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LSST Data Management Base Package
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A space separated list of all the conditions names recognized by the Warnings attribute. (string)
The following conditions are currently recognised (all are case-insensitive):
CTYPE keywords.CTYPE keyword, and the illegal code can be converted to an equivalent legal code.PCi_j, CDi_j, CROTA, etc. For example, the matrix will not be invertable if any rows or columns consist entirely of zeros. The FITS-WCS Paper I "Representation of World Coordinates in FITS" by Greisen & Calabretta requires that this matrix be invertable. Many operations (such as grid plotting) will not be possible if the matrix cannot be inverted.PVi_m header is found that refers to a projection parameter that is not used by the projection type specified by CTYPE, or the PV values are otherwise inappropriate for the projection type.CTYPE keywords specify an unsupported distortion code using the "4-3-3" format specified in FITS-WCS paper IV. Such distortion codes are ignored.CTYPE value is used within astRead, due to no value being present in the supplied FitsChan. This condition is only tested for when using non-Native encodings.TNX projection which includes terms not supproted by AST. Such terms are ignored and so the resulting FrameSet may be inaccurate.ZPX projection which includes "lngcor" or "latcor" correction terms. These terms are not supported by AST and are ignored. The resulting FrameSet may therefore be inaccurate.The index of the "current" FITS header card within a FitsChan, the first card having an index of 1. (int)
The choice of current card affects the behaviour of functions that access the contents of the FitsChan, such as FitsChan::delFits, FitsChan::findFits and FitsChan::putFits.
A value assigned to Card will position the FitsChan at any desired point, so that a particular card within it can be accessed. Alternatively, the value of Card may be enquired in order to determine the current position of a FitsChan.
The default value of Card is 1. This means that clearing this attribute (using FitsChan::clearCard) effectively "rewinds" the FitsChan, so that the first card is accessed next. If Card is set to a value which exceeds the total number of cards in the FitsChan (as given by its NCard attribute), it is regarded as pointing at the "end-of-file". In this case, the value returned in response to an enquiry is always one more than the number of cards in the FitsChan.
The comment for the current card of the FitsChan. (string)
A zero-length string is returned if the card has no comment or the card does not exist (Card points to a non-existent card).
The name of the keyword for the current card of the FitsChan. (string)
A zero-length string is returned if the card does not exist (Card points to a non-existent card).
The data type of the keyword value for the current card of the FitsChan as a CardType enum.
Ignore spherical rotations on CAR projections? (bool)
CarLin is a boolean value which specifies how FITS CAR (plate carree, or "Cartesian") projections should be treated when reading a FrameSet from a foreign encoded FITS header.
If CarLin is false (the default), it is assumed that the CAR projection conforms to the conventions described in the FITS world coordinate system (FITS-WCS) paper II "Representation of Celestial Coordinates in FITS" by M. Calabretta & E.W. Greisen. If CarLin is true, then these conventions are ignored, and it is assumed that the mapping from pixel coordinates to celestial coordinates is a simple linear transformation (hence the attribute name CarLin). This is appropriate for some older FITS data which claims to have a CAR projection, but which in fact do not conform to the conventions of the FITS-WCS paper.
The FITS-WCS paper specifies that headers which include a CAR projection represent a linear mapping from pixel coordinates to "native spherical
coordinates", NOT celestial coordinates. An extra mapping is then required from native spherical to celestial. This mapping is a 3D rotation and so the overall Mapping from pixel to celestial coordinates is NOT linear. See the FITS-WCS papers for further details.
Use CDi_j keywords to represent pixel scaling, rotation, etc? (bool)
CDMatrix is a boolean value which specifies how the linear transformation from pixel coordinates to intermediate world coordinates should be represented within a FitsChan when using FITS-WCS encoding. This transformation describes the scaling, rotation, shear, etc., of the pixel axes.
If the attribute has a non-zero value then the transformation is represented by a set of CDi_j keywords representing a square matrix (where "i" is the index of an intermediate world coordinate axis and "j" is the index of a pixel axis). If the attribute has a zero value the transformation is represented by a set of PCi_j keywords (which also represent a square matrix) together with a corresponding set of CDELTi keywords representing the axis scalings. See FITS-WCS paper II "Representation of Celestial Coordinates in FITS" by M. Calabretta & E.W. Greisen, for a complete description of these two schemes.
The default value of the CDMatrix attribute is determined by the contents of the FitsChan at the time the attribute is accessed. If the FitsChan contains any CDi_j keywords then the default value is non-zero. Otherwise it is zero. Note, reading a FrameSet from a FitsChan will in general consume any CDi_j keywords present in the FitsChan. Thus the default value for CDMatrix following a read will usually be zero, even if the FitsChan originally contained some CDi_j keywords. This behaviour is similar to that of the Encoding attribute, the default value for which is determined by the contents of the FitsChan at the time the attribute is accessed. If you wish to retain the original value of the CDMatrix attribute (that is, the value before reading the FrameSet) then you should enquire the default value before doing the read, and then set that value explicitly.
