NAME

wcs2dr -- Convert WCS header information into drizzle parameters

          for image registration

USAGE

wcs2dr image refim

PARAMETERS

images = "" [file name]

A list of input image files which are to be registered with the reference image. Only the image header is read from these to obtain the WCS information. This list can be specified using wildcards, a comma-separated list or by using an "@file".

refim = "" [file name]

The name of the reference image file with which the input image is to be registered. Only the image header is read to obtain the WCS information.

(style = "line") [string]

The style for the output listing. It can be either "para" in which case the result will be a series of lines each of which can be given to the IRAF cl to set the appropriate drizzle parameter or "line" in which case the results will be displayed as a table with column headings.

DESCRIPTION

This task uses the world coordinate information in the headers of a list of images and reference image to deduce the parameters which must be given to the "drizzle" task to register the input images with the reference.

Only the linear part of the WCS is used. This is assumed to consist of eight values - CRVAL1/2, CRPIX1/2 and the four elements of the CD matrix (CD1/2_1/2). It is further assumed that the two images are in TAN projection (although nor necessarily with the same reference point).

The transform is deduced by starting with a unit square at the center of the input image. The position of each corner of this square is then rigorously transformed, via equatorial coodinates, into a pixel position on the reference grid. A check is made to see whether the result is significantly non-square (ie, where there are differences in the X and Y scale or skew is present) and if so the user is informed. The shift of the center of the pixel, along with the measured rotation and scale change are then displayed in the form which the drizzle task needs. The user is also informed the values for the drizzle parameters shft_un, shft_fr and align which must be used.

EXAMPLES

If "files" is a list of images which are to be aligned with the reference
image "ref" then suitable drizzle parameters can be deduced using:

cl> wcs2dr @files ref style="para"
#
drizzle.data="test1"
drizzle.shft_fr="output"
drizzle.shft_un="output"
drizzle.align="center"
drizzle.xsh=   552.733
drizzle.ysh=   482.749
drizzle.scale=     1.009
drizzle.rot=    -2.871
#
drizzle.data="test2"
drizzle.shft_fr="output"
drizzle.shft_un="output"
drizzle.align="center"
drizzle.xsh=   532.142
drizzle.ysh=   498.043
drizzle.scale=     1.009
drizzle.rot=    -2.871
#
drizzle.data="test3"
drizzle.shft_fr="output"
drizzle.shft_un="output"
drizzle.align="center"
drizzle.xsh=   519.552
drizzle.ysh=   378.566 
drizzle.scale=     1.009
drizzle.rot=    -2.871

or alternatively:

cl> wcs2dr test ref style="line"
# Image             xsh     ysh      scale    rot
# Note: shft_fr="output" shft_un="output" align="center"
test1            552.733  482.749    1.009   -2.871
test2            532.142  498.043    1.009   -2.871
test3            519.552  378.566    1.009   -2.871
cl> 

where in this case "files" contains:

test1
test2
test3

TIMINGS

It is, as one would expect, very quick!

BUGS

1. Only the linear WCS is considered.

2. Only the TAN projection can be handled.

SEE ALSO

drizzle, traxy, tranback