ACRE
Name
acre
Purpose
Automatic Cosmic Ray Extraction
Description
This program will attempt to identify and remove Cosmic Ray strikes from
an image. This program was developed and tested on HST PC data prior
to the refurbishment mission. It may work for other types of data but
it is as of now untested elsewhere.
The simplest usage is the single pass mode where the same parameters are
used for the entire image. First, the image is smoothed with a median
filter using a box filter size given by width. This smoothed image is
then subtracted from the original image. A robust mean of a portion of
the image is calculated and subtracted from the image though this mean
should be near zero. Any pixel see to deviate by THRESH standard
deviations from this average is marked for removal and replaced by its
corresponding value in the smoothed image.
This initial step works very well on the sky. I've found that THRESH=3
WIDTH=7 work pretty well on all but the largest CRS's. Using a value
for width less than 7 seems to leave residual "rings" of hot pixels from
around the edges of a strike.
The draw back to these parameters is that it is much too agressive in and
around actual objects in the frame. The cores of the PSF will be removed
and numerous pixels will be tagged in the wings of the PSF.
To get around this problem, use the EXCLUDE keyword. This is a 5xN array
containing circular regions to scan with different parameters. The array
holds N such regions. For each region you must specify the following:
(0,n) - x location of region
(1,n) - y location of region
(2,n) - radius of region
(3,n) - sigma threshold to use in this region
(4,n) - width of median smoothing, (no smoothing if set to 0).
The procedure used is to first restore all the pixels in the region to
their original values (in case they were changed in the first step).
If the width is set to zero, nothing more is done. If the width is a
meaningful value, then the original image is smoothed with that width
and the region is scanned for deviant pixels again and replaces any
found.
The effects of each region are cumulative on the image and done in order
they appear in the array. So any final steps of restoring small locations
should be done last. Also, the step of smoothing the image is very cpu
intensive. It will run much faster if you can group all regions with
similar smoothing values together.
In practice, a smaller width (~3) and higher thresh (~4) seems to work in
the wings of the PSF, but it will still take out the core. So, you
need to specify two zones, one for cleaning that is nearly the size
of the outer fringes of the PSF and one for pretecting the image that
is smaller and centered on the core. If you happen to get a strike
near the core of the PSF, this routine won't help and you're probably
screwed anyway.
Category
CCD data processing
Calling Sequence
pro acre,dirty_im,clean_im,thresh,width, $
BLFINAL=blfinal,BLMASK=blmask, $
EXCLUDE=exclude,MASK=mask,VERBOSE=verbose
Inputs
dirty_im - Original input image to be cleaned.
thresh - Deviation threshold, in sigma, from background to cause
pixel to be fixed.
width - Median smoothing width to get local background reference.
Optional Input Parameters
Keyword Input Parameters
BLFINAL - Flag, if true, brings up ITOOL to blink between the original
and cleaned images.
BLMASK - Flag, if true, brings up ITOOL to blink between the original
and the mask showing pixels that are being replaced.
EXCLUDE - Array that controls special extraction behavior in select
regions of the image. See DESCRIPTION for details.
VERBOSE - Flag, if true, generates a wordy output of progress and
action as routine progresses.
Outputs
clean_im - Final cleaned up image.
Keyword Output Parameters
MASK - Return of the mask image.
Common Blocks
Side Effects
Restrictions
Procedure
Modification History
94/04/05 - Written by Marc W. Buie, Lowell Observatory