World Coordinate System
The term "World Coordinate System", or "WCS" refers
to a coordinate system that describes the positions of objects in
the sky rather than in the columns and rows of an image. Mira
adopts the WCS definition of the FITS format standard. This
standard proscribes both the way in which world coordinates are
expressed in the image header and also the image coordinate systems
available for FITS format images. Any FITS image meeting the FITS
WCS standard will be properly interpreted by Mira. The WCS is
implemented using a specific set of header
keywords.
For images containing WCS information, Mira shows
image coordinates, measurements, and plots in either WCS units or
the native array system of (column,row). For WCS calibrated images,
you can select which
plot coordinate system to use.
Note
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Mira implements FITS WCS support using the dynamic
link library FitsWcs.dll. This library resides in the Modules\
subdirectory. If this DLL is not present at startup, Mira will
operate normally but will not display world coordinates for FITS
images even though they have WCS information.
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WCS Projection Types
The mathematical relationship between pixel
location and world coordinates is known as the world coordinate
projection. The projection type is represented in the FITS
header using the CTYPEn
keyword, where n refers
to the axis dimension. Equatorial calibration usually employs the
"Tangent Plane (gnomonic)" projection with CTYPEn Code =
TAN. In the FITS header, this equatorial projection for right
ascension (axis 1) and declination (axis 2) appears as CTYPE1 = 'RA---TAN' and CTYPE2
= 'DEC--TAN'.
Mira supports the types of WCS projection listed in
the table below. The projection type may be obtained using the
class method
CImage:GetWcsType() in
the Mira Pro x64 Script language. In the table below,
nWcsProj is returned by
CImage:GetWcsType().The
CTYPEn Code is listed in FITS header as the string value of
the CTYPEn keyword. The
Description is reported in the output from the
List WCS Keywords command.
nWcsProj
|
CTYPEn Code
|
Description
|
-1
|
|
Pixel WCS
(default, no WCS)
|
0
|
LIN
|
Linear
projection
|
1
|
AZP
|
Zenithal /
Azimuthal Perspective
|
2
|
TAN
|
Tangent Plane
(gnomonic)
|
3
|
SIN
|
Orthographic /
synthesis
|
4
|
STG
|
Stereographic
|
5
|
ARC
|
Zenithal /
azimuthal equidistant
|
6
|
ZPN
|
Zenithal /
azimuthal Polymomial
|
7
|
ZEA
|
Zenithal /
azimuthal Equal Area
|
8
|
AIR
|
Airy
|
9
|
CYP
|
Cylindrical
Perspective
|
10
|
CAR
|
Cartesian
|
11
|
MER
|
Mercator
|
12
|
CEA
|
Cylindrical
Equal Area
|
13
|
COP
|
Conic
Perspective(COP)
|
14
|
COD
|
Conic
equidistant
|
15
|
COE
|
Conic Equal
Area
|
16
|
COO
|
Conic
Orthomorphic
|
17
|
BON
|
Bonne
|
18
|
PCO
|
Polyconic
|
19
|
GLS
|
Sanson -
Flamsteed(Global Sinusoidal)
|
20
|
PAR
|
Parabolic
|
21
|
AIT
|
Hammer -
Aitoff
|
22
|
MOL
|
Mollweide
|
23
|
CSC
|
COBE
quadrilateralized Spherical Cube
|
24
|
QSC
|
Quadrilateralized
Spherical Cube
|
25
|
TSC
|
Tangential
Spherical Cube
|
26
|
NCP
|
Special case
of Orthographic / synthesis
|
27
|
DSS
|
Digitized Sky
Survey plate solution
|
28
|
PLT
|
Plate fit
polynomials(SAO)
|
29
|
TNX
|
Tangent Plane
(gnomonic, NOAO with corrections)
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Related Topics
Contents
Image Coordinate System
Plot Coordinate Systems
Image Coordinate Readout
Point Measurements
Image Window
Astrometric Calibration
Set Equatorial Scale
Set Arcsecond Scale
World Coordinate System Keywords
Coordinate Systems
List WCS Keywords
Mira Pro x64 User's Guide, Copyright Ⓒ 2023 Mirametrics, Inc. All
Rights Reserved.
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