AIPS NRAO AIPS HELP file for FSHIFT in 31DEC18



As of Tue Sep 18 9:09:35 2018



HELP SECTION

FSHIFT
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
       FSHIFT distances measured from an image are NOT exactly the
       number of pixels of offset multiplied by the pixel scale.

       This version of the SHIFT adverb is used for the output from
       operations that fit a source model.

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.

Verbs:
   MAXFIT...returns pixel position and image intensity at a maximum
            FSHIFT is the ra/dec shift needed to put the peak on the
            nearest integer pixel.
Tasks:
   IMFIT....Fits Gaussians to portions of an image
            FSHIFT is the ra/dec shift needed to put the peak on the
            nearest integer pixel.
   JMFIT....Fits Gaussians to portions of an image
            FSHIFT is the ra/dec shift needed to put the peak on the
            nearest integer pixel.

EXPLAIN SECTION


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