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

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.

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.