Remove cards used whilst reading even if an error occurs? (bool)
A succesful read on a FitsChan always results in the removal of the cards which were involved in the description of the returned Object. However, in the event of an error during the read (for instance if the cards in the FitsChan have illegal values, or if some required cards are missing) no cards will be removed from the FitsChan if the Clean attribute is false (the default). If Clean is true then any cards which were used in the aborted attempt to read an object will be removed.
This provides a means of "cleaning" a FitsChan of WCS-related cards which works even in the event of the cards not forming a legal WCS description.
Use FK4 B1950 as default equatorial coordinates? (bool)
DefB1950 specifies a default equinox and reference frame to use when reading a FrameSet from a FitsChan with a foreign (i.e. non-native) encoding.
It is only used if the FITS header contains RA and DEC axes but contains no information about the reference frame or equinox. If this is the case, then values of FK4 and B1950 are assumed if the DefB1950 attribute has a non-zero value and ICRS is assumed if DefB1950 is zero. The default value for DefB1950 depends on the value of the Encoding attribute: for FITS-WCS encoding the default is zero, and for all other encodings it is one.
Encoding affects the behaviour of the Channel::write and Channel::read functions when they are used to transfer any AST Object to or from an external representation consisting of FITS header cards (i.e. whenever a write or read operation is performed using a FitsChan as the I/O Channel).
There are several ways (conventions) by which coordinate system information may be represented in the form of FITS headers and the Encoding attribute is used to specify which of these should be used. The encoding options available are outlined in the "Encodings Available" section below, and in more detail in the sections which follow.
Encoding systems differ in the range of possible Objects (e.g. classes) they can represent, in the restrictions they place on these Objects (e.g. compatibility with some externally-defined coordinate system model) and in the number of Objects that can be stored together in any particular set of FITS header cards (e.g. multiple Objects, or only a single Object). The choice of encoding also affects the range of external applications which can potentially read and interpret the FITS header cards produced.
The encoding options available are not necessarily mutually exclusive, and it may sometimes be possible to store multiple Objects (or the same Object several times) using different encodings within the same set of FITS header cards. This possibility increases the likelihood of other applications being able to read and interpret the information.
By default, a FitsChan will attempt to determine which encoding system is already in use, and will set the default Encoding value accordingly (so that subsequent I/O operations adopt the same conventions). It does this by looking for certain critical FITS keywords which only occur in particular encodings. For details of how this works, see the "Choice of Default Encoding" section below. If you wish to ensure that a particular encoding system is used, independently of any FITS cards already present, you should set an explicit Encoding value yourself.
The Encoding attribute can take any of the following (case insensitive) string values to select the corresponding encoding system:
DSS: Encodes coordinate system information in FITS header cards using the convention developed at the Space Telescope Science Institute (STScI) for the Digitised Sky Survey (DSS) astrometric plate calibrations. The main advantages of this encoding are that FITS images which use it are widely available and it is understood by a number of important and well-established astronomy applications. For further details, see the section "The `DSS` Encoding" below.FITS-WCS: Encodes coordinate system information in FITS header cards using the conventions described in the FITS world coordinate system (FITS-WCS) papers by E.W. Greisen, M. Calabretta, et al. The main advantages of this encoding are that it should be understood by any FITS-WCS compliant application and is likely to be adopted widely for FITS data in future. For further details, see the section "The `FITS-WCS` Encoding" below.FITS-PC: Encodes coordinate system information in FITS header cards using the conventions described in an earlier draft of the FITS world coordinate system papers by E.W. Greisen and M. Calabretta. This encoding uses a combination of CDELTi and PCiiijjj keywords to describe the scale and rotation of the pixel axes. This encoding is included to support existing data and software which uses these now superceded conventions. In general, the FITS-WCS encoding (which uses CDi_j or PCi_j keywords to describe the scale and rotation) should be used in preference to FITS-PC.FITS-IRAF: Encodes coordinate system information in FITS header cards using the conventions described in the document "World Coordinate Systems Representations Within the FITS
Format" by R.J. Hanisch and D.G. Wells, 1988. This encoding is currently employed by the IRAF data analysis facility, so its use will facilitate data exchange with IRAF. Its main advantages are that it is a stable convention which approximates to a subset of the propsed FITS-WCS encoding (above). This makes it suitable as an interim method for storing coordinate system information in FITS headers until the FITS-WCS encoding becomes stable. Since many datasets currently use the FITS-IRAF encoding, conversion of data from FITS-IRAF to the final form of FITS-WCS is likely to be well supported.FITS-AIPS: Encodes coordinate system information in FITS header cards using the conventions originally introduced by the AIPS data analysis facility. This is base on the use of CDELTi and CROTAi keuwords to desribe the scale and rotation of each axis. These conventions have been superceded but are still widely used.FITS-AIPS++: Encodes coordinate system information in FITS header cards using the conventions used by the AIPS++ project. This is an extension of FITS-AIPS which includes some of the features of FITS-IRAF and FITS-PC.FITS-CLASS: Encodes coordinate system information in FITS header cards using the conventions used by the CLASS project. CLASS is a software package for reducing single-dish radio and sub-mm spectroscopic data. See the section "CLASS FITS format" at http://www.iram.fr/IRAMFR/GILDAS/doc/html/class-html/.NATIVE: Encodes AST Objects in FITS header cards using a convention which is private to the AST library (but adheres to the general FITS standard) and which uses FITS keywords that will not clash with other encoding systems. The main advantages of this are that any class of AST Object may be encoded, and any (reasonable) number of Objects may be stored sequentially in the same FITS header. This makes FITS headers an almost loss-less communication path for passing AST Objects between applications (although all such applications must, of course, make use of the AST library to interpret the information). For further details, see the section "The `NATIVE` Encoding" below.If the Encoding attribute of a FitsChan is not set, the default value it takes is determined by the presence of certain critical FITS keywords within the FitsChan. The sequence of decisions
used to arrive at the default value is as follows:
NATIVE encoding is used,FITS-CLASS is used if the FitsChan contains a DELTAV keyword and a keyword of the form VELO-xxx, where xxx indicates one of the rest frames used by class (e.g. "VELO-LSR"), or "VLSR".FITS-AIPS or FITS-AIPS++ encoding is used. FITS-AIPS++ is used if any of the keywords CDi_j, PROJP, LONPOLE or LATPOLE are found in the FitsChan. Otherwise FITS-AIPS is used.FITS-PC encoding is used,FITS-IRAF encoding is used,FITS-IRAF encoding is used.FITS-PC encoding is used.FITS-AIPS encoding is used.FITS-WCS encoding is used.DSS encoding is used,NATIVE encoding is used.Except for the NATIVE and DSS encodings, all the above checks also require that the header contains at least one CTYPE, CRPIX and CRVAL keyword (otherwise the checking process continues to the next case).
