1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
|
.help cosmicrays Dec87 noao.imred.ccdred
.ih
NAME
cosmicrays -- Detect and replace cosmic rays
.ih
USAGE
cosmicrays input output
.ih
PARAMETERS
.ls input
List of input images in which to detect cosmic rays.
.le
.ls output
List of output images in which the detected cosmic rays will be replaced
by an average of neighboring pixels. If the output image name differs
from the input image name then a copy of the input image is made with
the detected cosmic rays replaced. If no output images are specified
then the input images are modified in place. In place modification of
an input image also occurs when the output image name is the same as
the input image name.
.le
.ls badpix = ""
List of bad pixel files to be created, one for each input image. If no
file names are given then no bad pixel file is created. The bad pixel
file is a simple list of pixel coordinates for each replaced cosmic ray.
This file may be used in conjunction with \fBbadpixelimage\fR to create
a mask image.
.le
.ls ccdtype = ""
If specified only the input images of the desired CCD image type will be
selected.
.le
.ls threshold = 25.
Detection threshold above the mean of the surrounding pixels for cosmic
rays. The threshold will depend on the noise characteristics of the
image and how weak the cosmic rays may be for detection. A typical value
is 5 or more times the sigma of the background.
.le
.ls fluxratio = 2.
The ratio (as a percent) of the mean neighboring pixel flux to the candidate
cosmic ray pixel for rejection. The value depends on the seeing and the
characteristics of the cosmic rays. Typical values are in the range
2 to 10 percent.
.le
.ls npasses = 5
Number of cosmic ray detection passes. Since only the locally strongest
pixel is considered a cosmic ray, multiple detection passes are needed to
detect and replace multiple pixel cosmic ray events.
.le
.ls window = 5
Size of cosmic ray detection window. A square window of either 5 by 5 or
7 by 7 is used to detect cosmic rays. The smaller window allows detection
in the presence of greater background gradients but is less sensitive at
discriminating multiple event cosmic rays from stars. It is also marginally
faster.
.le
.ls interactive = yes
Examine parameters interactively? A plot of the mean flux within the
detection window (x100) vs the flux ratio (x100) is plotted and the user may
set the flux ratio threshold, delete and undelete specific events, and
examine specific events. This is useful for new data in which one is
uncertain of an appropriate flux ratio threshold. Once determined the
task need not be used interactively.
.le
.ls answer
This parameter is used for interactive queries when processing a list of
images. The responses may be "no", "yes", "NO", or "YES". The upper case
responses permanently enable or disable the interactive review while
the lower case reponses allow selective examination of certain input
images.
.le
.ih
OTHER PARAMETERS
There are other parameters which may be defined by the package, as is the
case with \fBccdred\fR, or as part of the task, as is the case with
standalone version in the \fBgeneric\fR package.
.ls verbose
If yes then a time stamped log of the operation is printed on the standard
output.
.le
.ls logfile
If a log file is specified then a time stamped log of the operation is
recorded.
.le
.ls plotfile
If a plot file is specified then the graph of the flux ratio (x100) vs
the mean flux (x100) is recorded as metacode. This may be spooled or examined
later.
.le
.ls graphics = "stdgraph"
Interactive graphic output device for interactive examination of the
detection parameters.
.le
.ls cursor = ""
Interactive graphics cursor input. If null the graphics display cursor
is used, otherwise a file containing cursor input may be specified.
.le
.ls instrument
The \fBccdred\fR instrument file is used for mapping header keywords and
CCD image types.
.le
.ih
CURSOR COMMANDS
.nf
d Mark candidate for replacement (applys to '+' points)
q Quit and replace the selected pixels
r Redraw the graph
s Make a surface plot for the candidate nearest the cursor
t Set the flux ratio threshold at the y cursor position
u Mark candidate to not be replaced (applys to 'x' points)
w Adjust the graph window (see \fBgtools\fR)
.fi
There are no colon commands except those for the windowing options (type
:\help or see \fBgtools\fR).
