mkfringecor input output
input
List of input images for making fringe correction images.
output
List of output fringe 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 background.
ccdproc (parameter set)
CCD processing parameters.
The input blank sky images are automatically processed up through the illumination correction before computing the fringe correction images. The fringe corrections are subset dependent. The slowly varying background is determined and subtracted leaving only the fringe pattern caused by the sky emission lines. These fringe images are then scaled and subtracted from the observations by ccdproc. The background is determined by heavily smoothing the image using a moving "boxcar" average. The effects of the objects and fringes in the image is minimized by using a sigma clipping algorithm to detect and exclude them from the average. Note, however, that objects left in the fringe image will affect the fringe corrected observations. Any objects in the sky image should be removed using skyreplace (not yet available).
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 the fringes and any objects in the blank sky calibration images a sigma clipping algorithm is used to detect and exclude features from the background. 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.
1. The two examples below make an fringe correction image from a blank sky image, "sky017". In the first example a separate fringe image is created and in the second the fringe image replaces the sky image.
cl> mkskycor sky017 Fringe
cl> mkskycor sky017 frg017