; SHIFT ;--------------------------------------------------------------- ;! specifies a position shift ;# ADVERB IMAGING COORDINATES ;----------------------------------------------------------------------- ;; Copyright (C) 1995, 1999, 2008 ;; 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 ;----------------------------------------------------------------------- SHIFT LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC ---------------------------------------------------------------- SHIFT Type: Adverb (Real array, 2 elements) Use: Specifies a change or shift of an image location. The first element specifies a shift Eastward (lower X-pixel value). The second element specifies a shift Northward (larger Y-pixel value). Thus if both are > 0, the resulting map center is to the Northeast of the input (or the source appears to move down and to the right). The shift is no longer in SIN projected coordinates, so the SHIFT distances measured from an image are NOT exaxctly the number of pixels of offset multiplied by the pixel scale. Null Value: NONE ( 0 => no shift) The shifts are now (15OCT99) simpler to specify since they are in arc seconds at the reference point. They are simply the apparent difference in position on the image ( (Xpix - Xpix_0) * Cell_X, (Ypix - Ypix_0) * Cell_Y ) at least for small differences. For large shifts, use IMXY to give you RA and Dec and then compute the shifts as: SHIFT(1) = cos (Dec_0) * (RA - RA_0) SHIFT(2) = Dec - Dec_0 in arc seconds. The _0 refers to the reference coordinate. ----------------------------------------------------------------