; OHGEO ;--------------------------------------------------------------- ;! Geometric interpolation with correction for 3-D effects ;# Task IMAGE-UTIL OOP ;----------------------------------------------------------------------- ;; Copyright (C) 1995-1996, 1998, 2000, 2005 ;; 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 ;----------------------------------------------------------------------- OHGEO LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC OHGEO: Interpolate an image to the geometry of another. Input image INNAME Image name (name) INCLASS Image name (class) INSEQ 0.0 9999.0 Image name (seq. #) INDISK 0.0 9.0 Image disk drive # Image defining geometry IN2NAME Image name (name) IN2CLASS Image name (class) IN2SEQ 0.0 9999.0 Image name (seq. #) IN2DISK 0.0 9.0 Image disk drive # Output image OUTNAME Image name (name) OUTCLASS Image name (class) OUTSEQ -1.0 9999.0 Image name (seq. #) OUTDISK 0.0 9.0 Image disk drive # BLC 0.0 4096.0 Bottom left corner of image 0=>entire image TRC 0.0 4096.0 Top right corner of image 0=>entire image IMSIZE 0.0 4096.0 Output image size in pixels REWEIGHT 0.0 4.0 (1) Interpolation halfwidth (2) Minimum fraction of good pixels required (0->1/3) AXREF X axis reference pixel AX2REF Y axis reference pixel APARM (1) >0 => do 3-D corr. (2) Parallactic angle (deg) (3) Zenith angle (deg) (4-8) radial scaling parms (9) Linear scaling ---------------------------------------------------------------- OHGEO Type: Task Use: OHGEO does an interpolation of one image to the geometry defined by another. Optionally corrections can be made for 3-D distortions caused by a misaligned, but coplanar array (e.g. VLA snapshots). Another option is radial scaling of the image to correct for the interaction of a finite bandpass and the antenna primary beam size. Interpolation is done only in the first 2 dimensions. Unlike HGEOM, OHGEO will interpolate over blanked pixels so that it can fill in small blanked regions and handle edges without having to discard image area. NOTE: the input subimage is read into dynamically allocated memory. Very large input arrays may cause swapping on your computer. This task does a reasonably straightforward interpolation from the input to the output image grid. This process cannot be completely accurate if, for example, one shifts and scales an image and then reverses that process. Greater accuracy will be achieved on images with more points per beam and with larger support sizes for the interpolator. Adverbs: INNAME......The input image name. Standard defaults. INCLASS.....The input image class. Standard defaults. INSEQ.......The input image sequence number. 0 => high INDISK......The input image disk drive no. 0 => any IN2NAME.....The template image name. blank => actual IN2CLASS....The template image class. Standard defaults IN2SEQ......The template image seq no. 0 => any IN2DISK.....The template image disk drive no. 0 => any OUTNAME.....The output image name. blank => Standard defaults based on INNAME. OUTCLASS....The output image class. Standard behavior. OUTSEQ......The output image seq. no., 0=> highest unique If >0; image will be created if new, overwritten if image name exists. OUTDISK.....Output disk drive no., 0=> highest with space BLC..........The bottom left-hand pixel of the input image which becomes the bottom left corner of the input 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 subimage. The value (0,0) means take the top right hand corner of the image. IMSIZE.......Output image size in pixels [1=columns, 2=rows] Default is second input image size. The output size is independent of the input size. If IMSIZE is all 0 and AXREF and AX2REF are zero, then AXREF and AX2REF are taken from the 2nd image. REWEIGHT.....Interpolation kernal parameters: (1) Half width of the interpolating kernel (1 - 4). Default = 1 Larger support sizes should produce more accurate results at the cost of increased computation. (2) Minimum fraction of pixels in interpolation kernal area required for non-blanked output. <= 0 or >= 1 => 0.333 AXREF........X-axis reference pixel; AX2REF.......Y-axis reference pixel; If BOTH AXREF and AX2REF are zero, but IMSIZE is not, then the numeric center of the input (sub)image is placed at the numeric center of the output image. If IMSIZE is all 0 and both AXREF and AX2REF are zero, then AXREF and AX2REF are taken as the refernce pixels of the 2nd image. APARM........Transformation parameters: (1) = if > 0 then apply 3-D corrections SEE EXPLAIN OHGEO (2) Parallactic angle for 3-D correction if not already a header keyword. (degrees) (3) Zenith angle for 3-D correction if not already a header keyword. (degrees) (4-8) Parameters for radial scaling for primary beam effects. (4) = Antenna FWHM at nominal sky frequency (deg) 0 => no scaling. (5) = Fractional bandwidth (6-8) C1, C2, C3 SEE EXPLAIN OHGEO (9) Linear scaling factor, 0 => 1.0 SEE EXPLAIN OHGEO ---------------------------------------------------------------- OHGEO: Task to interpolate one image to the geometry of another Documentor: W. D. Cotton, NRAO Related Programs: HGEOM, UVADC, WFCLN 3-D Corrections: Corrections can be made for the distortion of an image made by a coplanar array which is not normal to the field center. This case includes snapshots made with the VLA or syntheses made with an east-west interferometer using u, v and ws in the sine (????