xnzap: Remove cosmis rays from images using averaging filter

Package: xdimsum

Usage

xnzap inlist omasks outlist crmasks

Parameters

inlist

The list of input sky subtracted images to be cosmic ray corrected.

omasks

The list of input object masks used to unzap cosmic rays detected in object regions or the input image keyword containing the name of the object mask, normally "CROBJMAS". Object masks contain 0's in object regions and 1's elsewhere. Note that this is the inverse of the usual definition of an XDIMSUM object mask.

outlist

The list of output cosmic ray corrected images. The output image list can be the same as the input image list.

crmasks = ".crm"

The name of the output cosmic ray masks or the string appended to the output image name to create the output cosmic ray mask name. Cosmic ray masks contain 1's in cosmic ray regions and 0's elsewhere. The name of the output cosmic ray mask is stored in the input and output image header keyword CRMASK.

zboxsz = 5

The size in pixels of the moving average filter. The central pixel plus nrejzap pixels are rejected from the average.

skyfiltsize = 15

The sky filter size in pixels. For each point the median of the pixels in an annulus of width (skyfiltsize - zboxsz) / 2 pixels is used to estimate the local background.

sigfiltsize = 25

Size of box used for local background sigma estimates. The sigma filter is not a moving filter. Sigmas are estimated using percentile points of of the pixels in the box.

nsigzap = 5.0

The cosmic ray detection threshold in units of sky sigma.

nsigneg = 0.0

The negative deviant pixel detection threshold in units of sky sigma. If nsigneg <= 0.0 negative feature detection is not enabled.

nrings = 0

The cosmic ray growing region half-width in pixels.

nsigobj = 2.0

The object detection threshold in units of sky sigma. If nsigobj <= 0.0 only cosmic ray detection is performed.

ngrowobj = 0

The number of pixels to flag as a buffer around objects detected in the object detection step.

del_crmask = no

Delete the cosmic ray mask at task termination ? By default the cosmic ray masks are stored and used in later processing steps.

verbose = no

Print messages about actions taken by the task ?

Description

XNZAP detects detects and removes cosmics rays from the input images inlist and writes the corrected images to outlist. The output image list may be the same as the input image list. If input object masks omasks are defined then only cosmic rays in sky regions are detected and removed. These object mask consist of 0's and 1's with 0's defining the object regions in contrast to the usual XDIMSUM convention. The output cosmic rays mask are written to crmasks. Cosmic ray masks consist of 1's and 0's with 1's defining the detected cosmic rays.

At each pixel position XNZAP computes a running average filter of size zboxsz excluding the central pixel and the nrejzap highest pixels from the average, and a running median in an annulus (skyfiltsize - zboxsz) / 2 pixels wide. The local sky sigmas are estimated by dividing the image into square blocks which are sigfiltsize pixels wide and estimating the percentile points of pixels in the box.

A pixel is considered part of an object if the difference between the average value and the median background > nsigobj times the background sigma. If nsigobj <= 0 then no object detection is performed. If the pixel is NOT considered to be part of an object and if the difference between the pixel value and the average value exceeds nsigzap times the background sigma it is identified as a cosmic ray. If nsigneg > 0.0 then pixels more than nsigneg times the background sigma are also considered to be cosmic rays and are added to the cosmic ray mask.

If nrings > 0 then XNZAP grows the detected cosmic rays to include pixels within a radius of nrings pixels of the detected cosmic ray. If nobjgrow is > 0 then the detected object regions are grown to include pixels with a radius of nobjgrow pixels of the target object. The output cosmic ray mask is not affected by the region growing parameters however.

XNZAP is a new cosmic ray detection program which can be used as an alternative to XZAP. It uses the CRAVERAGE task to detect and remove cosmic rays. Normally CRAVERAGE is part of the addon CRUTIL package but a private copy has been installed in XDIMSUM. Users can find out more about CRAVERAGE by consulting the CRAVERAGE help page.

If del_crmask = yes the output cosmic ray mask is deleted on task termination

If verbose = yes then XNZAP prints messages on the terminal about the progress of the task.

Examples

1. Detect cosmic rays in the demo sky subtracted images.

cl> type sdemo.list
demo01.sub.imh
demo02.sub.imh
...
demo25.sub.imh

cl> xnzap @sdemo.list "" @sdemo.list ".crm"

2. Repeat example 1 but specify an output cosmic ray mask list.

cl> xnzap @sdemo.list "" @sdemo.list @crmlist

3. Repeat example 1 but specify an input inverse object core mask using the keyword "CROBJMAS".

cl> xnzap @sdemo.list "CROBJMAS" @sdemo.list ".crm"

4. Repeat example 2 but specify the input inverse object core mask list.

cl> xnzap @sdemo.list @ocrmlist @sdemo.list @crmlist

Time requirements

Bugs

See also

xzap, craverage