Wcsprm¶

class
astropy.wcs.
Wcsprm
(header=None, key=' ', relax=False, naxis=2, keysel=0, colsel=None)¶ Bases:
object
Wcsprm
performs the core WCS transformations.Note
The members of this object correspond roughly to the key/value pairs in the FITS header. However, they are adjusted and normalized in a number of ways that make performing the WCS transformation easier. Therefore, they can not be relied upon to get the original values in the header. For that, use
astropy.io.fits.Header
directly.The FITS header parsing enforces correct FITS “keyword = value” syntax with regard to the equals sign occurring in columns 9 and 10. However, it does recognize freeformat character (NOST 1002.0, Sect. 5.2.1), integer (Sect. 5.2.3), and floatingpoint values (Sect. 5.2.4) for all keywords.
Warning
Many of the attributes of this class require additional processing when modifying underlying C structure. When needed, this additional processing is implemented in attribute setters. Therefore, for mutable attributes, one should always set the attribute rather than a slice of its current value (or its individual elements) since the latter may lead the class instance to be in an invalid state. For example, attribute
crpix
of a 2D WCS’Wcsprm
objectwcs
should be set aswcs.crpix = [crpix1, crpix2]
instead ofwcs.crpix[0] = crpix1; wcs.crpix[1] = crpix2]
. Parameters
 header
Header
,str
, or None. If
None
, the object will be initialized to default values. key
str
, optional The key referring to a particular WCS transform in the header. This may be either
' '
or'A'
'Z'
and corresponds to the"a"
part of"CTYPEia"
. (key may only be provided if header is also provided.) relaxbool or
int
, optional Degree of permissiveness:
False
: Recognize only FITS keywords defined by the published WCS standard.True
: Admit all recognized informal extensions of the WCS standard.int
: a bit field selecting specific extensions to accept. See Headerreading relaxation constants for details.
 naxis
int
, optional The number of world coordinates axes for the object. (naxis may only be provided if header is
None
.) keyselsequence of flag bits, optional
Vector of flag bits that may be used to restrict the keyword types considered:
WCSHDR_IMGHEAD
: Image header keywords.WCSHDR_BIMGARR
: Binary table image array.WCSHDR_PIXLIST
: Pixel list keywords.
If zero, there is no restriction. If 1, the underlying wcslib function
wcspih()
is called, rather thanwcstbh()
. colselsequence of
int
A sequence of table column numbers used to restrict the keywords considered.
None
indicates no restriction.
 header
 Raises
MemoryError
Memory allocation failed.
ValueError
Invalid key.
KeyError
Key not found in FITS header.
Attributes Summary
str
Character code for alternate coordinate descriptions.Auxprm
Auxiliary coordinate system information of a specialist nature.int array[naxis]
An array of fourdigit type codes for each axis.double
Equivalent toDATEOBS
.double array[naxis][naxis]
TheCDi_ja
linear transformation matrix.double array[naxis]
Coordinate increments (CDELTia
) for each coord axis.Celprm
Information required to transform celestial coordinates.boolean
Is there an offset?list of strings
A list of the coordinate axis names, fromCNAMEia
.int array[naxis]
An array recording the column numbers for each axis in a pixel list.int
Column of FITS binary table associated with this WCS.double array[naxis]
period of a phase axis, CPERIia.double array[naxis]
The random error in each coordinate axis,CRDERia
.double array[naxis]
CROTAia
keyvalues for each coordinate axis.double array[naxis]
Coordinate reference pixels (CRPIXja
) for each pixel axis.double array[naxis]
Coordinate reference values (CRVALia
) for each coordinate axis.double array[naxis]
The systematic error in the coordinate value axes,CSYERia
.list of strings[naxis]
List ofCTYPEia
keyvalues.int
Index into thepixcrd
(pixel coordinate) array for theCUBEFACE
axis.list of astropy.UnitBase[naxis]
List ofCUNITia
keyvalues asastropy.units.UnitBase
instances.double array[naxis]
The time at the zero point of a phase axis,CSPHSia
.string
Representative midpoint of the date of observation.string
Date at the start of the observation.string
Date at the end of the observation.string
Start of the date of observation.string
Date of a reference epoch relative to which other time measurements refer.double
The equinox associated with dynamical equatorial or ecliptic coordinate systems.double array[2][2]
(readonly) Inverse of theCDELT
orPC
matrix.double
Equivalent toDATEOBS
.int
(readonly) The index into the world coord array containing latitude values.double
The native latitude of the celestial pole,LATPOLEa
(deg).string
(readonly) Celestial axis type for latitude.int
(readonly) The index into the world coord array containing longitude values.string
(readonly) Celestial axis type for longitude.double
The native longitude of the celestial pole.double
Modified Julian Date corresponding toDATEAVG
.double
Modified Julian Date corresponding toDATEBEG
.double
Modified Julian Date corresponding toDATEEND
.double
Modified Julian Date corresponding toDATEOBS
.double
Modified Julian Date corresponding toDATEREF
.string
The name given to the coordinate representationWCSNAMEa
.int
(readonly) The number of axes (pixel and coordinate).double array[3]
Location of the observer in a standard terrestrial reference frame.string
URI, URL, or name of an orbit ephemeris file giving spacecraft coordinates relating to TREFPOS.double array[naxis][naxis]
ThePCi_ja
(pixel coordinate) transformation matrix.double
The native latitude of the fiducial point.double array[2][2]
(readonly) Matrix containing the product of theCDELTia
diagonal matrix and thePCi_ja
matrix.string
The Solar System ephemeris used for calculating a pathlength delay.string
The equatorial or ecliptic coordinate system type,RADESYSa
.double
Rest frequency (Hz) fromRESTFRQa
.double
Rest wavelength (m) fromRESTWAVa
.int
(readonly) The index containing the spectral axis values.string
Spectral reference frame (standard of rest),SPECSYSa
.string
Spectral reference frame.string
Spectral reference frame for redshift.list of Tabprm
Tabular coordinate objects.double
equivalent to the elapsed time between DATEBEG and DATEEND, in units of TIMEUNIT.double
The native longitude of the fiducial point.double
the resolution of the time stamps.double
Time offset, which may be used, for example, to provide a uniform clock correctiondouble
relative position of the time stamps in binned time intervals, a value between 0.0 and 1.0.string
Time scale (UTC, TAI, etc.) in which all other timerelated auxiliary header values are recorded.string
Time units in which the following header values are expressed:TSTART
,TSTOP
,TIMEOFFS
,TIMSYER
,TIMRDER
,TIMEDEL
.double
the accuracy of time stamps relative to each other, in units of TIMEUNIT.double
the absolute error of the time values, in units of TIMEUNIT.string
Reference direction used in calculating a pathlength delay.string
Location in space where the recorded time is valid.double
equivalent to DATEBEG expressed as a time in units of TIMEUNIT relative to DATEREF+TIMEOFFS.double
equivalent to DATEEND expressed as a time in units of TIMEUNIT relative to DATEREF+TIMEOFFS.double
Velocity angle.double
Relative radial velocity.int
AIPS velocity code.list of Wtbarr
objects to construct coordinate lookup tables from BINTABLE.double
effective exposure time in units of TIMEUNIT.double
The redshift,ZSOURCEa
, of the source.Methods Summary
bounds_check
(pix2world, world2pix)Enable/disable bounds checking.
cdfix
()Fix erroneously omitted
CDi_ja
keywords.Translates AIPSconvention celestial projection types,
NCP
andGLS
.compare
(other[, cmp, tolerance])Compare two Wcsprm objects for equality.
cylfix
()Fixes WCS keyvalues for malformed cylindrical projections.
datfix
()Translates the old
DATEOBS
date format to year2000 standard form(yyyymmddThh:mm:ss)
and derivesMJDOBS
from it if not already set.fix
([translate_units, naxis])Applies all of the corrections handled separately by
datfix
,unitfix
,celfix
,spcfix
,cylfix
andcdfix
.Coordinate increments (
CDELTia
) for each coord axis asdouble array[naxis]
.get_pc
()Returns the
PC
matrix in readonly form asdouble array[naxis][naxis]
.get_ps
()Returns
PSi_ma
keywords for each i and m as list of tuples.get_pv
()Returns
PVi_ma
keywords for each i and m as list of tuples.has_cd
()Returns
True
ifCDi_ja
is present.Alias for
has_cd
.Returns
True
ifCROTAia
is present.Alias for
has_crota
.has_pc
()Returns
True
ifPCi_ja
is present.Alias for
has_pc
.is_unity
()Returns
True
if the linear transformation matrix (cd
) is unity.mix
(mixpix, mixcel, vspan, vstep, viter, …)Given either the celestial longitude or latitude plus an element of the pixel coordinate, solves for the remaining elements by iterating on the unknown celestial coordinate element using
s2p
.p2s
(pixcrd, origin)Converts pixel to world coordinates.
Print the contents of the
Wcsprm
object to stdout.s2p
(world, origin)Transforms world coordinates to pixel coordinates.
set
()Sets up a WCS object for use according to information supplied within it.
set_ps
(ps)Sets
PSi_ma
keywords for each i and m.set_pv
(pv)Sets
PVi_ma
keywords for each i and m.spcfix
()Translates AIPSconvention spectral coordinate types.
sptr
(ctype[, i])Translates the spectral axis in a WCS object.
sub
(axes)Extracts the coordinate description for a subimage from a
WCS
object.to_header
([relax])to_header
translates a WCS object into a FITS header.unitfix
([translate_units])Translates nonstandard
CUNITia
keyvalues.Attributes Documentation

