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.help wcspars Jul93 tables
.ih
NAME
wcspars -- Edit the parameter set that defines a world coordinate
system.
.ih
USAGE
wcspars
.ih
DESCRIPTION
The parameters in this pset are used to define a simple world
coordinate system (WCS) for use by various tasks that require such
information, such as 'wcslab' or 'siaper'.
Note that this is a pset, not an executable task; it defines a set of
parameters used by other tasks. Invoking the pset by name runs 'eparm'
on the parameter set, allowing the user to modify the parameters.
Alternatively, the parameters may be modified on the command linne by
specifying the pset name and parameter name. For example, you can
type 'wcspars.ctype="ra---tan"'. Parameters may also be edited by
using 'eparam' on the calling task. An example is the task 'wcslab'. By
typing 'eparam wcslab', positioning the cursor on the parameter
'wcspars', and type ':e', the user would then be editing the parameters
in this pset.
A WCS is used to transform coordinates from one system to another.
For example, for converting from pixel coordinates to celestial
coordinates. To perform such transformations, certain information is
required, such as the type of system. Below is a brief description of
the IRAF implementation of WCS and how to the parameters in this pset
to define a WCS.
The IRAF implementation defines a transformation from some "logical"
system (e.g., pixel space) to some "world" system (e.g., RA and DEC).
The first piece of information required is the type of world system is
being dealt with. At the moment, there are two general systems
defined: 'linear' which provides a linear mapping from the logical to
world systems, and the celestial projects which provide a mapping from
pixel space to celestial coordinate space. The parameters 'crtype1' and
'ctype2' are used to specify the type of system. If a linear system is
desired, both parameters will have the value "linear". If the TANGENT
plane projection is desired where the first axis represents RA and the
second represents DEC, then the parameters would have the values,
'ctype1 = "ra---tan"', 'ctype2 = "dec--tan"'. There are also a sine
projection (SIN) and arc (ARC) projection provided.
The scale factor and rotation between the two systems are defined by
a coordinate transformation (CD) matrix. Through matrix
multiplication, the logical coordinates are multiplied by the CD
matrix to produce the world coordinates. The matrix is represented in
the parameters as follows:
.nf
|---------------|
| cd1_1 cd1_2 |
| |
| cd2_1 cd2_2 |
|---------------|
.fi
To construct the CD matrix, the following definitions may be used:
.nf
cd1_1 = Sx * cos(PA)
cd1_2 = -Sy * sin(PA)
cd2_1 = Sx * sin(PA)
cd2_2 = Sy * cos(PA)
.fi
where Sx and Sy are the scale factors from the logical to world
systems and PA is the angle of rotation between the two systems
(positive rotation is counterclockwise).
There is a special case for the transformation to RA and DEC. Since RA
increases "to the left", opposite of standard convention, -1 needs
to be multiplied through the CD matrix for the first axis. This
results in the formulas below:
.nf
cd1_1 = -Sx * cos(PA)
cd1_2 = Sy * sin(PA)
cd2_1 = Sx * sin(PA)
cd2_2 = Sy * cos(PA)
.fi
Finally, the origins of the logical and world systems must be defined.
The parameters 'crpix1' and 'crpix2' define the coordinate in the logical
space that corresponds to the coordinate in world space defined by the
parameters 'crval1' and 'crval2'. Quite simply, the coordinate (crpix1,
crpix2) in the logical space, when transformed to the world space,
would be the coordinate (crval1, crval2).
The last set of parameters, 'log_x1', 'log_x2', 'log_y1', 'log_y2', define a
region in the logical space over which the transformation is valid.
.ih
PARAMETERS
.ls (crtype1 = "linear") [string]
The system type of the first axis. Possible values depend on what
transformations have been implemented in the IRAF system. To date the
following values represent valid transformations: linear, xxx--tan,
xxx-sin, xxx-arc (where xxx is either "ra-" or "dec"). Note that if any
of the celestial transformations are used, the "ra" must appear in one
of 'crtype1' or 'crtype2' and "dec" must appear in the other parameter.
.le
.ls (crtype2 = "linear") [string]
The system type of the second axis. See above for values.
.le
.ls (crpix1 = 0.) [real]
The X coordinate of the reference point in logical space that
corresponds to the reference point in the world space.
.le
.ls (crpix2 = 0.) [real]
The Y coordinate of the reference point in logical space that
corresponds to the reference point in the world space.
.le
.ls (crval1 = 0.) [real]
The X coordinate of the reference point in world space that
corresponds to the reference point in the logical space.
.le
.ls (crval2 = 0.) [real]
The Y coordinate of the reference point in world space that
corresponds to the reference point in the logical space.
.le
.ls (cd1_1 = 1.) [real]
Entry in the CD matrix. Usually has the value <xscale * cos(angle)>,
or for RA and DEC systems, <-xscale * cos(angle)>.
.le
.ls (cd1_2 = 0.) [real]
Entry in the CD matrix. Usually has the value <-yscale * sin(angle)>,
or for RA and DEC systems, <yscale * sin(angle)>.
.le
.ls (cd2_1 = 0.) [real]
Entry in the CD matrix. Usually has the value <xscale * sin(angle)>.
.le
.ls (cd2_2 = 1.) [real]
Entry in the CD matrix. Usually has the value <yscale * cos(angle)>.
.le
.ls (log_x1 = 0.) [real]
The lower X axis extent in logical space over which the transformation
is valid.
.le
.ls (log_x2 = 0.) [real]
The upper X axis extent in logical space over which the transformation
is valid.
.le
.ls (log_y1 = 0.) [real]
The lower Y axis extent in logical space over which the transformation
is valid.
.le
.ls (log_y2 = 0.) [real]
The upper Y axis extent in logical space over which the transformation
is valid.
.le
.ih
EXAMPLES
1. The following example
is for an image that does not contain any WCS information.
The image is 512x512 pixels, where the pixels are approximately 1/10th
an arcsecond in size, whose center pixel is located at 9h 22m 30.5s
and -15o 5m 42s and is rotated 30 degrees towards the west:
.nf
ctype1 = 'ra---tan'
ctype2 = 'dec--tan'
crpix1 = 256.0
crpix2 = 256.0
crval1 = 140.62708
crval2 = -15.09500
cd1_1 = -2.405626e-5
cd1_2 = 1.388889e-5
cd2_1 = 1.388889e-5
cd2_2 = 2.405626e-5
log_x1 = 1.
log_x2 = 512.
log_y1 = 1.
log_y2 = 512.
.fi
.ih
BUGS
.ih
SEE ALSO
.endhelp
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