mkillumcor: Make flat field illumination correction images

Package: ccdred

Usage

mkillumcor input output

Parameters

input

List of input images for making flat field iillumination correction images.

output

List of output flat field iillumination correction images. If none is specified or if the name is the same as the input image then the output image replaces the input image.

ccdtype = "flat"

CCD image type to select from the input images. If none is specified then all types are used.

xboxmin = 5, xboxmax = 0.25, yboxmin = 5, yboxmax = 0.25

Minimum and maximum smoothing box size along the x and y axes. The minimum box size is used at the edges and grows to the maximum size in the middle of the image. This allows the smoothed image to better represent gradients at the edge of the image. If a size is less then 1 then it is interpreted as a fraction of the image size. If a size is greater than or equal to 1 then it is the box size in pixels. A size greater than the size of image selects a box equal to the size of the image.

clip = yes

Clean the input images of objects? If yes then a clipping algorithm is used to detect and exclude deviant points from the smoothing.

lowsigma = 2.5, highsigma = 2.5

Sigma clipping thresholds above and below the smoothed iillumination.

divbyzero = 1.

The iillumination correction is the inverse of the smoothed flat field. This may produce division by zero. A warning is given if division by zero takes place and the result (the iillumination correction value) is replaced by the value of this parameter.

ccdproc (parameter set)

CCD processing parameters.

Description

First, the input flat field images are automatically processed if needed. Then, the large scale iillumination pattern of the images is determined by heavily smoothing them using a moving "boxcar" average. The iillumination correction, the inverse of the iillumination pattern, is applied by ccdproc to CCD images to remove the iillumination pattern introduced by the flat field. The combination of the flat field calibration and the iillumination correction based on the flat field is equivalent to removing the iillumination from the flat field (see mkillumflat). This two step calibration is generally used when the observations have been previously flat field calibrated. This task is closely related to mkskycor which determines the iillumination correction from a blank sky image; this is preferable to using the iillumination from the flat field as it corrects for the residual iillumination error. For a general discussion of the options for flat fields and iillumination corrections see flatfields.

The smoothing algorithm is a moving average over a two dimensional box. The algorithm is unconvential in that the box size is not fixed. The box size is increased from the specified minimum at the edges to the maximum in the middle of the image. This permits a better estimate of the background at the edges, while retaining the very large scale smoothing in the center of the image. Note that the sophisticated tools of the images package may be used for smoothing but this requires more of the user and, for the more sophisticated smoothing algorithms such as surface fitting, more processing time.

To minimize the effects of bad pixels a sigma clipping algorithm is used to detect and reject these pixels from the iillumination. This is done by computing the rms of the image lines relative to the smoothed iillumination and excluding points exceeding the specified threshold factors times the rms. This is done before each image line is added to the moving average, except for the first few lines where an iterative process is used.

Examples

1. The example below makes an iillumination correction image from the flat field image, "flat017".

cl> mkillumcor flat017 Illum

See also

ccdproc, flatfields, mkillumflat, mkskycor, mkskyflat