Setting an explicit value for the Encoding attribute always over-rides this default behaviour.
Note that when writing information to a FitsChan, the choice of encoding will depend greatly on the type of application you expect to be reading the information in future. If you do not know this, there may sometimes be an advantage in writing the information several times, using a different encoding on each occasion.
DSS Encoding:The DSS encoding uses FITS header cards to store a multi-term polynomial which relates pixel positions on a digitised photographic plate to celestial coordinates (right ascension and declination). This encoding may only be used to store a single AST Object in any set of FITS header cards, and that Object must be a FrameSet which conforms to the STScI/DSS coordinate system model (this means the Mapping which relates its base and current Frames must include either a DssMap or a WcsMap with type AST__TAN or AST__TPN).
When reading a DSS encoded Object (using Channel::read), the FitsChan concerned must initially be positioned at the first card (its Card attribute must equal 1) and the result of the read, if successful, will always be a pointer to a FrameSet. The base Frame of this FrameSet represents DSS pixel coordinates, and the current Frame represents DSS celestial coordinates. Such a read is always destructive and causes the FITS header cards required for the construction of the FrameSet to be removed from the FitsChan, which is then left positioned at the "end-of-file". A subsequent read using the same encoding will therefore not return another FrameSet, even if the FitsChan is rewound.
When Channel::write is used to store a FrameSet using DSS encoding, an attempt is first made to simplify the FrameSet to see if it conforms to the DSS model. Specifically, the current Frame must be a FK5 SkyFrame; the projection must be a tangent plane (gnomonic) projection with polynomial corrections conforming to DSS requirements, and north must be parallel to the second base Frame axis.
If the simplification process succeeds, a description of the FrameSet is written to the FitsChan using appropriate DSS FITS header cards. The base Frame of the FrameSet is used to form the DSS pixel coordinate system and the current Frame gives the DSS celestial coordinate system. A successful write operation will over-write any existing DSS encoded data in the FitsChan, but will not affect other (non-DSS) header cards. If a destructive read of a DSS encoded Object has previously occurred, then an attempt will be made to store the FITS header cards back in their original locations.
If an attempt to simplify a FrameSet to conform to the DSS model fails (or if the Object supplied is not a FrameSet), then no data will be written to the FitsChan and Channel::write will return zero. No error will result.
FITS-WCS Encoding:The FITS-WCS convention uses FITS header cards to describe the relationship between pixels in an image (not necessarily 2-dimensional) and one or more related "world coordinate systems". The FITS-WCS encoding may only be used to store a single AST Object in any set of FITS header cards, and that Object must be a FrameSet which conforms to the FITS-WCS model (the FrameSet may, however, contain multiple Frames which will be result in multiple FITS "alternate axis descriptions"). Details of the use made by this Encoding of the conventions described in the FITS-WCS papers are given in the appendix FITS-WCS Coverage" of this document. A few
main points are described below.
The rotation and scaling of the intermediate world coordinate system
can be specified using either "CDi_j" keywords, or "PCi_j" together
with "CDELTi" keywords. When writing a @ref FrameSet to a @ref FitsChan, the
the value of the CDMatrix attribute of the @ref FitsChan determines
which system is used.