-SIN) projection. In these cases the array elements are confined to a plane (or are nearly so) but the normal to this plane is oriented in a direction other than the center of the image produced. This will cause a distortion of the geometry but not of the image. OHGEO will correct for this distortion if APARM(1) > 0 and two parameters ("parallactic" angle and "zenith" angle are provided. These are the parallactic and zenith angles of the image center. For east-west arrays these values are from a "zenith" of the appropriate celestial pole; for the VLA these are with respect to the instrumental zenith. The parallactic and zenith angles can be provided in degrees as either catalog header keywords 'PARANGLE' and 'ZENANGLE' or as APARM(2&3) in which case they will be converted into header keywords. Note: for VLA snapshots task WFCLN can provide the necessary catalog keywords. The 3D correction is based on the distance from the tangent point which may be the same as the pointing position (DO3DIMAG false) or different (DO3DIMAG true in IMAGR). The coordinate reference pixel must give the tangent point position. The x and y coordinates must be RA and Declination or vice versa or this program will not work properly. Note: this correction will not correct for image distortion caused by a noncoplanar array such as an image made from VLA data consisting of multiple snapshots or an extended synthesis. Radial Scaling for Primary Beam Effects. In synthesis observations the variation of the primary antenna gain over the observed bandpass can cause the effective observing frequency to vary radially from the antenna pointing position. This will cause a radial variation in apparent image scale; usually a contraction of the scale with increasing radius. A correction can be made for this using parameters APARM(4-8). Note there may also be a constant scale error due to an incorrect assumed central frequency. This correction will not correct for this effect. The radial corrections are based on the position offset from the original pointing center and this information MUST be in the catalog header. GETHEAD can obtain these values using keywords 'OBSRA' and 'OBSDEC' to see if non-zero values are present. APARM(4) If APARM(4) is larger than zero the a radial scaling is done. This value is the antenna primary beam FWHM in degrees at the nomimal sky frequency. For the VLA (25 m antenna) this is 7.203E8/Freq (Hz). APARM(5) This value is the fractional bandpass which is the true bandpass divided by the nominal frequency. APARM(6-8) These coefficients, called C1, C2 and C3, parameterize the beam shape. For a Gaussian beam C1 = 2*log(2)/3 = 0.4621, C2=0 and C3=0. For a uniformly illuminated circular aperture (a good approximation for the VLA) C1=0.46, C2=0 and C3=0.58 gives a good approximation out to a distance of FWHM. NOTE: THIS CORRECTION CAN GIVE VERY VERY WRONG ANSWERS OUTSIDE OF THE FWHM!!! Linear scaling: An error in the assumed center frequency of data used to make a synthesis image will cause a misscaling of the image as discussed in the previous section. If the assumed bandpass shape is incorrect there will be a constant scaling error over the entire image. This effect can be corrected in a number of other tasks (UVADC, WFCLN) and if this correction has already been applied it should NOT be reapplied here. This factor is the ratio of the true centroid frequency to the assumed frequency. If APARM(9) is greater than 0 then it is used as an overall scaling factor which is used in addition to any scaling from APARM(4-8). On the Relation of Input Image Size to Output Image Size: The adverb IMSIZE specifies the dimensions of the output image. If it is set to zero then the default applies and the output image is the same size as the second input. But if it is nonzero it means EXACTLY what it says: the output will have dimensions specified by IMSIZE regardless of what the dimensions of the input image happen to be. Suppose the input image is 800-square and BLC and TRC are zero (i.e., all of the image). If IMSIZE is 100-square then the output is 100-square. On the other hand, if IMSIZE is bigger than the input, say 1000-square, the result is that the input images to the output and regions which are outside the input image will be blank in the output. Note that the portion of the input which is used is delimited by BLC and TRC completely independently of IMSIZE. The pixel position of the "reference pixel" is specified by the header of the second image. It is not affected by BLC, TRC. If IMSIZE is smaller than the second input image then the output image will be centered on the center (NOT reference pixel) of the input subimage (as defined by BLC, TRC). This will affect the value of the reference pixel which in this case may not be inside the output image.