mkillumflat: Make illumination corrected flat fields

Package: quadred

Usage

mkillumflat input output

Parameters

input

List of input flat field images to be illumination corrected.

output

List of output illumination corrected flat field 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.

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 objects from the smoothing.

lowsigma = 2.5, highsigma = 2.5

Sigma clipping thresholds above and below the smoothed illumination.

divbyzero = 1.

The illumination flat field is the ratio of the flat field to a smoothed flat field. This may produce division by zero. A warning is given if division by zero takes place and the result (the illumination corrected flat field value) is replaced by the value of this parameter.

ccdproc (parameter set)

CCD processing parameters.

Description

First, the input flat field images are processed as needed. Then the large scale illumination pattern of the images is removed. The illumination pattern is determined by heavily smoothing the image using a moving "boxcar" average. The output image is the ratio of the input image to the illumination pattern. The illumination pattern is normalized by its mean to preserve the mean level of the input image.

When this task is applied to flat field images only the small scale response effects are retained. This is appropriate if the flat field images have illumination effects which differ from the astronomical images and blank sky images are not available for creating sky corrected flat fields. When a high quality blank sky image is available the related task mkskyflat should be used. Note that the illumination correction, whether from the flat field or a sky image, may be applied as a separate step by using the task mkillumcor or mkskycor and applying the illumination correction as a separate operation in ccdproc. However, creating an illumination corrected flat field image before processing is more efficient since one less operation per image processed is needed. For more discussion about flat fields and illumination 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 illumination. This is done by computing the rms of the image lines relative to the smoothed illumination 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. Two examples in which a new image is created and in which the input flat fields are corrected in place are:

cl> mkllumflat flat004 FlatV
cl> mkillumflat flat* ""

See also

ccdproc, flatfields, mkfringecor, mkillumcor, mkskycor, mkskyflat