compare

NAME

compare -- align the surface of images using a polynomial fit


USAGE

compare [flags] image(s)

kappa k 4.0
nan n 0.0
order i 2
radius l 10
reference f
region r 0:0,0:0
verbose v 0


PARAMETERS

kappa = 4.0
Cutting parameter. Number of sigmas for clipping outlying pixels (caused e.g. by stars).
nan = 0.0
Not a number value. Pixel with this value are not taken into account and stay at "nan" in the output image.
order = 2
Order of polynomial fit:
0: const.
1: const. + a*x1 + b*y1
2: const. + a*x1 + b*y1 + c*x2 + d*x1y1 + e*y2
...
radius = 10
Cutting radius. If a pixel's value p(x,y) > min_3 + kappa * e(x,y) or if it's value p(x,y) < max_3 - kappa * e(x,y) the pixel is not taken into account in the step of the polynomial surface fit. min_3 is the the third minimum, max_3 the third maximum value of all neighbouring pixels closer than the cutting radius and e(x,y) is the pixel's error value (measured in the errorframe).
reference
Image to which the comparison is made. If no image is specified all image(s) are compared to the first.
region = 0:0,0:0
Normalization Region. The median of this region stays the same for the input image and the compared output image. If region is equal to "0:0,0:0" the region will be a centered rectangle with a quarter of the image's lengths.
verbose = 0
Verbose level. If verbose >= 1, for each input image three additional output images are written showing the cutted pixels and the fitted polynomial surface.
image(s)
Image(s) to be processed.

DESCRIPTION

The task compare is designed for aligning the surface (e.g. the illumination of a flatfield) of one or more image(s) to the surface of a reference image. The alignment is done by multiplication with a two-dimentional surface of a given order. To determine this surface the task first divides the reference image by the single image and normalizes the result to the median of all pixels in a given region (in order to have roughly the same flux before and after the alignment). After that remaining stars originating from either the reference image or the single image are cutted: within a radius it is checked if a pixel's value p(x,y) > min_3 + kappa * e(x,y) or if it's value p(x,y) < max_3 - kappa * e(x,y). min_3 is the the third lowest, max_3 the third highest value of all neighbouring pixels closer than the cutting radius and e(x,y) is the pixel's error value (measured in the errorframe). Using all the remaining pixels the two-dimentional surface is determined and finaly multiplied with the single image.
If verbose >= 1, for each input image three additional output images are written showing the cutted pixels and the fitted polynomial surface.


OUTPUT

c...
aligned images
e.c...
errorframes of aligned images

If verbose >= 1:
div-...
This output images are the result of a division of the reference image by each of the input images. A border of 10 pixels is added.
cutdiv-...
Same as div-... but all the cutted pixels are set to the value "0".
fit-...
Polynomial surface fits of the normalized division of the reference image by each of the input images. The input images are multiplied by this in order to get the aligned images.

EXAMPLE

compare -i 2 -k 4 -f b-sky_r_040529_001.fits b-sky_r_040529_*.fits


Task index
This task was implemented by A. Riffeser
Dokumentation maintained by J. Koppenhöfer
This file was last time updated on 21 Feb 2005