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
|
.help flat1d Mar93 noao.imred.generic
.ih
NAME
flat1d -- Make flat fields by fitting a 1D function to the image
.ih
USAGE
flat1d input output
.ih
PARAMETERS
.ls input
Calibration images to be used to make the flat fields. The images may
contain image sections. Only the region covered by the section will be
modified in the output image.
.le
.ls output
Flat field images to be created or modified. The number of output images
must match the number of input images. If an output image does not exist
it is first created and initialized to unit response.
.le
.ls axis = 1
Axis along which the one dimensional fitting is done. Axis 1 corresponds
to fitting the image lines and axis 2 corresponds to fitting the columns.
.le
.ls interactive = yes
Set the fitting parameters interactively?
.le
.ls sample = "*"
Lines or columns to be used in the fits.
.le
.ls naverage = 1
Number of sample points to combined to create a fitting point.
A positive value specifies an average and a negative value specifies
a median.
.le
.ls function = spline3
Function to be fit to the image lines or columns. The functions are
"legendre" (legendre polynomial), "chebyshev" (chebyshev polynomial),
"spline1" (linear spline), and "spline3" (cubic spline). The functions
may be abbreviated.
.le
.ls order = 1
The order of the polynomials or the number of spline pieces.
.le
.ls low_reject = 2.5, high_reject = 2.5
Low and high rejection limits in units of the residual sigma.
.le
.ls niterate = 1
Number of rejection iterations.
.le
.ls grow = 1.
When a pixel is rejected, pixels within this distance of the rejected pixel
are also rejected.
.le
.ls minflat = 0.
When the fitted value is less than the value of this parameter the flat
field value is set to unity.
.le
.ls graphics = "stdgraph"
Graphics device for interactive graphics output.
.le
.ls cursor = ""
Graphics cursor input
.le
.ih
DESCRIPTION
Flat fields are created containing only the small scale variations in the
calibration images. The large scale variations in the images are modeled
by fitting a function to each image line or column with deviant pixel rejection.
The flat field values are obtained by taking the ratio of the image values
to the function fit. However, if the fitted value is less than the
parameter \fIminflat\fR the flat field value is set to unity.
The function fitting parameters may be set interactively when the interactive
flag is set using the interactive curve fitting package \fBicfit\fR.
The cursor mode commands for this package are described in a separate
help entry under "icfit". For two dimensional images the user is
prompted for the sample line or column or a blank-separated range to be
averaged and graphed.
Note that the lines or columns are relative the input image section; for
example line 1 is the first line of the image section and not the first
line of the image. Any number of lines or columns may be examined.
When satisfied with the fit parameters the user
responds with a carriage return to the line or column prompt.
The function is then fit to all the lines or columns and the flat field
ratios are determined.
If the output image does not exist initially it is created with the same
size as the input image \fIwithout\fR an image section and initialized
to unit response. Subsequently the flat field data modifies the pixel
values in the output image. Input image sections may be used to restrict
the region in which the flat field response is determined leaving the
rest of the output image unmodified. This ability is particularly useful
when dealing with multi-aperture data.
This task is very similar to \fBfit1d\fR with the addition of the
parameter \fIminflat\fR and the deletion of the parameter \fItype\fR
which is always "ratio".
.ih
EXAMPLES
1. Create a flat field from the calibration image "quartz" with the
spectrum running along the lines. Exclude the first and last columns,
use a spline fit of 25 pieces (a width of 32 pixels over 800 columns),
and set grow to 4 pixels.
.nf
cl> flat1d quartz flat order=25 sample="2:799" grow=4 \
>>> interactive=no
or
cl> flat1d quartz[2:799,*] flat order=25 grow=4 inter-
.fi
The fitting parameters may be set interactively in which case the fitting
parameters need not be specified. The command would be
.nf
cl> flat1d quartz flat
quartz: Fit column = 1 10
quartz: Fit column =
.fi
The user selects sample columns to be fit interactively with the interactive
curve fitting package. When satisfied with the fit parameters
respond with a carriage return to the prompt. The function is then fit to
all the columns and the flat field ratios are determined.
2. As an example for multi-slit spectra the locations of the slits are
determined and a file containing the image sections is created.
Since there must be the same number of output images another file
containing the output images is also created. For
example the files might contain
.nf
File quartzs File flats
_______________ __________
quartz[23:40,*] flat
quartz[55:61,*] flat
quartz[73:84,*] flat
.fi
A flat field for the slits is then obtained with the command
cl> flat1d @quartzs flats axis=2
.ih
REVISIONS
.ls FLAT1D V2.10.3
The image header keyword "CCDMEAN = 1." is now added or updated.
.le
.ih
BUGS
The creation of multi-slit files and the need for an equal number of
repeated output files is annoying. It will be worked on in the future.
.ih
SEE ALSO
fit1d, icfit
.endhelp
|