In addition, this encoding supports the "TAN with polynomial correction terms" projection which was included in a draft of the <tt>FITS-WCS</tt> paper,
but was not present in the final version. A "TAN with polynomial correction terms" projection is represented using a WcsMap with type
AST__TPN (rather than AST__TAN which is used to represent simple
TAN projections). When reading a FITS header, a CTYPE keyword value
including a "-TAN" code results in an AST__TPN projection if there are
any projection parameters (given by the PVi_m keywords) associated with
the latitude axis, or if there are projection parameters associated
with the longitude axis for m greater than 4. When writing a
@ref FrameSet to a FITS header, an AST__TPN projection gives rise to a
CTYPE value including the normal "-TAN" code, but the projection
parameters are stored in keywords with names "QVi_m", instead of the
usual "PVi_m". Since these QV parameters are not part of the
<tt>FITS-WCS</tt> standard they will be ignored by other non-AST software,
resulting in the WCS being interpreted as a simple TAN projection
without any corrections. This should be seen as an interim solution
until such time as an agreed method for describing projection
distortions within <tt>FITS-WCS</tt> has been published.
AST extends the range of celestial coordinate systems which may be
described using this encoding by allowing the inclusion of
"AZ--" and "EL--" as the coordinate specification within CTYPE
values. These form a longitude/latitude pair of axes which describe
azimuth and elevation. The geographic position of the observer
should be supplied using the OBSGEO-X/Y/Z keywords described in <tt>FITS-WCS</tt>
paper III. Currently, a simple model is used which includes diurnal
aberration, but ignores atmospheric refraction, polar motion, etc.
These may be added in a later release.
If an AST SkyFrame that represents offset rather than absolute
coordinates (see attribute @ref SkyFrame_SkyRefIs "SkyRefIs") is written to a @ref FitsChan using
<tt>FITS-WCS</tt> encoding, two alternate axis descriptions will be created.
One will describe the offset coordinates, and will use "OFLN" and
"OFLT" as the axis codes in the CTYPE keywords. The other will
describe absolute coordinates as specified by the System attribute
of the SkyFrame, using the usual CTYPE codes ("RA--"/"DEC-", etc).
In addition, the absolute coordinates description will contain
AST-specific keywords (SREF1/2, SREFP1/2 and SREFIS) that allow the
header to be read back into AST in order to reconstruct the original
SkyFrame.
When reading a <tt>FITS-WCS</tt> encoded @ref Object (using @ref Channel.read), the @ref FitsChan
concerned must initially be positioned at the first card (its
Card attribute must equal 1) and the result of the read, if
successful, will always be a pointer to a @ref FrameSet. The base
Frame of this @ref FrameSet represents <tt>FITS-WCS</tt> pixel coordinates,
and the current Frame represents the physical coordinate system
described by the <tt>FITS-WCS</tt> primary axis descriptions. If
secondary axis descriptions are also present, then the @ref FrameSet
may contain additional (non-current) Frames which represent
these. Such a read is always destructive and causes the FITS
header cards required for the construction of the @ref FrameSet to be
removed from the @ref FitsChan, which is then left positioned at the
"end-of-file". A subsequent read using the same encoding will
therefore not return another @ref FrameSet, even if the @ref FitsChan is
rewound.
When @ref Channel.write is used to store a @ref FrameSet using <tt>FITS-WCS</tt>
encoding, an attempt is first made to simplify the @ref FrameSet to
see if it conforms to the <tt>FITS-WCS</tt> model. If this simplification
process succeeds (as it often should, as the model is reasonably
flexible), a description of the @ref FrameSet is written to the
@ref FitsChan using appropriate FITS header cards. The base Frame of
the @ref FrameSet is used to form the <tt>FITS-WCS</tt> pixel coordinate
system and the current Frame gives the physical coordinate
system to be described by the <tt>FITS-WCS</tt> primary axis
descriptions. Any additional Frames in the @ref FrameSet may be used
to construct secondary axis descriptions, where appropriate.
A successful write operation will over-write any existing
<tt>FITS-WCS</tt> encoded data in the @ref FitsChan, but will not affect other
(non-<tt>FITS-WCS</tt>) header cards. If a destructive read of a <tt>FITS-WCS</tt>
encoded @ref Object has previously occurred, then an attempt will be
made to store the FITS header cards back in their original
locations. Otherwise, the new cards will be inserted following
any other <tt>FITS-WCS</tt> related header cards present or, failing
that, in front of the current card (as given by the Card
attribute).
If an attempt to simplify a @ref FrameSet to conform to the <tt>FITS-WCS</tt>
model fails (or if the @ref Object supplied is not a @ref FrameSet), then
no data will be written to the @ref FitsChan and @ref Channel.write will
return zero. No error will result.
<h3>The <tt>FITS-IRAF</tt> Encoding:</h3>
The <tt>FITS-IRAF</tt> encoding can, for most purposes, be considered as
a subset of the <tt>FITS-WCS</tt> encoding (above), although it differs
in the details of the FITS keywords used. It is used in exactly
the same way and has the same restrictions, but with the
addition of the following:
- The only celestial coordinate systems that may be represented
are equatorial, galactic and ecliptic.
- Sky projections can be represented only if any associated
projection parameters are set to their default values.
- Secondary axis descriptions are not supported, so when writing
a @ref FrameSet to a @ref FitsChan, only information from the base and
current Frames will be stored.
Note that this encoding is provided mainly as an interim measure to
provide a more stable alternative to the <tt>FITS-WCS</tt> encoding until the
FITS standard for encoding WCS information is finalised. The name
<tt>FITS-IRAF</tt> indicates the general keyword conventions used and does
not imply that this encoding will necessarily support all features of
the WCS scheme used by IRAF software. Nevertheless, an attempt has
been made to support a few such features where they are known to be
used by important sources of data.
