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LSST Applications
21.0.0-172-gfb10e10a+18fedfabac,22.0.0+297cba6710,22.0.0+80564b0ff1,22.0.0+8d77f4f51a,22.0.0+a28f4c53b1,22.0.0+dcf3732eb2,22.0.1-1-g7d6de66+2a20fdde0d,22.0.1-1-g8e32f31+297cba6710,22.0.1-1-geca5380+7fa3b7d9b6,22.0.1-12-g44dc1dc+2a20fdde0d,22.0.1-15-g6a90155+515f58c32b,22.0.1-16-g9282f48+790f5f2caa,22.0.1-2-g92698f7+dcf3732eb2,22.0.1-2-ga9b0f51+7fa3b7d9b6,22.0.1-2-gd1925c9+bf4f0e694f,22.0.1-24-g1ad7a390+a9625a72a8,22.0.1-25-g5bf6245+3ad8ecd50b,22.0.1-25-gb120d7b+8b5510f75f,22.0.1-27-g97737f7+2a20fdde0d,22.0.1-32-gf62ce7b1+aa4237961e,22.0.1-4-g0b3f228+2a20fdde0d,22.0.1-4-g243d05b+871c1b8305,22.0.1-4-g3a563be+32dcf1063f,22.0.1-4-g44f2e3d+9e4ab0f4fa,22.0.1-42-gca6935d93+ba5e5ca3eb,22.0.1-5-g15c806e+85460ae5f3,22.0.1-5-g58711c4+611d128589,22.0.1-5-g75bb458+99c117b92f,22.0.1-6-g1c63a23+7fa3b7d9b6,22.0.1-6-g50866e6+84ff5a128b,22.0.1-6-g8d3140d+720564cf76,22.0.1-6-gd805d02+cc5644f571,22.0.1-8-ge5750ce+85460ae5f3,master-g6e05de7fdc+babf819c66,master-g99da0e417a+8d77f4f51a,w.2021.48
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 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 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 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 later 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). 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 FITS-WCS 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 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 Channel::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 Channel::write will return zero. No error will result.
FITS-IRAF Encoding:The FITS-IRAF encoding can, for most purposes, be considered as a subset of the FITS-WCS 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:
Note that this encoding is provided mainly as an interim measure to provide a more stable alternative to the FITS-WCS encoding until the FITS standard for encoding WCS information is finalised. The name FITS-IRAF 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 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.
FITS-PC Encoding:The FITS-PC encoding can, for most purposes, be considered as equivalent to the FITS-WCS 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.
FITS-AIPS Encoding:The FITS-AIPS encoding can, for most purposes, be considered as equivalent to the FITS-WCS 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:
LSRK, LSRD, barycentric or geocentric.AIT, SFL and MER projections can only be written if the CRVAL keywords are zero for both longitude and latitude axes.FITS-AIPS++ Encoding:The FITS-AIPS++ encoding is based on the FITS-AIPS encoding, but includes some features of the FITS-IRAF and FITS-PC encodings. Specifically, any celestial projections supported by FITS-PC 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 CROTA/CDELT keywords if possible, otherwise CDi_j keywords will be used instead.
FITS-CLASS Encoding:The FITS-CLASS 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 FITS-AIPS with the following restrictions:
FITS-CLASS 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 FrameSet has been created.FITS-CLASS header, the current Frame in the 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 FrameSet refers to some other system (e.g. radio velocity, etc), then it will be converted to frequency.TAN, ARC, STG, AIT, SFL, SIN) can be used. For AIT and SFL, the reference point must be at the origin of longitude and latitude. For SIN, the associated projection parameters must both be zero.SourceVel and SourceVRF must have been assigned values.FITS-CLASS 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 FrameSet out using FITS-CLASS 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.
NATIVE Encoding:The NATIVE encoding may be used to store a description of any class of AST Object in the form of FITS header cards, and (for most practical purposes) any number of these Object descriptions may be stored within a single set of FITS cards. If multiple Object descriptions are stored, they are written and read sequentially. The NATIVE 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 NATIVE encoded object from a FitsChan (using Channel::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). If further NATIVE encoded Object descriptions exist in the FitsChan, subsequent calls to Channel::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 Channel::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, Channel::write should (barring errors) always transfer data and return a value of 1.
The order for the WCS axes in any new FITS-WCS headers created using Channel::write. (string)
The value of the FitsAxisOrder attribute can be either "<auto>" (the default value), "<copy>" or a space-separated list of axis symbols:
Digits of precision for floating-point FITS values. (int)
FitsDigits gives the number of significant decimal digits to use when formatting floating point values for inclusion in the FITS header cards within a 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 FitsChan::findFits or by the action of the FitsChan's sink function when it is finally deleted.
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.
Add a Frame describing Intermediate World Coords? (bool)
Iwcs is used when a FrameSet is read from a FitsChan with a foreign FITS encoding (e.g. FITS-WCS) using Channel::read.
If Iwcs 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 false.
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 PCi_j, CDi_j, etc). For instance, for a 2-D FITS-WCS header describing projected celestial longitude and latitude, the intermediate world coordinates represent offsets in degrees from the reference point within the plane of projection.
The total number of FITS header cards stored in a FitsChan. (int)
NCard is updated as cards are added or deleted.
The total number of unique FITS keywords stored in a FitsChan. (int)
Nkey is updated as cards are added or deleted. If no keyword occurrs more than once in the FitsChan, the NCard and Nkey attributes will be equal. If any keyword occurrs more than once, the Nkey attribute value will be smaller than the NCard attribute value.
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 PolyTan). A FITS-WCS header could in principle, contain keywords for both schemes although this is unlikely.
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.
Should the FITS "-TAB" algorithm be recognised? (bool)
TabOK is an integer value which indicates if the -TAB 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 -TAB axes (i.e. axes that have -TAB in their CTYPE keyword value). In this case, the Channel::write method will return zero if the write operation would required the use of the -TAB algorithm, and the Channel::read method will fail if any axis in the supplied header uses the -TAB algorithm.
If TabOK is set to a non-zero positive integer, these methods will recognise and convert axes described by the -TAB algorithm, as follows:
The Channel::write method will generate headers that use the -TAB 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 INDEX<i> (where <i> is the index of the corresponding FITS WCS axis). Note, index vectors are only created if required. The EXTNAME value will be set to the value of the AST__TABEXTNAME constant (currently "WCS-TAB"). The EXTVER header will be set to the positive integer value assigned to the TabOK attribute. No value will be stored for the EXTLEVEL header, and should therefore be considered to default to 1.
The Channel::read method will generate a FrameSet from headers that use the -TAB 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 Channel::read method. Secondly, if the application does not know which FITS binary tables will be needed by 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 Channel::read if and when a -TAB 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 Channel::read. Note, currently Channel::read can only handle -TAB headers that describe 1-dimensional (i.e. separable) axes.
Use PVi_m keywords to define distorted TAN projection? (bool)
PolyTan 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 PVi_m 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 PVi_m 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 PolyTan attribute is -1. A negative values causes the used convention to depend on the contents of the FitsChan. If the FitsChan contains any PVi_m keywords for the latitude axis, or if it contains PVi_m keywords for the longitude axis with "m" greater than 4, then the distorted TAN convention is used. Otherwise, the standard convention is used.
Controls the issuing of warnings about selected conditions when an Object or keyword is read from or written to a FitsChan. (string)
The value supplied for the Warnings 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.
1.9.1