alt
¶ str
Character code for alternate coordinate descriptions.For example, the
"a"
in keyword names such asCTYPEia
. This is a space character for the primary coordinate description, or one of the 26 uppercase letters, AZ.

axis_types
¶ int array[naxis]
An array of fourdigit type codes for each axis.First digit (i.e. 1000s):
0: Nonspecific coordinate type.
1: Stokes coordinate.
2: Celestial coordinate (including
CUBEFACE
).3: Spectral coordinate.
Second digit (i.e. 100s):
0: Linear axis.
1: Quantized axis (
STOKES
,CUBEFACE
).2: Nonlinear celestial axis.
3: Nonlinear spectral axis.
4: Logarithmic axis.
5: Tabular axis.
Third digit (i.e. 10s):
0: Group number, e.g. lookup table number
The fourth digit is used as a qualifier depending on the axis type.
For celestial axes:
0: Longitude coordinate.
1: Latitude coordinate.
2:
CUBEFACE
number.
For lookup tables: the axis number in a multidimensional table.
CTYPEia
in"43"
form with unrecognized algorithm code will have its type set to 1 and generate an error.

bepoch
¶ double
Equivalent toDATEOBS
.Expressed as a Besselian epoch.
See also

cd
¶ double array[naxis][naxis]
TheCDi_ja
linear transformation matrix.For historical compatibility, three alternate specifications of the linear transformations are available in wcslib. The canonical
PCi_ja
withCDELTia
,CDi_ja
, and the deprecatedCROTAia
keywords. Although the latter may not formally coexist withPCi_ja
, the approach here is simply to ignore them if given in conjunction withPCi_ja
.has_pc
,has_cd
andhas_crota
can be used to determine which of these alternatives are present in the header.These alternate specifications of the linear transformation matrix are translated immediately to
PCi_ja
byset
and are nowhere visible to the lowerlevel routines. In particular,set
resetscdelt
to unity ifCDi_ja
is present (and noPCi_ja
). If noCROTAia
is associated with the latitude axis,set
reverts to a unityPCi_ja
matrix.