When writing a @ref FrameSet using the <tt>FITS-IRAF</tt> encoding, axis rotations
are specified by a matrix of FITS keywords of the form "CDi_j", where
"i" and "j" are single digits. The alternative form "CDiiijjj", which
is also in use, is recognised when reading an @ref Object, but is never
written.
In addition, the experimental IRAF "ZPX" and "TNX" sky projections will
be accepted when reading, but will never be written (the corresponding
FITS <tt>ZPN</tt> or "distorted TAN" projection being used instead). However,
there are restrictions on the use of these experimental projections.
For <tt>ZPX</tt>, longitude and latitude correction surfaces (appearing as
"lngcor" or "latcor" terms in the IRAF-specific <tt>WAT</tt> keywords) are
not supported. For <tt>TNX</tt> projections, only cubic surfaces encoded as
simple polynomials with "half cross-terms" are supported. If an
un-usable <tt>TNX</tt> or <tt>ZPX</tt> projection is encountered while reading
from a @ref FitsChan, a simpler form of <tt>TAN</tt> or <tt>ZPN</tt> projection is used
which ignores the unsupported features and may therefore be
inaccurate. If this happens, a warning message is added to the
contents of the @ref FitsChan as a set of cards using the keyword <tt>ASTWARN</tt>.
You should not normally attempt to mix the foreign FITS encodings within
the same @ref FitsChan, since there is a risk that keyword clashes may occur.
<h3>The <tt>FITS-PC</tt> Encoding:</h3>
The <tt>FITS-PC</tt> encoding can, for most purposes, be considered as
equivalent to the <tt>FITS-WCS</tt> encoding (above), although it differs
in the details of the FITS keywords used. It is used in exactly
the same way and has the same restrictions.
<h3>The <tt>FITS-AIPS</tt> Encoding:</h3>
The <tt>FITS-AIPS</tt> encoding can, for most purposes, be considered as
equivalent to the <tt>FITS-WCS</tt> encoding (above), although it differs
in the details of the FITS keywords used. It is used in exactly
the same way and has the same restrictions, but with the
addition of the following:
- The only celestial coordinate systems that may be represented
are equatorial, galactic and ecliptic,
- Spectral axes can only be represented if they represent
frequency, radio velocity or optical velocity, and are linearly
sampled in frequency. In addition, the standard of rest
must be <tt>LSRK</tt>, <tt>LSRD</tt>, barycentric or geocentric.
- Sky projections can be represented only if any associated
projection parameters are set to their default values.
- The <tt>AIT</tt>, <tt>SFL</tt> and <tt>MER</tt> projections can only be written if the <tt>CRVAL</tt>
keywords are zero for both longitude and latitude axes.
- Secondary axis descriptions are not supported, so when writing
a @ref FrameSet to a @ref FitsChan, only information from the base and
current Frames will be stored.
- If there are more than 2 axes in the base and current Frames, any
rotation must be restricted to the celestial plane, and must involve
no shear.
<h3>The <tt>FITS-AIPS++</tt> Encoding:</h3>
The <tt>FITS-AIPS++</tt> encoding is based on the <tt>FITS-AIPS</tt> encoding, but
includes some features of the <tt>FITS-IRAF</tt> and <tt>FITS-PC</tt> encodings.
Specifically, any celestial projections supported by <tt>FITS-PC</tt> may be
used, including those which require parameterisation, and the axis
rotation and scaling may be specified using CDi_j keywords. When
writing a FITS header, rotation will be specified using <tt>CROTA</tt>/<tt>CDELT</tt>
keywords if possible, otherwise <tt>CDi_j</tt> keywords will be used instead.
<h3>The <tt>FITS-CLASS</tt> Encoding:</h3>
The <tt>FITS-CLASS</tt> encoding uses the conventions of the CLASS project.
These are described in the section "Developer Manual"/"CLASS FITS Format" contained in the CLASS documentation at:
http://www.iram.fr/IRAMFR/GILDAS/doc/html/class-html/class.html.
This encoding is similar to <tt>FITS-AIPS</tt> with the following restrictions:
- When a @ref SpecFrame is created by reading a <tt>FITS-CLASS</tt> header, the
attributes describing the observer's position (ObsLat, ObsLon and
ObsAlt) are left unset because the CLASS encoding does not specify
these values. Conversions to or from the topocentric standard of rest
will therefore be inaccurate (typically by up to about 0.5 km/s)
unless suitable values are assigned to these attributes after the
@ref FrameSet has been created.
- When writing a @ref FrameSet to a <tt>FITS-CLASS</tt> header, the current Frame
in the @ref FrameSet must have at least 3 WCS axes, of which one must be
a linear spectral axis. The spectral axis in the created header will
always describe frequency. If the spectral axis in the supplied
@ref FrameSet refers to some other system (e.g. radio velocity, etc),
then it will be converted to frequency.
- There must be a pair of celestial axes - either (RA,Dec) or
(GLON,GLAT). RA and Dec must be either FK4/B1950 or FK5/J2000.
