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	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.

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)

WCS Projection Types

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