; PBCOR ;--------------------------------------------------------------- ;! Task to apply or correct an image for a primary beam ;# TASK IMAGING ANALYSIS ;----------------------------------------------------------------------- ;; Copyright (C) 1995, 1997, 2000-2001, 2003-2005, 2009 ;; Associated Universities, Inc. Washington DC, USA. ;; ;; This program is free software; you can redistribute it and/or ;; modify it under the terms of the GNU General Public License as ;; published by the Free Software Foundation; either version 2 of ;; the License, or (at your option) any later version. ;; ;; This program is distributed in the hope that it will be useful, ;; but WITHOUT ANY WARRANTY; without even the implied warranty of ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the ;; GNU General Public License for more details. ;; ;; You should have received a copy of the GNU General Public ;; License along with this program; if not, write to the Free ;; Software Foundation, Inc., 675 Massachusetts Ave, Cambridge, ;; MA 02139, USA. ;; ;; Correspondence concerning AIPS should be addressed as follows: ;; Internet email: aipsmail@nrao.edu. ;; Postal address: AIPS Project Office ;; National Radio Astronomy Observatory ;; 520 Edgemont Road ;; Charlottesville, VA 22903-2475 USA ;----------------------------------------------------------------------- PBCOR LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC PBCOR: Task to apply the primary beam correction 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 drive # BLC 0.0 4096.0 Bottom Left Corner TRC 0.0 4096.0 Top Right Corner OUTNAME Output image name (name) OUTCLASS Output image name (class) OUTSEQ -1.0 9999.0 Output image name (seq. #) OUTDISK 0.0 9.0 Output disk drive # PBPARM Beam parameters: (1) Cutoff 0 -> 0.023 (2) > 0 -> Use (3)-(7) (3)-(7) Beam shape COORDINA RA and DEC in form: HH MM SS.SS, DD MM SS.S ** used if not all 0 ** DOINVERS -1.0 1.0 > 0 => multiply image by beam ---------------------------------------------------------------- PBCOR Type: Task Use: PBCOR will apply a primary beam correction (divide by the primary beam gain factor) to images. It can also multiply an image by a primary beam pattern to, for example, compare an optical or single-dish image with an interferometer image. (One multiplies the former by the beam of the latter rather than raising the appearance of noise in the latter by dividing by its beam.) The image is required to have RA, DEC and FREQ axes in order to compute the correction. However, they may be in any order and "cubes" are fully supported. Adverbs: INNAME......Input name of image(name). Standard defaults. INCLASS.....Input name of image(class). Standard defaults. INSEQ.......Input name of image(seq. #). 0 => highest. INDISK......Disk drive # of image. 0 => any. BLC.........The bottom left-hand pixel of the input image which becomes the bottom left corner of the corrected subimage. The value (0,0) means (1,1). TRC.........The top right-hand pixel of the input image which becomes the top right corner of the correct- ed subimage. The value (0,0) means take the top right hand corner of the image. OUTNAME.....Output name of image(name). Standard defaults. OUTCLASS....Output name of image(class). Standard defaults. OUTSEQ......Output name of image(seq. #). 0 => highest unique. OUTDISK.....Disk drive # of Output image. 0 => highest w room. PBPARM......Primary beam parameters: (1) Lowest beam value to believe: <= 0.001 -> 0.023 Image pixels outside this range are blanked. (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. For the VLA, these parms are, by default, 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 See explain for details COORDINA....The RA and DEC coordinates for the pointing position are found as: RA = abs(CO(1)) + abs(CO(2))/60 + abs(CO(3))/3600 if any of CO(1), CO(2), CO(3) < 0: Xpos = -Xpos DEC = abs(CO(4)) + abs(CO(5))/60 + abs(CO(6))/3600 if any of CO(4), CO(5), CO(6) < 0: Ypos = -Ypos RA is in hours, minutes, seconds of time DEC is in degrees, minutes, seconds of arc WARNING: if any of these 6 are not zero, then COORDINA defines the pointing direction for the image. Otherwise the pointing position in the header (or if that is 0, the source coordinate in the header) is used. DOINVERS....> 0 Multiply the image by the beam pattern. <= 0 Divide the image by the beam pattern. ---------------------------------------------------------------- PBCOR: Task to apply the primary beam correction RELATED PROGRAMS: PRTIM PURPOSE PBCOR corrects an image for the primary beam attenuation of the 25-meter antennas used at the VLA. 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% 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%. The cutoff level may be specified with DPARM(1). RICK PERLEY'S 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% 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. COMMENTS COORDINA: If COORDINA is all zero, the pointing position is taken to be the pointing position given in the header or, if that is 0, the position of the reference pixel (equal to the map making position which is equal to the phase tracking position if the map was made with no shift). Any other pointing position can be specified in COORDINA, an array of dimension 6 in the format COORDINA= HH MM SS.SS DD MM SS.SS where the RA is HHh MMm SS.SSs and the DEC is DDd MM' SS.SS". Set some or all of COORDINA to non-zero to have it used by the program. ACCURACY: The accuracy of the correction is about 1% within the 50% primary beam sensitivity and about 2% beyond this region. Thus the errors of the correction F(x) become quite large near the edges of the primary beam response. WHEN TO USE PBCOR: Do NOT use PBCOR on the dirty map before running APCLN or any of the other deconvolution processes. PBCOR is usually run near the end of data processing. It may now be run on data "cubes" but it does require that frequency, right ascension, and declination axes appear in the header. The speed of the task is dependent on the axis order and the accuracy of the position computations. In general, with linear computations, the RA-DEC-FREQ order of axes is faster. With non-linear position computations (needed for 1-degree fields and larger), the FREQ-RA-DEC order appears to be significantly faster than RA-DEC-FREQ. Any order will give correct results, however, and non-linear is always rather slower than linear.