saltcrclean

Name

saltcrclean – Clean multiple CCD extensions for cosmic rays

Usage

saltcrclean images outimages outpref (crtype) (thresh) (mbox) (flux_ratio) (bbox) (bthresh) (gain) (rdnoise) (fthresh) (bfactor) (gbox) (maxiter) (multithread) (clobber) (logfile) (verbose)

Parameters

images

String. List of input images including, if necessary, absolute or relative paths to the data. Data can be provided as a comma-separated list, or a string with a wildcard (e.g. ‘images=S20061210*.fits’), or a foreign file containing an ascii list of image filenames. For ascii list option, the filename containing the list must be provided preceded by a ‘@’ character, e.g. 'images=@listoffiles.lis‘. Do not provide any files that have already been mosaiced.

outimages

String. A list of images. Data can be provided as a comma-separated list, or a string with a wildcard (e.g. ‘outimages=rS20061210*.fits’), or a foreign file containing an ascii list of image filenames. For the ascii list option, the filename containing the list must be provided preceded by a ‘@’ character, e.g. 'outimages=@listoffiles.lis‘. This list must be of the same size as the images argument list. If the output is intended for a different directory the absolute or relative path must be supplied with the file name.

outpref

String. If the outpref string is non-zero in length and contains characters other than a blank space, it will override any value of the outimages argument. Output file names will use the name list provided in the images argument, but adding a prefix to the basename of each output file defined by outpref. An absolute or relative directory path can be included in the prefix, e.g. ‘outpref=/Volumes/data/p’.

crtype

String. The type of cosmic ray cleaning to perform. The options are fast, median, and edge. Fast cleaning identifies cosmic rays from statistical deviations above the global background. Median cleaning smooths the image first and uses local background determinations Edge cleaning looks for cosmic rays using a Laplacian filter.

(thresh)

Real. The detection threshhold for the identification of cosmic rays. This is a factor above the background deviation used to detect cosmic rays.

(mbox)

Int. Window size for searching for cosmic rays. For fast detection, a detected pixel will be compared to neighbors within this box. For median detection, it is the size of median filter. It is not used for edge detection.

(flux_ratio)

Real. The flux_ratio is used to confirm cosmic rays in the fast method. If the ratio of the flux in the peak pixel to the median flux from the neighboring pixels within a window of size mbox around that pixel is greater than the flux_ratio, the pixel is identified as a cosmic ray.

(bbox)

Int. For median cleaning, the bbox is the background box size used to determine the local background statistics. The bbox should be equal or greater than 10 in order to have sufficient pixel statistics.

(bthresh)

Real. Sigma-clipping threshhold for the determination of the background statistics. Used in the fast and median types of cleaning.

(gain)

Real. Gain of the CCD image. This is only used for calculation of noise statistics in the edge identification scheme.

(rdnoise)

Real. Read noise of the CCD image.This is only used for calculation of noise statistics in the edge identification scheme.

(fthresh)

Real. Threshhold for excluding compact sources in the edge detection scheme. Any sources below this threshhold will not be identified as cosmic rays.

(bfactor)

Int. Factor to sub-pixel sample the images. This is only used in edge detection scheme.

(gbox)

Int. Window size for growing sources. Additional sources will not be searched for if gbox=0. #Growth of sources are dependent on the #detection method, but sources which are above gthresh will also be #flagged as cosmic rays.

(maxiter)

Int. Maximum number of times to repeat the cosmic ray detection

(multithread)

Bool. If set to ‘yes’, the program will use a different thread for processing each of the SCI extensions in the image

(clobber)

Hidden boolean. If set to ‘yes’ files contained within the outpath directory will be overwritten by newly created files of the same name.

(logfile)

String. Name of an ascii file for storing log and error messages written by the task. The file may be new, or messages can also be appended to a pre-existing file.

(verbose)

Boolean. If verbose=n, log messages will be suppressed.

Description

SALTCRCLEAN is a task capable of cleaning cosmic rays form single or multi-extension fits data. The task gives users a choice of three different methods of cleaning cosmic rays that provide different levels of performance. The three choices the user has are between fast, median and edge cosmic ray detection.

Fast cosmic ray detection performs very fast identification of cosmic rays in a CCD image. The algorithm will calculate the sigma-clipped background statistics, where all pixels above bthresh are ignored from the calculations. Next, it will detect all pixels above thresh times the background deviation. These pixels will only be identified as cosmic rays if: (1) They are the peak pixel inside the window size of mbox, and (2) if the ratio of the median flux of all the pixels within the window of size mbox to the flux of the identified pixel is less than flux_ratio. Pixels identified as cosmic rays will be replaced with the median value of all of the neighbors within the window specified by mbox. The process will repeat for maxiter. If the grow parameter is set, it will identify all the pixels around an identified cosmic ray and will then replace all of the cosmic rays identified with the median of their neighbors.

Median cosmic ray detection detects cosmic rays through median smoothing the image. The first thing the algorithm does is measure the local background statistics for each pixel. Within a window of bbox size, the background statistics are calculated for each pixel. Then the image is median smoothed with a bin size given by mbox. The median smoothed image is subtracted from the original image, and any pixel thresh times greater than its local background deviation is flagged as a cosmic ray. If gbox > 0, cosmic rays above gthresh will be identified if they are neighboring an already identified cosmic ray. The process will repeat for the number of iterations set by the maxiter parameter.

Edge cosmic ray detection uses a Laplacian filter to detect cosmic rays in an image. This algorithm uses the process outlines in van Dokkem (2001). The first step is that the image is sub-pixel sampled. Then the image is convolved with a Laplacian filter and then returned to its original pixel sampling. A noise image is created by median smoothing the image and applying a realistic estimate for the gain and rdnoise. Cosmic rays are then identified from a signal to noise image after extended sources have been removed. Finally, compact sources are removed by setting the fthresh parameter. Once complete, additional cosmic rays can be found by setting gbox>0. The whole process will iterate until maxiter or until no more cosmic rays are removed.

Examples

1. To clean a sequence of images:

   --> saltcrclean images='/Volumes/data1/bxpP*.fits' outimages=''
   outpref='/Volumes/data2/m'
   clobber='yes' logfile='salt.log' verbose='yes'

Time and disk requirements

For a fast linux machine with a 2.8 Ghz processor and 2 Gb of RAM, one 1043x1024 FITS image can be processed with the following times: fast (4 sec), median (120 sec), and edge (45 sec). For this test, the parameters for SALTCRCLEAN were thresh=5, gbox=3, and maxiter=3 for all of the different methods.

Bugs and limitations

None

Send feedback and bug reports to salthelp@saao.ac.za

See also

iraf.noao.imred.crutil.cosmicrays iraf.noao.imred.crutil.crmedian