cdelt
¶ double array[naxis]
Coordinate increments (CDELTia
) for each coord axis.If a
CDi_ja
linear transformation matrix is present, a warning is raised andcdelt
is ignored. TheCDi_ja
matrix may be deleted by:del wcs.wcs.cd
An undefined value is represented by NaN.

cel_offset
¶ boolean
Is there an offset?If
True
, an offset will be applied to(x, y)
to force(x, y) = (0, 0)
at the fiducial point, (phi_0, theta_0). Default isFalse
.

cname
¶ list of strings
A list of the coordinate axis names, fromCNAMEia
.

colax
¶ int array[naxis]
An array recording the column numbers for each axis in a pixel list.

colnum
¶ int
Column of FITS binary table associated with this WCS.Where the coordinate representation is associated with an imagearray column in a FITS binary table, this property may be used to record the relevant column number.
It should be set to zero for an image header or pixel list.

cperi
¶ double array[naxis]
period of a phase axis, CPERIia.An undefined value is represented by NaN.

crder
¶ double array[naxis]
The random error in each coordinate axis,CRDERia
.An undefined value is represented by NaN.

crota
¶ double array[naxis]
CROTAia
keyvalues for each coordinate axis.For historical compatibility, three alternate specifications of the linear transformations are available in wcslib. The canonical
PCi_ja
withCDELTia
,CDi_ja
, and the deprecatedCROTAia
keywords. Although the latter may not formally coexist withPCi_ja
, the approach here is simply to ignore them if given in conjunction withPCi_ja
.has_pc
,has_cd
andhas_crota
can be used to determine which of these alternatives are present in the header.These alternate specifications of the linear transformation matrix are translated immediately to
PCi_ja
byset
and are nowhere visible to the lowerlevel routines. In particular,set
resetscdelt
to unity ifCDi_ja
is present (and noPCi_ja
). If noCROTAia
is associated with the latitude axis,set
reverts to a unityPCi_ja
matrix.

crpix
¶ double array[naxis]
Coordinate reference pixels (CRPIXja
) for each pixel axis.

crval
¶ double array[naxis]
Coordinate reference values (CRVALia
) for each coordinate axis.

csyer
¶ double array[naxis]
The systematic error in the coordinate value axes,CSYERia
.An undefined value is represented by NaN.

ctype
¶ list of strings[naxis]
List ofCTYPEia
keyvalues.The
ctype
keyword values must be in upper case and there must be zero or one pair of matched celestial axis types, and zero or one spectral axis.

cubeface
¶ int
Index into thepixcrd
(pixel coordinate) array for theCUBEFACE
axis.This is used for quadcube projections where the cube faces are stored on a separate axis.
The quadcube projections (
TSC
,CSC
,QSC
) may be represented in FITS in either of two ways:The six faces may be laid out in one plane and numbered as follows:
0 4 3 2 1 4 3 2 5
Faces 2, 3 and 4 may appear on one side or the other (or both). The worldtopixel routines map faces 2, 3 and 4 to the left but the pixeltoworld routines accept them on either side.
The
COBE
convention in which the six faces are stored in a threedimensional structure using aCUBEFACE
axis indexed from 0 to 5 as above.
These routines support both methods;
set
determines which is being used by the presence or absence of aCUBEFACE
axis inctype
.p2s
ands2p
translate theCUBEFACE
axis representation to the single plane representation understood by the lowerlevel projection routines.

cunit
¶ list of astropy.UnitBase[naxis]
List ofCUNITia
keyvalues asastropy.units.UnitBase
instances.These define the units of measurement of the
CRVALia
,CDELTia
andCDi_ja
keywords.As
CUNITia
is an optional header keyword,cunit
may be left blank but otherwise is expected to contain a standard units specification as defined by WCS Paper I.unitfix
is available to translate commonly used nonstandard units specifications but this must be done as a separate step before invokingset
.For celestial axes, if
cunit
is not blank,set
useswcsunits
to parse it and scalecdelt
,crval
, andcd
to decimal degrees. It then resetscunit
to"deg"
.For spectral axes, if
cunit
is not blank,set
useswcsunits
to parse it and scalecdelt
,crval
, andcd
to SI units. It then resetscunit
accordingly.set
ignorescunit
for other coordinate types;cunit
may be used to label coordinate values.

czphs
¶ double array[naxis]
The time at the zero point of a phase axis,CSPHSia
.An undefined value is represented by NaN.

dateavg
¶ string
Representative midpoint of the date of observation.In ISO format,
yyyymmddThh:mm:ss
.See also

datebeg
¶ string
Date at the start of the observation.In ISO format,
yyyymmddThh:mm:ss
.See also

dateend
¶ string
Date at the end of the observation.In ISO format,
yyyymmddThh:mm:ss
.See also

dateobs
¶ string
Start of the date of observation.In ISO format,
yyyymmddThh:mm:ss
.See also

dateref
¶ string
Date of a reference epoch relative to which other time measurements refer.See also

equinox
¶ double
The equinox associated with dynamical equatorial or ecliptic coordinate systems.EQUINOXa
(orEPOCH
in older headers). Not applicable to ICRS equatorial or ecliptic coordinates.An undefined value is represented by NaN.

imgpix_matrix
¶ double array[2][2]
(readonly) Inverse of theCDELT
orPC
matrix.Inverse containing the product of the
CDELTia
diagonal matrix and thePCi_ja
matrix.

jepoch
¶ double
Equivalent toDATEOBS
.Expressed as a Julian epoch.
See also

lat
¶ int
(readonly) The index into the world coord array containing latitude values.

latpole
¶ double
The native latitude of the celestial pole,LATPOLEa
(deg).

