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.
used to arrive at the default value is as follows:
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.
When reading a DSS encoded Object (using AST_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 AST_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 AST_WRITE will return zero. No error will result.
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 FrameSet to a FitsChan, the the value of the CDMatrix attribute of the 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 FITS-WCS 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 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 FITS-WCS 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 FITS-WCS has been published.
AST extends the range of celestial coordinate sytstems which may be described using this encoding by inclusion of the allowing the use 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 FITS-WCS 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 leter release.
If an AST SkyFrame that represents offset rather than absolute coordinates (see attribute SkyRefIs) is written to a FitsChan using FITS-WCS 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). Inaddition, 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 FITS-WCS encoded Object (using AST_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 FITS-WCS pixel coordinates, and the current Frame represents the physical coordinate system described by the FITS-WCS primary axis descriptions. If secondary axis descriptions are also present, then the 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 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 AST_WRITE is used to store a FrameSet using FITS-WCS encoding, an attempt is first made to simplify the FrameSet to see if it conforms to the FITS-WCS model. If this simplification process succeeds (as it often should, as the model is reasonably flexible), a description of the FrameSet is written to the FitsChan using appropriate FITS header cards. The base Frame of the FrameSet is used to form the FITS-WCS pixel coordinate system and the current Frame gives the physical coordinate system to be described by the FITS-WCS primary axis descriptions. Any additional Frames in the FrameSet may be used to construct secondary axis descriptions, where appropriate.
A successful write operation will over-write any existing FITS-WCS encoded data in the FitsChan, but will not affect other (non-FITS-WCS) header cards. If a destructive read of a FITS-WCS encoded 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 FITS-WCS 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 FrameSet to conform to the FITS-WCS model fails (or if the Object supplied is not a FrameSet), then no data will be written to the FitsChan and AST_WRITE will return zero. No error will result.
addition of the following:
When writing a FrameSet using the FITS-IRAF 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 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 "ZPN" or "distorted TAN" projection being used instead). However, there are restrictions on the use of these experimental projections. For "ZPX", longitude and latitude correction surfaces (appearing as "lngcor" or "latcor" terms in the IRAF-specific "WAT" keywords) are not supported. For "TNX" projections, only cubic surfaces encoded as simple polynomials with "half cross-terms" are supported. If an un-usable "TNX" or "ZPX" projection is encountered while reading from a FitsChan, a simpler form of TAN or ZPN 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 FitsChan as a set of cards using the keyword "ASTWARN".
You should not normally attempt to mix the foreign FITS encodings within the same FitsChan, since there is a risk that keyword clashes may occur.
addition of the following:
Format" contained in the CLASS documentation at:
This encoding is similar to FITS-AIPS with the following restrictions:
When reading a NATIVE encoded object from a FitsChan (using AST_READ), FITS header cards are read, starting at the current card (as determined by the Card attribute), until the start of the next Object description is found. This description is then read and converted into an AST Object, for which a pointer is returned. Such a read is always destructive and causes all the FITS header cards involved in the Object description to be removed from the FitsChan, which is left positioned at the following card.
The 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 using AST_GETC). If further NATIVE encoded Object descriptions exist in the FitsChan, subsequent calls to AST_READ will return the Objects they describe in sequence (and destroy their descriptions) until no more remain between the current card and the "end-of-file".
When AST_WRITE is used to write an Object using NATIVE encoding, a description of the Object is inserted immediately before the current card (as determined by the Card attribute). Multiple 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 NATIVE encoding does not over-write previously written Object descriptions. Note, however, that subsequent behaviour is undefined if an Object description is written inside a previously-written description, so this should be avoided.
When an Object is written to a FitsChan using NATIVE encoding, AST_WRITE should (barring errors) always transfer data and return a value of 1.
AST A Library for Handling World Coordinate Systems in Astronomy