- A limited range of projection codes (<tt>TAN</tt>, <tt>ARC</tt>, <tt>STG</tt>, <tt>AIT</tt>, <tt>SFL</tt>, <tt>SIN</tt>)
can be used. For <tt>AIT</tt> and <tt>SFL</tt>, the reference point must be at the
origin of longitude and latitude. For <tt>SIN</tt>, the associated projection
parameters must both be zero.
- No rotation of the celestial axes is allowed, unless the spatial
axes are degenerate (i.e. cover only a single pixel).
- The frequency axis in the created header will always describe
frequency in the source rest frame. If the supplied @ref FrameSet uses
some other standard of rest then suitable conversion will be applied.
- The source velocity must be defined. In other words, the @ref SpecFrame
attributes <tt>SourceVel</tt> and <tt>SourceVRF</tt> must have been assigned values.
- The frequency axis in a <tt>FITS-CLASS</tt> header does not represent
absolute frequency, but instead represents offsets from the rest
frequency in the standard of rest of the source.
When writing a @ref FrameSet out using <tt>FITS-CLASS</tt> encoding, the current
Frame may be temporarily modified if this will allow the header
to be produced. If this is done, the associated pixel->WCS Mapping
will also be modified to take account of the changes to the Frame.
The modifications performed include re-ordering axes (WCS axes, not
pixel axes), changing spectral coordinate system and standard of
rest, changing the celestial coordinate system and reference equinox,
and changing axis units.
<h3>The <tt>NATIVE</tt> Encoding:</h3>
The <tt>NATIVE</tt> encoding may be used to store a description of any
class of AST @ref Object in the form of FITS header cards, and (for
most practical purposes) any number of these @ref Object descriptions
may be stored within a single set of FITS cards. If multiple
@ref Object descriptions are stored, they are written and read
sequentially. The <tt>NATIVE</tt> encoding makes use of unique FITS
keywords which are designed not to clash with keywords that have
already been used for other purposes (if a potential clash is
detected, an alternative keyword is constructed to avoid the
clash).
When reading a <tt>NATIVE</tt> encoded object from a @ref FitsChan (using
@ref Channel.read), FITS header cards are read, starting at the current
card (as determined by the Card attribute), until the start of
the next @ref Object description is found. This description is then
read and converted into an AST @ref Object, for which a pointer is
returned. Such a read is always destructive and causes all the
FITS header cards involved in the @ref Object description to be
removed from the @ref FitsChan, which is left positioned at the
following card.
The @ref Object returned may be of any class, depending on the
description that was read, and other AST routines may be used to
validate it (for example, by examining its Class or ID attribute).
If further <tt>NATIVE</tt> encoded @ref Object descriptions
exist in the @ref FitsChan, subsequent calls to @ref Channel.read will return
the @ref Object "Objects" they describe in sequence (and destroy their
descriptions) until no more remain between the current card and
the "end-of-file".
When @ref Channel.write is used to write an @ref Object using <tt>NATIVE</tt> encoding,
a description of the @ref Object is inserted immediately before the
current card (as determined by the Card attribute). Multiple
@ref Object descriptions may be written in this way and are stored
separately (and sequentially if the Card attribute is not
modified between the writes). A write operation using the <tt>NATIVE</tt>
encoding does not over-write previously written @ref Object
descriptions. Note, however, that subsequent behaviour is
undefined if an @ref Object description is written inside a
previously-written description, so this should be avoided.
When an @ref Object is written to a @ref FitsChan using <tt>NATIVE</tt> encoding,
@ref Channel.write should (barring errors) always transfer data and
return a value of 1.
@subsection FitsChan_FitsAxisOrder FitsAxisOrder
The order for the WCS axes in any new FITS-WCS headers
created using @ref Channel.write. (string)
The value of the <tt>FitsAxisOrder</tt> attribute can be either "<auto>"
(the default value), "<copy>" or a space-separated list of axis
symbols:
- "<auto>": causes the WCS axis order to be chosen automatically so that
the i'th WCS axis in the new FITS header is the WCS axis which is
more nearly parallel to the i'th pixel axis.
- "<copy>": causes the WCS axis order to be set so that the i'th WCS
axis in the new FITS header is the i'th WCS axis in the current
Frame of the FrameSet being written out to the header.
- "Sym1 Sym2...": the space-separated list is seached in turn for
the Symbol attribute of each axis in the current Frame of the
FrameSet. The order in which these Symbols occur within the
space-separated list defines the order of the WCS axes in the
new FITS header. An error is reported if Symbol for a current
Frame axis is not present in the supplied list. However, no error
is reported if the list contains extra words that do not correspond
to the Symbol of any current Frame axis.
@subsection FitsChan_FitsDigits FitsDigits
Digits of precision for floating-point FITS values. (int)
<tt>FitsDigits</tt> gives the number of significant decimal digits to
use when formatting floating point values for inclusion in the
FITS header cards within a @ref FitsChan.
By default, a positive value is used which results in no loss of
information, assuming that the value's precision is double.
Usually, this causes no problems.