lattyp
¶ string
(readonly) Celestial axis type for latitude.For example, “RA”, “DEC”, “GLON”, “GLAT”, etc. extracted from “RA–”, “DEC“, “GLON”, “GLAT”, etc. in the first four characters of
CTYPEia
but with trailing dashes removed.

lng
¶ int
(readonly) The index into the world coord array containing longitude values.

lngtyp
¶ string
(readonly) Celestial axis type for longitude.For example, “RA”, “DEC”, “GLON”, “GLAT”, etc. extracted from “RA–”, “DEC“, “GLON”, “GLAT”, etc. in the first four characters of
CTYPEia
but with trailing dashes removed.

lonpole
¶ double
The native longitude of the celestial pole.LONPOLEa
(deg).

mjdavg
¶ double
Modified Julian Date corresponding toDATEAVG
.(MJD = JD  2400000.5)
.An undefined value is represented by NaN.
See also

mjdbeg
¶ double
Modified Julian Date corresponding toDATEBEG
.(MJD = JD  2400000.5)
.An undefined value is represented by NaN.
See also

mjdend
¶ double
Modified Julian Date corresponding toDATEEND
.(MJD = JD  2400000.5)
.An undefined value is represented by NaN.
See also

mjdobs
¶ double
Modified Julian Date corresponding toDATEOBS
.(MJD = JD  2400000.5)
.An undefined value is represented by NaN.
See also

mjdref
¶ double
Modified Julian Date corresponding toDATEREF
.(MJD = JD  2400000.5)
.An undefined value is represented by NaN.
See also

name
¶ string
The name given to the coordinate representationWCSNAMEa
.

naxis
¶ int
(readonly) The number of axes (pixel and coordinate).Given by the
NAXIS
orWCSAXESa
keyvalues.The number of coordinate axes is determined at parsing time, and can not be subsequently changed.
It is determined from the highest of the following:
NAXIS
WCSAXESa
The highest axis number in any parameterized WCS keyword. The keyvalue, as well as the keyword, must be syntactically valid otherwise it will not be considered.
If none of these keyword types is present, i.e. if the header only contains auxiliary WCS keywords for a particular coordinate representation, then no coordinate description is constructed for it.
This value may differ for different coordinate representations of the same image.

obsgeo
¶ double array[3]
Location of the observer in a standard terrestrial reference frame.OBSGEOX
,OBSGEOY
,OBSGEOZ
(in meters).An undefined value is represented by NaN.

obsorbit
¶ string
URI, URL, or name of an orbit ephemeris file giving spacecraft coordinates relating to TREFPOS.See also

pc
¶ double array[naxis][naxis]
ThePCi_ja
(pixel coordinate) transformation matrix.The order is:
[[PC1_1, PC1_2], [PC2_1, PC2_2]]
For historical compatibility, three alternate specifications of the linear transformations are available in wcslib. The canonical
PCi_ja
withCDELTia
,CDi_ja
, and the deprecatedCROTAia
keywords. Although the latter may not formally coexist withPCi_ja
, the approach here is simply to ignore them if given in conjunction withPCi_ja
.has_pc
,has_cd
andhas_crota
can be used to determine which of these alternatives are present in the header.These alternate specifications of the linear transformation matrix are translated immediately to
PCi_ja
byset
and are nowhere visible to the lowerlevel routines. In particular,set
resetscdelt
to unity ifCDi_ja
is present (and noPCi_ja
). If noCROTAia
is associated with the latitude axis,set
reverts to a unityPCi_ja
matrix.

phi0
¶ double
The native latitude of the fiducial point.The point whose celestial coordinates are given in
ref[1:2]
. If undefined (NaN) the initialization routine,set
, will set this to a projectionspecific default.See also

piximg_matrix
¶ double array[2][2]
(readonly) Matrix containing the product of theCDELTia
diagonal matrix and thePCi_ja
matrix.

plephem
¶ string
The Solar System ephemeris used for calculating a pathlength delay.See also

radesys
¶ string
The equatorial or ecliptic coordinate system type,RADESYSa
.

restfrq
¶ double
Rest frequency (Hz) fromRESTFRQa
.An undefined value is represented by NaN.

restwav
¶ double
Rest wavelength (m) fromRESTWAVa
.An undefined value is represented by NaN.

spec
¶ int
(readonly) The index containing the spectral axis values.

specsys
¶ string
Spectral reference frame (standard of rest),SPECSYSa
.

ssysobs
¶ string
Spectral reference frame.The spectral reference frame in which there is no differential variation in the spectral coordinate across the fieldofview,
SSYSOBSa
.

ssyssrc
¶ string
Spectral reference frame for redshift.The spectral reference frame (standard of rest) in which the redshift was measured,
SSYSSRCa
.

tab
¶ list of Tabprm
Tabular coordinate objects.A list of tabular coordinate objects associated with this WCS.

telapse
¶ double
equivalent to the elapsed time between DATEBEG and DATEEND, in units of TIMEUNIT.See also

theta0
¶ double
The native longitude of the fiducial point.The point whose celestial coordinates are given in
ref[1:2]
. If undefined (NaN) the initialization routine,set
, will set this to a projectionspecific default.See also

timedel
¶ double
the resolution of the time stamps.See also

timeoffs
¶ double
Time offset, which may be used, for example, to provide a uniform clock correctionfor times referenced to DATEREF.
See also

timepixr
¶ double
relative position of the time stamps in binned time intervals, a value between 0.0 and 1.0.See also

timesys
¶ string
Time scale (UTC, TAI, etc.) in which all other timerelated auxiliary header values are recorded. Also defines the time scale for an image axis with CTYPEia set to ‘TIME’.See also

timeunit
¶ string
Time units in which the following header values are expressed:TSTART
,TSTOP
,TIMEOFFS
,TIMSYER
,TIMRDER
,TIMEDEL
.It also provides the default value for
CUNITia
for time axes.See also

timrder
¶ double
the accuracy of time stamps relative to each other, in units of TIMEUNIT.See also

timsyer
¶ double
the absolute error of the time values, in units of TIMEUNIT.See also

trefdir
¶ string
Reference direction used in calculating a pathlength delay.See also

trefpos
¶ string
Location in space where the recorded time is valid.See also

tstart
¶ double
equivalent to DATEBEG expressed as a time in units of TIMEUNIT relative to DATEREF+TIMEOFFS.See also

tstop
¶ double
equivalent to DATEEND expressed as a time in units of TIMEUNIT relative to DATEREF+TIMEOFFS.See also

velangl
¶ double
Velocity angle.The angle in degrees that should be used to decompose an observed velocity into radial and transverse components.
An undefined value is represented by NaN.

