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 (see "World Coordinate System Keywords" in the Mira User's Guide). The WCS calibration stored in image headers may be used to align them at the subpixel level using the CImCombine:AlignWcs method.
|
Note |
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. |
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 WCS projections listed in the table below. The projection type nWcsProj is returned by the class method CImage:GetWcsProj. 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 menu command of the Mira user interface.
|
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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