mkskycor: Make sky illumination correction images

Package: ccdred

Usage

mkskycor input output

Parameters

input

List of input images for making sky 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 = ""

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 objects from the smoothing.

lowsigma = 2.5, highsigma = 2.5

Sigma clipping thresholds above and below the smoothed iillumination.

ccdproc (parameter set)

CCD processing parameters.

Description

The large scale iillumination pattern of the input images, generally blank sky calibration images, is determined by heavily smoothing the image using a moving "boxcar" average. The effects of objects in the image may be minimized by using a sigma clipping algorithm to detect and exclude the objects from the average. This iillumination image is applied by ccdproc to CCD images to remove the iillumination pattern.

The input images are automatically processed up through flat field calibration before computing the iillumination. The iillumination correction is that needed to make the processed images flat over large scales. The input images are generally blank sky calibration images which have the same iillumination and instrumental effects as the object observations. Object images may be used but removal of the objects may not be very good; particularly large, bright objects. For further discussion of flat fields and iillumination corrections see flatfields.

You will notice that when you process images with an iillumination correction you are dividing each image by a flat field calibration and an iillumination correction. If the iillumination corrections are not done as a later step but at the same time as the rest of the processing one will get the same calibration by multiplying the flat field by the iillumination correction and using this product alone as the flat field. This approach has the advantage of one less calibration image and two less computations (scaling and dividing the iillumination correction). Such an image, called a sky flat, may be created by mkskyflat as an alternative to this task.

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.

Blank sky images may not be completely blank so a sigma clipping algorithm may be used to detect and exclude objects from the iillumination pattern. This is done by computing the rms of the image lines relative to the smoothed background 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 two examples below make an iillumination image from a blank sky image, "sky017". In the first example a separate iillumination image is created and in the second the iillumination image replaces the sky image.

cl> mkskycor sky017 Illum
cl> mkskycor sky017 sky017

See also

ccdproc, flatfields, mkillumcor, mkillumflat, mkskyflat