velosys
¶ double
Relative radial velocity.The relative radial velocity (m/s) between the observer and the selected standard of rest in the direction of the celestial reference coordinate,
VELOSYSa
.An undefined value is represented by NaN.

velref
¶ int
AIPS velocity code.From
VELREF
keyword.

wtb
¶ list of Wtbarr
objects to construct coordinate lookup tables from BINTABLE.

xposure
¶ double
effective exposure time in units of TIMEUNIT.See also

zsource
¶ double
The redshift,ZSOURCEa
, of the source.An undefined value is represented by NaN.
Methods Documentation

bounds_check
(pix2world, world2pix)¶ Enable/disable bounds checking.
 Parameters
Notes
Note that by default (without calling
bounds_check
) strict bounds checking is enabled.

cdfix
()¶ Fix erroneously omitted
CDi_ja
keywords.Sets the diagonal element of the
CDi_ja
matrix to unity if allCDi_ja
keywords associated with a given axis were omitted. According to Paper I, if anyCDi_ja
keywords at all are given in a FITS header then those not given default to zero. This results in a singular matrix with an intersecting row and column of zeros. Returns
 success
int
Returns
0
for success;1
if no change required.
 success

celfix
()¶ Translates AIPSconvention celestial projection types,
NCP
andGLS
. Returns
 success
int
Returns
0
for success;1
if no change required.
 success

compare
(other, cmp=0, tolerance=0.0)¶ Compare two Wcsprm objects for equality.
 Parameters
 other
Wcsprm
The other Wcsprm object to compare to.
 cmp
int
, optional A bit field controlling the strictness of the comparison. When 0, (the default), all fields must be identical.
The following constants, defined in the
astropy.wcs
module, may be or’ed together to loosen the comparison.WCSCOMPARE_ANCILLARY
: Ignores ancillary keywords that don’t change the WCS transformation, such asXPOSURE
orEQUINOX
. Note that this also ignoresDATEOBS
, which does change the WCS transformation in some cases.WCSCOMPARE_TILING
: Ignore integral differences inCRPIXja
. This is the ‘tiling’ condition, where two WCSes cover different regions of the same map projection and align on the same map grid.WCSCOMPARE_CRPIX
: Ignore any differences at all inCRPIXja
. The two WCSes cover different regions of the same map projection but may not align on the same grid map. OverridesWCSCOMPARE_TILING
.
 tolerance
float
, optional The amount of tolerance required. For example, for a value of 1e6, all floatingpoint values in the objects must be equal to the first 6 decimal places. The default value of 0.0 implies exact equality.
 other
 Returns
 equalbool

cylfix
()¶ Fixes WCS keyvalues for malformed cylindrical projections.
 Returns
 success
int
Returns
0
for success;1
if no change required.
 success

datfix
()¶ Translates the old
DATEOBS
date format to year2000 standard form(yyyymmddThh:mm:ss)
and derivesMJDOBS
from it if not already set.Alternatively, if
mjdobs
is set anddateobs
isn’t, thendatfix
derivesdateobs
from it. If both are set but disagree by more than half a day thenValueError
is raised. Returns
 success
int
Returns
0
for success;1
if no change required.
 success

fix
(translate_units='', naxis=0)¶ Applies all of the corrections handled separately by
datfix
,unitfix
,celfix
,spcfix
,cylfix
andcdfix
. Parameters
 translate_units
str
, optional Specify which potentially unsafe translations of nonstandard unit strings to perform. By default, performs all.
Although
"S"
is commonly used to represent seconds, its translation to"s"
is potentially unsafe since the standard recognizes"S"
formally as Siemens, however rarely that may be used. The same applies to"H"
for hours (Henry), and"D"
for days (Debye).This string controls what to do in such cases, and is caseinsensitive.
If the string contains
"s"
, translate"S"
to"s"
.If the string contains
"h"
, translate"H"
to"h"
.If the string contains
"d"
, translate"D"
to"d"
.
Thus
''
doesn’t do any unsafe translations, whereas'shd'
does all of them. naxis
int
array
, optional Image axis lengths. If this array is set to zero or
None
, thencylfix
will not be invoked.
 translate_units
 Returns

get_cdelt
() → numpy.ndarray¶ Coordinate increments (
CDELTia
) for each coord axis asdouble array[naxis]
.Returns the
CDELT
offsets in readonly form. Unlike thecdelt
property, this works even when the header specifies the linear transformation matrix in one of the alternativeCDi_ja
orCROTAia
forms. This is useful when you want access to the linear transformation matrix, but don’t care how it was specified in the header.

get_pc
() → numpy.ndarray¶ Returns the
PC
matrix in readonly form asdouble array[naxis][naxis]
. Unlike thepc
property, this works even when the header specifies the linear transformation matrix in one of the alternativeCDi_ja
orCROTAia
forms. This is useful when you want access to the linear transformation matrix, but don’t care how it was specified in the header.

