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seam stsdas.hst_calib.wfpc


NAME · USAGE · DESCRIPTION · PARAMETERS · EXAMPLES · REFERENCES
SEE_ALSO

NAME

seam -- Smooth the boundaries between the 4 WF/PC frames after they are mosaicked into one image. This task does NOT work for WFPC2 images.

USAGE

seam infile outfile length width delta npbg

DESCRIPTION

After the four groups of a WF/PC image are mosaicked (using the task WMOSAIC) into one image, brightness discontinuities may still show up near the boundaries between the four frames and thus the name seam. This task is trying to airbrush the seams such that the whole image has a better integral visual appearance. This implies that fluxes near the seams after running this task may have little scientific validity and should not be used for precise photometric data reduction.

This task only works on WF/PC images, not WFPC2 images. The reason is that WF/PC has sharp drop-offs at the boundaries and this task is designed to smooth out possible discontinuities. WFPC2 images have smooth role-offs at the boundaries and therefore can benefit little from this task.

As an example to explain the algorithm of this task, imagine there are two frames, one on top of the other and the seam between them runs horizontally. The smoothing is carried out for each column, i.e. each column contains a stitch which is a segment of pixels vertical to the direction of the seam. The boundary between these two frames crosses all stitches at the middle points. The length of the stitches is specified by the parameter LENGTH.

We start from the bottom of each stitch: the bottom NPBKGD pixels of the stitch are used to determine the background flux level. The median flux of these pixels is regarded as the background and we then examine each pixel upward till one having a flux level of either greater or less than this background flux by a fraction of DELTA. This pixel will be marked as the beginning of the flux discontinuity. Repeat the same from the other end and we get the ending point of the discontinuity. Now take the median flux of the NPBKGD pixels just below the beginning point (call it F1) and the median flux of the NPBKGD pixels just above the ending point (call it F2), replace all fluxes of the discontinuity pixels by linearly interpolating between F1 and F2.

The user can also choose to include more points to determine the background level without increasing the stitch length. This can be done by using a larger number of WIDTH. For example, if WIDTH = 3, one extra stitch on both sides are used to determine the background level as well as F1 and F2.

PARAMETERS

infile [string]
Input image. This must be the output image from the task WMOSAIC.
outfile [string]
Output image. It has the same size and header keywords as the input image.
length = [int, min = 3]
Length of each stitch. Must be an odd number. If an even number is specified, the next higher odd number will be used.
width = [int, min = 1]
Total number of stitches used to determine background level. Must be an odd number. If an even number is specified, the next higher odd number will be used.
delta = 0.[real, min = 0.]
Discrimination level used to detect the beginning of bad pixels.
npbg = [int, min = 1]
Number of pixels used to determine the background level in each stitch.

EXAMPLES

1. Use a stitch 21 pixels long, use three stitches for background determination, use three pixels in each stitch for background determination, and use 0.05 as the discrimination level.

wf> seam wr1mos.hhh wr1seam.hhh length=21 width=3 delta=0.05 npbg=3 

REFERENCES

SEE ALSO

wmosaic


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