.ih
DESCRIPTION
Cosmic ray events in each input image are detected and replaced by the
average of the four neighbors. The replacement may be performed
directly on the input image if no output image is specified or if the
output image name is the same as the input image name. If a new image
is created it is a copy of the input image except for the replaced
pixels. The processing keyword CRCOR is added to the output image
header. Optional output includes a log file to which a processing log
is appended, a verbose log output to the standard output (the same as
that in the log file), a plot file showing the parameters of the
detected cosmic ray candidates and the flux ratio threshold used, and a
bad pixel file containing the coordinates of the replaced pixels. The
bad pixel file may be used for plotting purposes or to create a mask
image for display and analysis using the task \fBbadpiximage\fR. This
bad pixel file will be replaced by the IRAF bad pixel facility when it
becomes available. If one wants more than a simple mask image then by
creating a different output image a difference image between the
original and the modified image may be made using \fBimarith\fR.
This task may be applied to an image previously processed to detect
additional cosmic rays. A warning will be given (because of the
CRCOR header parameter) and the previous processing header keyword will
be overwritten.
The cosmic ray detection algorithm consists of the following steps.
First a pixel must be the brightest pixel within the specified
detection window (either 5x5 or 7x7). The mean flux in the surrounding
pixels with the second brightest pixel excluded (which may also be a
cosmic ray event) is computed and the candidate pixel must exceed this
mean by the amount specified by the parameter \fIthreshold\fR. A plane
is fit to the border pixels of the window and the fitted background is
subtracted. The mean flux (now background subtracted) and the ratio of
this mean to the cosmic ray candidate (the brightest pixel) are
computed. The mean flux (x100) and the ratio (x100) are recorded for
interactive examination if desired.
Once the list of cosmic ray candidates has been created and a threshold
for the flux ratio established (either by the parameter \fIfluxratio\fR
or modified interactively) the pixels with ratios below the threshold
are replaced in the output by the average of the four neighboring pixels
(with the second strongest pixel in the detection window excluded if it is
one of these pixels). Additonal pixels may then be detected and replaced
in further passes as specified by the parameter \fInpasses\fR. Note that
only pixels in the vicinity of replaced pixels need be considered in
further passes.
The division between the peaks of real objects and cosmic rays is made
based on the flux ratio between the mean flux (excluding the center
pixel and the second strongest pixel) and the candidate pixel. This
threshold depends on the point spread function and the distribution of
multiple cosmic ray events and any additional neighboring light caused
by the events. This threshold is not strongly coupled to small changes
in the data so that once it is set for a new type of image data it may
be used for similar images. To set it initially one may examine the
scatter plot of the flux ratio as a function of the mean flux. This
may be done interactively or from the optional plot file produced.
When the interactive flag is set the user is queried for each image.
Responses may be made for specific images or for all images by using
lower or upper case answers respectively. When the parameters are
examined interactively the user may change the flux ratio threshold
('t' key). Changes made are stored in the parameter file and, thus,
learned for further images. Pixels to be deleted are marked by crosses
and pixels which are peaks of objects are marked by pluses. The user
may explicitly delete or undelete any point if desired but this is only
for special cases near the threshold. In the future keys for
interactive display of the specific detections will be added.
Currently a surface plot of any candidate may be displayed graphically
in four 90 degree rotated views using the 's' key. Note that the
initial graph does not show all the points some of which are clearly
cosmic rays because they have negative mean flux or flux ratio. To
view all data one must rewindow the graph with the 'w' key or ":/"
commands (see \fBgtools\fR).
.ih
EXAMPLES
1. To replace cosmic rays in a set of images ccd*:
.nf
cl> cosmicrays ccd* new//ccd*
ccd001: Examine parameters interactively? (yes):
[A scatter plot graph is made. One can adjust the threshold.]
[Looking at a few points using the 's' key can be instructive.]
[When done type 'q'.]
ccd002: Examine parameters interactively? (yes): NO
[No further interactive examination is done.]
.fi
After cleaning one typically displays the images and possibly blinks them.
A difference image or mask image may also be created.
2. To create a mask image a bad pixel file must be specified. In the
following we replace the cosmic rays in place and create a bad pixel
file and mask image:
.nf
cl> cosmicrays ccd001 ccd001 badpix=ccd001.bp
cl> badpiximage ccd001.bp ccd001 ccd001bp
.fi
.ih
SEE ALSO
badpixelimage gtools
.endhelp
|