get_ps
() → list¶ Returns
PSi_ma
keywords for each i and m as list of tuples. Returns
 ps
list
Returned as a list of tuples of the form (i, m, value):
i: int. Axis number, as in
PSi_ma
, (i.e. 1relative)m: int. Parameter number, as in
PSi_ma
, (i.e. 0relative)value: string. Parameter value.
 ps
See also
astropy.wcs.Wcsprm.set_ps
Set
PSi_ma
values

get_pv
() → list¶ Returns
PVi_ma
keywords for each i and m as list of tuples. Returns
See also
astropy.wcs.Wcsprm.set_pv
Set
PVi_ma
values
Notes
Note that, if they were not given,
set
resets the entries forPVi_1a
,PVi_2a
,PVi_3a
, andPVi_4a
for longitude axis i to match (phi_0
,theta_0
), the native longitude and latitude of the reference point given byLONPOLEa
andLATPOLEa
.

has_cd
() → bool¶ Returns
True
ifCDi_ja
is present.CDi_ja
is an alternate specification of the linear transformation matrix, maintained for historical compatibility.Matrix elements in the IRAF convention are equivalent to the product
CDi_ja = CDELTia * PCi_ja
, but the defaults differ from that of thePCi_ja
matrix. If one or moreCDi_ja
keywords are present then all unspecifiedCDi_ja
default to zero. If noCDi_ja
(orCROTAia
) keywords are present, then the header is assumed to be inPCi_ja
form whether or not anyPCi_ja
keywords are present since this results in an interpretation ofCDELTia
consistent with the original FITS specification.While
CDi_ja
may not formally coexist withPCi_ja
, it may coexist withCDELTia
andCROTAia
which are to be ignored.See also
astropy.wcs.Wcsprm.cd
Get the raw
CDi_ja
values.

has_crota
() → bool¶ Returns
True
ifCROTAia
is present.CROTAia
is an alternate specification of the linear transformation matrix, maintained for historical compatibility.In the AIPS convention,
CROTAia
may only be associated with the latitude axis of a celestial axis pair. It specifies a rotation in the image plane that is applied after theCDELTia
; any otherCROTAia
keywords are ignored.CROTAia
may not formally coexist withPCi_ja
.CROTAia
andCDELTia
may formally coexist withCDi_ja
but if so are to be ignored.See also
astropy.wcs.Wcsprm.crota
Get the raw
CROTAia
values

has_pc
() → bool¶ Returns
True
ifPCi_ja
is present.PCi_ja
is the recommended way to specify the linear transformation matrix.See also
astropy.wcs.Wcsprm.pc
Get the raw
PCi_ja
values

mix
(mixpix, mixcel, vspan, vstep, viter, world, pixcrd, origin)¶ Given either the celestial longitude or latitude plus an element of the pixel coordinate, solves for the remaining elements by iterating on the unknown celestial coordinate element using
s2p
. Parameters
 mixpix
int
Which element on the pixel coordinate is given.
 mixcel
int
Which element of the celestial coordinate is given. If mixcel =
1
, celestial longitude is given inworld[self.lng]
, latitude returned inworld[self.lat]
. If mixcel =2
, celestial latitude is given inworld[self.lat]
, longitude returned inworld[self.lng]
. vspan(
float
,float
) Solution interval for the celestial coordinate, in degrees. The ordering of the two limits is irrelevant. Longitude ranges may be specified with any convenient normalization, for example
(120,+120)
is the same as(240,480)
, except that the solution will be returned with the same normalization, i.e. lie within the interval specified. vstep
float
Step size for solution search, in degrees. If
0
, a sensible, although perhaps nonoptimal default will be used. viter
int
If a solution is not found then the step size will be halved and the search recommenced. viter controls how many times the step size is halved. The allowed range is 5  10.
 world
ndarray
World coordinate elements as
double array[naxis]
.world[self.lng]
andworld[self.lat]
are the celestial longitude and latitude, in degrees. Which is given and which returned depends on the value of mixcel. All other elements are given. The results will be written to this array inplace. pixcrd
ndarray
Pixel coordinates as
double array[naxis]
. The element indicated by mixpix is given and the remaining elements will be written inplace. origin
int
Specifies the origin of pixel values. The Fortran and FITS standards use an origin of 1. Numpy and C use array indexing with origin at 0.
 mixpix
 Returns
 result
dict
Returns a dictionary with the following keys:
phi (
double array[naxis]
)theta (
double array[naxis]
)Longitude and latitude in the native coordinate system of the projection, in degrees.
imgcrd (
double array[naxis]
)Image coordinate elements.
imgcrd[self.lng]
andimgcrd[self.lat]
are the projected x and ycoordinates, in decimal degrees.
world (
double array[naxis]
)Another reference to the world argument passed in.
 result
 Raises
MemoryError
Memory allocation failed.
SingularMatrixError
Linear transformation matrix is singular.
InconsistentAxisTypesError
Inconsistent or unrecognized coordinate axis types.
ValueError
Invalid parameter value.
InvalidTransformError
Invalid coordinate transformation parameters.
InvalidTransformError
Illconditioned coordinate transformation parameters.
InvalidCoordinateError
Invalid world coordinate.
NoSolutionError
No solution found in the specified interval.
See also
astropy.wcs.Wcsprm.lat
,astropy.wcs.Wcsprm.lng
Get the axes numbers for latitude and longitude
Notes
Initially, the specified solution interval is checked to see if it’s a “crossing” interval. If it isn’t, a search is made for a crossing solution by iterating on the unknown celestial coordinate starting at the upper limit of the solution interval and decrementing by the specified step size. A crossing is indicated if the trial value of the pixel coordinate steps through the value specified. If a crossing interval is found then the solution is determined by a modified form of “regula falsi” division of the crossing interval. If no crossing interval was found within the specified solution interval then a search is made for a “noncrossing” solution as may arise from a point of tangency. The process is complicated by having to make allowance for the discontinuities that occur in all map projections.
Once one solution has been determined others may be found by subsequent invocations of
mix
with suitably restricted solution intervals.Note the circumstance that arises when the solution point lies at a native pole of a projection in which the pole is represented as a finite curve, for example the zenithals and conics. In such cases two or more valid solutions may exist but
mix
only ever returns one.Because of its generality,
mix
is very computeintensive. For computelimited applications, more efficient specialcase solvers could be written for simple projections, for example nonoblique cylindrical projections.

