percent of the value at
the beam center. Corrections factors < 1 are forced to be 1.
The estimated error of the algorithm is about 0.02 in (1/F(x))
and thus leads to very large errors for x>1500, or at areas
outside of the primary response of 20 percent. The cutoff level
may be specified with DPARM(1).
Default values of PBPARM for the VLA are given by Perley's fits:
0.0738 GHz -0.897 2.71 -0.242
0.3275 -0.935 3.23 -0.378
1.465 -1.343 6.579 -1.186
4.885 -1.372 6.940 -1.309
8.435 -1.306 6.253 -1.100
14.965 -1.305 6.155 -1.030
22.485 -1.417 7.332 -1.352
43.315 -1.321 6.185 -0.983
For the ATCA, these are by default:
1.5 GHz -1.049 4.238 -0.8473 0.09073 -5.004E-3
2.35 -0.9942 3.932 -0.7772 0.08239 -4.429E-3
5.5 -1.075 4.651 -1.035 0.12274 -6.125E-3
8.6 -0.9778 3.875 -0.8068 0.09414 -5.841E-3
20.5 -0.9579 3.228 -0.3807 0.0 0.0
For the Karl G Jansky VLA ("EVLA"), the defaults are frequency
dependent. If the observing frequency is between two tabulated
frequencies, then the beam is computed for each of the tabulated
frequencies and then interpolated to the observing frequency. The
values used are far too numerous to give here, see EVLA Memo 195,
"Jansky Very Large Array Primary Beam Characteristics" by Rick Perley,
revision dated June 2016. Obtain it from
http://library.nrao.edu/evla.shtml
RICK PERLEY'S (OLD) REPORT
Polynomial Coefficients from LSq Fit to VLA Primary
Beam raster scans.
Functional form fitted:
1 + G1.X^2 + G2.X^4 + G3.X^6
where X = r.F,
and r = radius in arcminutes
F = frequency in GHz.
Fits were made to 3 percent cutoff in power for 24 antennas.
Poor fits, and discrepant fits were discarded, and the most
consistent subset of antennas had their fitted coefficients
averaged to produce the following 'best' coefficients.
Freq. G1 G2 G3
1.285 -1.329E-3 6.445E-7 -1.146E-10 *
1.465 -1.343 6.579 -1.186 "
4.885 -1.372 6.940 -1.309
8.435 -1.306 6.253 -1.100
14.965 -1.305 6.155 -1.030
22.485 (old) -1.350 6.526 -1.090 *
22.485 (new) -1.417 7.332 -1.352
43.315 -1.321 6.185 -0.983
The estimated errors (from the scatter in the fitted
coefficients) are generally very small:
G1: .003 at all bands except Q (.014)
G2: .03 to .07 at all bands except Q (.15)
G3: .01 to .02 at all bands except Q (.04)
R. Perley 21/Nov/00
* The 1.285 and 22.485 old feed values are not used.