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POSTAGESTAMP

POSTAGESTAMP

Name


  POSTAGESTAMP

Purpose


  This function extracts a subregion from a 2 or 3-dimensional FITS
  image. It uses a nearest-neighbor regridding scheme.

Category


  image processing

Calling Sequence


  result=POSTAGESTAMP(im, head, cen, wid, scale, [/NAN])

Inputs


  IM: A 2- or 3-dimensional image array
  HEAD: The FITS header corresponding to IM (as is generated by
        MRDFITS, for example)
  CEN: A 2 or 3 element array giving the center of the desired
      subfield in 'real sky units' (i.e. whatever real sky units are
      assumed within the CRVAL keywords of HEAD)
  WID: A 2 or 3 element array giving the size of the extracted
      subfield in 'real sky units' (see note for CEN)
  SCALE: A 2 or 3 element array giving the size of the output pixels
        in real sky units (whatever real sky units are assumed in
        the CD or CDELT keywords of HEAD)

Keyword Parameters


  NAN: If set and nonzero, regions of the subimage which lie outside
  the boundaries of the input image are output as NaN. Otherwise,
  they are output as zero.
  NPIX: If set, this is a 2 or 3 element array giving the pixel size
  of the output image. It replaces the role of the WID parameter.
  OUTHEAD: A named variable to hold the postage stamp header

Outputs


  The postage stamp described by cen, wid, and scale

Example


  Given im and head, extract a .25 x .25 degree field centered at
  (a,d)=(10.,20.) where each pixel is .002 degrees
    result=postagestamp(in,head,[10.,20.],[.25,.25],.[.002,.002])
 
  Repeat, but this time preserve the convention that RA increases to
  the left in sky images:
    result=postagestamp(in,head,[10.,20.],[.25,.25],[-.002,.002])

Restrictions


  The coordinates of CEN will always be the exact center of the
  output image, and the pixels will always be exactly the size given
  by scale. However, the output images have an odd number of pixels
  along each axis, and thus may be a pixel or two different from the
  size requested by WID.
  This function uses SKY2PIX, which provides rather limited
  astrometry capabilities (e.g. no corrections for field curvature or
  other higher order distortion terms). If you need more robust
  astrometry, consider using the IDL astro library routine
  HEXTRACT. However, that routine will not handle 3D images.

Modification History


  Written by: Chris Beaumont August 9, 2008
  December 17 2008: Added NPIX keyword. cnb
  Feb 16, 2009: Added a warning if the requested postage stamp lies
                outside the bounds of the input image. cnb.
  Feb 16, 2009: Added the OUTHEAD keyword. cnb.



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