p2s
(pixcrd, origin)¶ Converts pixel to world coordinates.
 Parameters
 Returns
 result
dict
Returns a dictionary with the following keys:
imgcrd: ndarray
Array of intermediate world coordinates as
double array[ncoord][nelem]
. For celestial axes,imgcrd[][self.lng]
andimgcrd[][self.lat]
are the projected x, and ycoordinates, in pseudo degrees. For spectral axes,imgcrd[][self.spec]
is the intermediate spectral coordinate, in SI units.
phi: ndarray
Array as
double array[ncoord]
.
theta: ndarray
Longitude and latitude in the native coordinate system of the projection, in degrees, as
double array[ncoord]
.
world: ndarray
Array of world coordinates as
double array[ncoord][nelem]
. For celestial axes,world[][self.lng]
andworld[][self.lat]
are the celestial longitude and latitude, in degrees. For spectral axes,world[][self.spec]
is the intermediate spectral coordinate, in SI units.
stat: ndarray
Status return value for each coordinate as
int array[ncoord]
.0
for success,1+
for invalid pixel coordinate.
 result
 Raises
MemoryError
Memory allocation failed.
SingularMatrixError
Linear transformation matrix is singular.
InconsistentAxisTypesError
Inconsistent or unrecognized coordinate axis types.
ValueError
Invalid parameter value.
ValueError
x and ycoordinate arrays are not the same size.
InvalidTransformError
Invalid coordinate transformation parameters.
InvalidTransformError
Illconditioned coordinate transformation parameters.
See also
astropy.wcs.Wcsprm.lat
,astropy.wcs.Wcsprm.lng
Definition of the latitude and longitude axes

print_contents
()¶ Print the contents of the
Wcsprm
object to stdout. Probably only useful for debugging purposes, and may be removed in the future.To get a string of the contents, use
repr
.

s2p
(world, origin)¶ Transforms world coordinates to pixel coordinates.
 Parameters
 Returns
 result
dict
Returns a dictionary with the following keys:
phi:
double array[ncoord]
theta:
double array[ncoord]
Longitude and latitude in the native coordinate system of the projection, in degrees.
imgcrd:
double array[ncoord][nelem]
Array of intermediate world coordinates. For celestial axes,
imgcrd[][self.lng]
andimgcrd[][self.lat]
are the projected x, and ycoordinates, in pseudo “degrees”. For quadcube projections with aCUBEFACE
axis, the face number is also returned inimgcrd[][self.cubeface]
. For spectral axes,imgcrd[][self.spec]
is the intermediate spectral coordinate, in SI units.
pixcrd:
double array[ncoord][nelem]
Array of pixel coordinates. Pixel coordinates are zerobased.
stat:
int array[ncoord]
Status return value for each coordinate.
0
for success,1+
for invalid pixel coordinate.
 result
 Raises
MemoryError
Memory allocation failed.
SingularMatrixError
Linear transformation matrix is singular.
InconsistentAxisTypesError
Inconsistent or unrecognized coordinate axis types.
ValueError
Invalid parameter value.
InvalidTransformError
Invalid coordinate transformation parameters.
InvalidTransformError
Illconditioned coordinate transformation parameters.
See also
astropy.wcs.Wcsprm.lat
,astropy.wcs.Wcsprm.lng
Definition of the latitude and longitude axes

set
()¶ Sets up a WCS object for use according to information supplied within it.
Note that this routine need not be called directly; it will be invoked by
p2s
ands2p
if necessary.Some attributes that are based on other attributes (such as
lattyp
onctype
) may not be correct until afterset
is called.set
strips off trailing blanks in all string members.set
recognizes theNCP
projection and converts it to the equivalentSIN
projection and it also recognizesGLS
as a synonym forSFL
. It does alias translation for the AIPS spectral types (FREQLSR
,FELOHEL
, etc.) but without changing the input header keywords. Raises
MemoryError
Memory allocation failed.
SingularMatrixError
Linear transformation matrix is singular.
InconsistentAxisTypesError
Inconsistent or unrecognized coordinate axis types.
ValueError
Invalid parameter value.
InvalidTransformError
Invalid coordinate transformation parameters.
InvalidTransformError
Illconditioned coordinate transformation parameters.

set_ps
(ps)¶ Sets
PSi_ma
keywords for each i and m. Parameters
See also

set_pv
(pv)¶ Sets
PVi_ma
keywords for each i and m. Parameters
See also

spcfix
() → int¶ Translates AIPSconvention spectral coordinate types. {
FREQ
,VELO
,FELO
}{OBS
,HEL
,LSR
} (e.g.FREQLSR
,VELOOBS
,FELOHEL
) Returns
 success
int
Returns
0
for success;1
if no change required.
 success

sptr
(ctype, i= 1)¶ Translates the spectral axis in a WCS object.
For example, a
FREQ
axis may be translated intoZOPTF2W
and vice versa. Parameters
 ctype
str
Required spectral
CTYPEia
, maximum of 8 characters. The first four characters are required to be given and are never modified. The remaining four, the algorithm code, are completely determined by, and must be consistent with, the first four characters. Wildcarding may be used, i.e. if the final three characters are specified as"???"
, or if just the eighth character is specified as"?"
, the correct algorithm code will be substituted and returned. i
int
Index of the spectral axis (0relative). If
i < 0
(or not provided), it will be set to the first spectral axis identified from theCTYPE
keyvalues in the FITS header.
 ctype
 Raises
MemoryError
Memory allocation failed.
SingularMatrixError
Linear transformation matrix is singular.
InconsistentAxisTypesError
Inconsistent or unrecognized coordinate axis types.
ValueError
Invalid parameter value.
InvalidTransformError
Invalid coordinate transformation parameters.
InvalidTransformError
Illconditioned coordinate transformation parameters.
InvalidSubimageSpecificationError
Invalid subimage specification (no spectral axis).