However, to adhere strictly to the recommendations of the FITS
standard, the width of the formatted value (including sign,
decimal point and exponent) ought not to be more than 20
characters. If you are concerned about this, you should set
FitsDigits to a negative value, such as -15. In this case, the
absolute value (+15) indicates the maximum number of significant
digits to use, but the actual number used may be fewer than this
to ensure that the FITS recommendations are satisfied. When
using this approach, the resulting number of significant digits
may depend on the value being formatted and on the presence of
any sign, decimal point or exponent.
The value of this attribute is effective when FITS header cards
are output, either using @ref FitsChan.findFits
or by the action of the @ref FitsChan's sink function
when it is finally deleted.
@subsection FitsChan_FitsTol FitsTol
Tolerance used for writing a FrameSet using a foreign encoding, such as FITS-WCS (double)
This attribute is used when attempting to write a FrameSet to a FitsChan using a foreign encoding.
It specifies the maximum departure from linearity allowed on any axis within the mapping from
pixel coordinates to Intermediate World Coordinates. It is expressed in units of pixels. If an axis
of the Mapping is found to deviate from linearity by more than this amount, the write operation fails.
If the linearity test succeeds, a linear approximation to the mapping is used to determine the
FITS keyword values to be placed in the FitsChan. The default value for "FitsTol" is 0.1.
@subsection FitsChan_Iwc Iwc
Add a Frame describing Intermediate World Coords? (bool)
<tt>Iwcs</tt> is used when a FrameSet is read from a FitsChan with a foreign FITS encoding
(e.g. <tt>FITS-WCS</tt>) using @ref Channel.read.
If <tt>Iwcs</tt> is true then the returned FrameSet will include
Frames representing "intermediate world coordinates" (IWC). These
Frames will have Domain name "IWC" for primary axis descriptions, and
"IWCa" for secondary axis descriptions, where "a" is replaced by
the single alternate axis description character, as used in the
FITS-WCS header. The default value for "Iwc" is <tt>false</tt>.
FITS-WCS paper 1 defines IWC as a Cartesian coordinate system with one
axis for each WCS axis, and is the coordinate system produced by the
rotation matrix (represented by FITS keyword <tt>PCi_j</tt>, <tt>CDi_j</tt>, etc).
For instance, for a 2-D <tt>FITS-WCS</tt> header describing projected
celestial longitude and latitude, the intermediate world
coordinates represent offsets in degrees from the reference point
within the plane of projection.
@subsection FitsChan_NCard NCard
The total number of FITS header cards stored in a @ref FitsChan. (int)
<tt>NCard</tt> is updated as cards are added or deleted.
@subsection FitsChan_Nkey Nkey
The total number of unique FITS keywords stored in a @ref FitsChan. (int)
<tt>Nkey</tt> is updated as cards are added or deleted.
If no keyword occurrs more than once in the @ref FitsChan,
the @ref FitsChan_NCard "NCard" and @ref FitsChan_Nkey "Nkey" attributes will be equal.
If any keyword occurrs more than once, the @ref FitsChan_Nkey "Nkey" attribute value
will be smaller than the @ref FitsChan_NCard "NCard" attribute value.
@subsection FitsChan_SipOK SipOK
Use Spitzer Space Telescope keywords to define distortion? (bool)
This attribute is a boolean value which specifies whether to include support for the "SIP" scheme,
which can be used to add distortion to basic FITS-WCS projections. This scheme was first defined by
the Spitzer Space Telescope and is described in the following document:
http://irsa.ipac.caltech.edu/data/SPITZER/d The default for SipOK is 1.
When using astRead to read a FITS-WCS encoded header, a suitable PolyMap will always be included
in the returned FrameSet if the header contains SIP keywords, regardless of the value of the SipOK attribute.
The PolyMap will be immediately before the MatrixMap that corresponds to the FITS-WCS PC or CD matrix.
When using astWrite to write a FrameSet to a FITS-WCS encoded header, suitable SIP keywords will be included
in the header if the FrameSet contains a PolyMap immediately before the MatrixMap that corresponds to the
FITS-WCS PC or CD matrix, but only if the SipOK attribute is non-zero. If the FrameSet contains a PolyMap
but SipOK is zero, then an attempt will be made to write out the FrameSet without SIP keywords using a
linear approximation to the pixel-to-IWC mapping. If this fails because the Mapping exceeds the linearity
requirement specified by attribute FitsTol, astWrite will return zero, indicating that the FrameSet
could not be written out. Note, SIP headers can only be produced for axes that form part of a SkyFrame.
Note, the SIP distortion scheme is independent of the TPV/TPN distortion schemes (see attribute
@ref FitsChan_PolyTan "PolyTan").
A FITS-WCS header could in principle, contain keywords for both schemes although this is unlikely.
@subsection FitsChan_SipReplace SipReplace
Ignore inverse SIP coefficients (replacing them with fit coefficients or an iterative inverse)? (bool)
If true then ignore SIP inverse coefficients. If false then attempt to fit
new inverse coefficients; if those cannot be determined to suitable accuracy
then use an iterative inverse. The default is false.
