newgeom stsdas.hst_calib.foc

NAME · USAGE · DESCRIPTION · PARAMETERS · EXAMPLES · BUGS · REFERENCES
SEE_ALSO

NAME

newgeom -- Perform geometric correction.

USAGE

newgeom input output geo_file

DESCRIPTION

This task performs geometric correction of FOC images using the algorithm developed by Robert Jedrzejewski and Bill Sparks. This is a replacement for the geomcorrx task. The GEOCORR header parameter in the output image will be set to "COMPLETE" when the task finishes.

The pixel coordinate convention used by p2geo and newgeom differs from the standard IRAF convention. Normally in IRAF, an integer coordinate refers to the center of the pixel, so the first pixel spans the range from 0.5 to 1.5. The convention adopted by p2geo and newgeom is that the first pixel runs from zero to one. For example, the actual pixel values in a geometric correction reference file with the identity mapping for a 2 x 2 image are as follows:

``` Sample:     1       2       3       4       5       6
Line
3      0.      1.      2.      2.      2.      2.

2      0.      1.      2.      1.      1.      1.

1      0.      1.      2.      0.      0.      0.
```

Each line of the geometric reference file gives the X coordinates of pixel corners followed by the Y coordinates of those corners. Note that the values are the coordinates in the input, distorted image, for each pixel of the output, undistorted image. For example, line two above gives the X coordinates and Y coordinates in the input image for the top of the first line (or the bottom of the second line) of the 2 x 2 output image. There are three such corners, with coordinates (0,1), (1,1) and (2,1).

PARAMETERS

input = "" [file name]
Image to be geometrically corrected.
output = "" [file name]
Output (corrected) image.
geo_file = "" [file name]
Geometric correction reference file. This is an image which can be created from a polynomial fit file using the task p2geo. See the DESCRIPTION section above and also the help for p2geo for further information about geo_file.
(defval = 0.) [real]
Default output value. If a given output pixel corresponds (via the geometric distortion) to a point that is outside the input image, defval will be assigned to that output pixel.
(coord_trans = "set_values") [string, allowed values: set_values |
apply_distortion | none]

How should the coordinate parameters be modified?

The default value of "set_values" is usually appropriate. With this option certain coordinate parameters in the image header are assigned specific values, overriding previous values. For long-slit spectrographic mode, all the coordinate parameters are replaced by values taken from the header of the geometric correction reference image. This is done because after geometric correction the coordinate type for the second axis (Y) is wavelength. For normal image mode, the CD matrix is replaced by values computed from the image position angle and pixel spacing. The pixel spacing is taken from the IMSCALE keyword in the geometric correction reference image; the CD matrix is not modified if IMSCALE is missing. The image orientation is taken from the ORIENTAT keyword in the input image (or from the input CD matrix if ORIENTAT is missing). Using PA for the position angle of the image Y axis (line direction) measured eastward from north, and using IMSCALE for the pixel spacing, the CD matrix is computed as follows:

```	CD1_1 = -IMSCALE * cos (PA)
CD1_2 = IMSCALE * sin (PA)
CD2_1 = IMSCALE * sin (PA)
CD2_2 = IMSCALE * cos (PA)
```

If the coordinate parameters of the input image really are a fair representation of the coordinate system of that image, then an alternative is to modify CRPIX1, CRPIX2, and the CD matrix of the geometrically corrected image by amounts that depend on the geometric distortion. To make this change, set coord_trans to "apply_distortion".

If the coordinate parameters should not be modified at all, set coord_trans to "none".

(box_check = no) [boolean]
Check computation of box overlap?
(verbose = yes) [boolean]
Print percent done?

EXAMPLES

1. Geometrically correct the image "x0dw0103t.d0h" using the geometric reference file "b191525mx.r5h", producing output image "x0dw0103_gc.hhh".

```   fo> newgeom x0dw0103t.d0h x0dw0103_gc.hhh b191525mx.r5h
```

REFERENCES

This task was written by Robert Jedrzejewski, with a subroutine by Bill Sparks. The polynomial evaluation routine was extracted to the separate task p2geo by Phil Hodge.