sub
(axes)¶ Extracts the coordinate description for a subimage from a
WCS
object.The world coordinate system of the subimage must be separable in the sense that the world coordinates at any point in the subimage must depend only on the pixel coordinates of the axes extracted. In practice, this means that the
PCi_ja
matrix of the original image must not contain nonzero offdiagonal terms that associate any of the subimage axes with any of the nonsubimage axes.sub
can also add axes to a wcsprm object. The new axes will be created using the defaults set by the Wcsprm constructor which produce a simple, unnamed, linear axis with world coordinates equal to the pixel coordinate. These default values can be changed before invokingset
. Parameters
 axes
int
ora
sequence. If an int, include the first N axes in their original order.
If a sequence, may contain a combination of image axis numbers (1relative) or special axis identifiers (see below). Order is significant;
axes[0]
is the axis number of the input image that corresponds to the first axis in the subimage, etc. Use an axis number of 0 to create a new axis using the defaults.If
0
,[]
orNone
, do a deep copy.
Coordinate axes types may be specified using either strings or special integer constants. The available types are:
'longitude'
/WCSSUB_LONGITUDE
: Celestial longitude'latitude'
/WCSSUB_LATITUDE
: Celestial latitude'cubeface'
/WCSSUB_CUBEFACE
: QuadcubeCUBEFACE
axis'spectral'
/WCSSUB_SPECTRAL
: Spectral axis'stokes'
/WCSSUB_STOKES
: Stokes axis'temporal'
/WCSSUB_TIME
: Time axis (requiresWCSLIB
version 7.8 or greater)'celestial'
/WCSSUB_CELESTIAL
: An alias for the combination of'longitude'
,'latitude'
and'cubeface'
.
 axes
 Returns
 Raises
MemoryError
Memory allocation failed.
InvalidSubimageSpecificationError
Invalid subimage specification (no spectral axis).
NonseparableSubimageCoordinateSystemError
Nonseparable subimage coordinate system.
Notes
Combinations of subimage axes of particular types may be extracted in the same order as they occur in the input image by combining the integer constants with the ‘binary or’ (

) operator. For example:wcs.sub([WCSSUB_LONGITUDE  WCSSUB_LATITUDE  WCSSUB_SPECTRAL])
would extract the longitude, latitude, and spectral axes in the same order as the input image. If one of each were present, the resulting object would have three dimensions.
For convenience,
WCSSUB_CELESTIAL
is defined as the combinationWCSSUB_LONGITUDE  WCSSUB_LATITUDE  WCSSUB_CUBEFACE
.The codes may also be negated to extract all but the types specified, for example:
wcs.sub([ WCSSUB_LONGITUDE, WCSSUB_LATITUDE, WCSSUB_CUBEFACE, (WCSSUB_SPECTRAL  WCSSUB_STOKES)])
The last of these specifies all axis types other than spectral or Stokes. Extraction is done in the order specified by
axes
, i.e. a longitude axis (if present) would be extracted first (viaaxes[0]
) and not subsequently (viaaxes[3]
). Likewise for the latitude and cubeface axes in this example.The number of dimensions in the returned object may be less than or greater than the length of
axes
. However, it will never exceed the number of axes in the input image.

to_header
(relax=False)¶ to_header
translates a WCS object into a FITS header.The details of the header depends on context:
The output header will almost certainly differ from the input in a number of respects:
The output header only contains WCSrelated keywords. In particular, it does not contain syntacticallyrequired keywords such as
SIMPLE
,NAXIS
,BITPIX
, orEND
.Deprecated (e.g.
CROTAn
) or nonstandard usage will be translated to standard (this is partially dependent on whetherfix
was applied).Quantities will be converted to the units used internally, basically SI with the addition of degrees.
Floatingpoint quantities may be given to a different decimal precision.
Elements of the
PCi_j
matrix will be written if and only if they differ from the unit matrix. Thus, if the matrix is unity then no elements will be written.Additional keywords such as
WCSAXES
,CUNITia
,LONPOLEa
andLATPOLEa
may appear.The original keycomments will be lost, although
to_header
tries hard to write meaningful comments.Keyword order may be changed.
Keywords can be translated between the image array, binary table, and pixel lists forms by manipulating the
colnum
orcolax
members of theWCS
object. Parameters
 Returns
 header
str
Raw FITS header as a string.
 header

unitfix
(translate_units='')¶ Translates nonstandard
CUNITia
keyvalues.For example,
DEG
>deg
, also stripping off unnecessary whitespace. Parameters
 translate_units
str
, optional Do potentially unsafe translations of nonstandard unit strings.
Although
"S"
is commonly used to represent seconds, its recognizes"S"
formally as Siemens, however rarely that may be translation to"s"
is potentially unsafe since the standard used. The same applies to"H"
for hours (Henry), and"D"
for days (Debye).This string controls what to do in such cases, and is caseinsensitive.
If the string contains
"s"
, translate"S"
to"s"
.If the string contains
"h"
, translate"H"
to"h"
.If the string contains
"d"
, translate"D"
to"d"
.
Thus
''
doesn’t do any unsafe translations, whereas'shd'
does all of them.
 translate_units
 Returns
 success
int
Returns
0
for success;1
if no change required.
 success