@subsection FitsChan_TabOK TabOK
Should the FITS "-TAB" algorithm be recognised? (bool)
<tt>TabOK</tt> is an integer value which indicates if the <tt>-TAB</tt>
algorithm, defined in FITS-WCS paper III, should be supported by
the FitsChan. The default value is zero. A zero or negative value
results in no support for <tt>-TAB</tt> axes (i.e. axes that have <tt>-TAB</tt>
in their CTYPE keyword value). In this case, the
@ref Channel.write
method will return zero if the write operation would required the
use of the <tt>-TAB</tt> algorithm, and the
@ref Channel.read method will fail
if any axis in the supplied header uses the <tt>-TAB</tt> algorithm.
If <tt>TabOK</tt> is set to a non-zero positive integer, these methods will
recognise and convert axes described by the <tt>-TAB</tt> algorithm, as
follows:
The @ref Channel.write
method will generate headers that use the <tt>-TAB</tt> algorithm (if
possible) if no other known FITS-WCS algorithm can be used to
describe the supplied FrameSet. This will result in a table of
coordinate values and index vectors being stored in the FitsChan.
After the write operation, the calling application should check to
see if such a table has been stored in the FitsChan. If so, the
table should be retrived from the FitsChan using the
astGetTables
method, and the data (and headers) within it copied into a new
FITS binary table extension. See
astGetTables
for more information. The FitsChan uses a FitsTable object to store
the table data and headers. This FitsTable will contain the required
columns and headers as described by FITS-WCS paper III - the
coordinates array will be in a column named "COORDS", and the index
vector(s) will be in columns named <tt>INDEX\<i\></tt> (where <tt>\<i\></tt> is the index
of the corresponding FITS WCS axis). Note, index vectors are only
created if required. The <tt>EXTNAME</tt> value will be set to the value of the
<tt>AST__TABEXTNAME</tt> constant (currently "WCS-TAB"). The <tt>EXTVER</tt> header
will be set to the positive integer value assigned to the <tt>TabOK</tt>
attribute. No value will be stored for the <tt>EXTLEVEL</tt> header, and should
therefore be considered to default to 1.
The @ref Channel.read
method will generate a FrameSet from headers that use the <tt>-TAB</tt>
algorithm so long as the necessary FITS binary tables are made
available. There are two ways to do this: firstly, if the application
knows which FITS binary tables will be needed, then it can create a
Fitstable describing each such table and store it in the FitsChan
(using method
astPutTables or astPutTable) before invoking the @ref Channel.read method.
Secondly, if the application does not know which FITS binary tables
will be needed by @ref Channel.read,
then it can register a call-back function with the FitsChan using
method astTableSource.
This call-back function will be called from within @ref Channel.read
if and when a <tt>-TAB</tt> header is encountered. When called, its arguments
will give the name, version and level of the FITS extension containing
a required table. The call-back function should read this table from
an external FITS file, and create a corresponding FitsTable which
it should then return to
@ref Channel.read. Note, currently @ref Channel.read
can only handle <tt>-TAB</tt> headers that describe 1-dimensional (i.e.
separable) axes.
@subsection FitsChan_PolyTan PolyTan
Use <tt>PVi_m</tt> keywords to define distorted TAN projection? (bool)
<tt>PolyTan</tt> specifies how FITS "TAN"
projections should be treated when reading a FrameSet from a foreign
encoded FITS header. If zero, the projection is assumed to conform
to the published FITS-WCS standard. If positive, the convention
for a distorted TAN projection included in an early draft version
of FITS-WCS paper II are assumed. In this convention the
coefficients of a polynomial distortion to be applied to
intermediate world coordinates are specified by the <tt>PVi_m</tt> keywords.
This convention was removed from the paper before publication and so
does not form part of the standard. Indeed, it is incompatible with
the published standard because it re-defines the meaning of the
first five <tt>PVi_m</tt> keywords on the longitude axis, which are reserved
by the published standard for other purposes. However, headers that
use this convention are still to be found, for instance the SCAMP
utility (http://www.astromatic.net/software/scamp) creates them.
The default value for the <tt>PolyTan</tt> attribute is <tt>-1</tt>. A negative
values causes the used convention to depend on the contents
of the FitsChan. If the FitsChan contains any <tt>PVi_m</tt> keywords for
the latitude axis, or if it contains <tt>PVi_m</tt> keywords for the
longitude axis with "m" greater than 4, then the distorted TAN
convention is used. Otherwise, the standard convention is used.
@subsection FitsChan_Warnings Warnings
Controls the issuing of warnings about selected conditions when an @ref Object or keyword
is read from or written to a @ref FitsChan. (string)
The value supplied for the <tt>Warnings</tt> attribute should
consist of a space separated list of condition names (see the
AllWarnings attribute for a list of the currently defined names).
Each name indicates a condition which should be reported. The default
value for Warnings is the string
"BadKeyName BadKeyValue Tnx Zpx BadCel BadMat BadPV BadCTYPE".
The text of any warning will be stored within the FitsChan in the form of one or more new header cards with keyword ASTWARN. If required, applications can check the FitsChan for ASTWARN cards (using FitsChan::findFits) after the call to Channel::read or Channel::write has been performed, and report the text of any such cards to the user. ASTWARN cards will be propagated to any output header unless they are deleted from the FitsChan using FitsChan::delFits.
1.9.5