Exports coordinate system information from images


This task exports coordinate system information from a set of images, writing it to an AST file. For each image a frameset is written containing information about how to map between a selected Base frame and the image’s Current frame. Each frameset is identified by a key which is derived from the image itself, and matches keys which can be derived from other images to which similar framesets ought to apply. The key should be generated in the same way when the AST file is used for importing the mapping information by ASTIMP or MAKESET. Currently these keys can be generated according to a FITS header card or the order in which the images are presented. Additional information may be written describing what use to make of FITS headers in the images.

Used together, the framesets written out to an AST file can thus contain information about the positioning of images in a set of related images.

AST files written out by this application can be applied to other images of similar origin using the ASTIMP or MAKESET programs, so that registration information present in the WCS components of one set of images (put there for instance by the REGISTER or WCSEDIT applications) can be transferred using ASTIMP and ASTEXP to another similar set. This "similar set" will typically be one from chips in the same mosaic camera instrument.

A 2-frame frameset is output for each image. The Base frame is one selected by the BASEFRAME parameter, and is identical in the exported frameset to the one in the original image. The Current frame in the exported frameset is the same as the Current frame in the original image, but may be given a different Domain name by the OUTDOMAIN parameter.

Under normal circumstances, the Current frames of all the input images should share the same Domain name, and so should the frames identified by the BASEFRAME parameter. A warning will be issued if this is not the case. Warnings will also be issued if the image identifiers are not all unique.


ASTEXP in astfile outdomain baseframe


The name of the AST file to be written.
If a "Sky Co-ordinate System" specification is supplied (using parameter BASEFRAME) for a celestial co-ordinate system, then an epoch value is needed to qualify it. This is the epoch at which the supplied sky positions were determined. It should be given as a decimal years value, with or without decimal places ("1996.8" for example). Such values are interpreted as a Besselian epoch if less than 1984.0 and as a Julian epoch otherwise.
This parameter specifies the WCS frame from the images relative to which the Current frames will be defined in the output AST file. To be useful, this must specify a frame which occurs in all the images in the IN list, and can be expected to occur in any image to which the AST file will later be applied using ASTIMP. AXIS is a good choice since this may be applicable to frames which have been modified, for instance by an application like KAPPA’s COMPAVE.

The value of the parameter can be one of the following:

  • A domain name such as SKY, AXIS, PIXEL, etc.
  • An integer value giving the index of the required Frame within the WCS component.
  • A "Sky Co-ordinate System" (SCS) value such as EQUAT(J2000) (see section "Sky Co-ordinate Systems" in SUN/95).
A domain name is usually the most suitable choice.

Unlike the Current frame, the frame selected using this parameter is copied to the AST file unmodified; in particular it retains the same Domain name. [PIXEL]

If the IDTYPE parameter has the value FITSID, this parameter gives the FITS header keyword whose value distinguishes frames with different coordinate system information. If any lower case characters are given, they are converted to upper case. This may be a compound name to handle hierarchical keywords, in which case it has the form keyword1.keyword2 etc. Each keyword must be no longer than 8 characters.
If this parameter is not null, it gives the name of a FITS header keyword whose value gives a number of degrees to rotate the coordinate system by when it is imported. If any lower case characters are given, they are converted to upper case. This may be a compound name to handle hierarchical keywords, in which case it has the form keyword1.keyword2 etc. Each keyword must be no longer than 8 characters. [!]
This parameter destermines the form of the ID value which distinguishes the framesets from each other in the exported AST file. It may have one of the following values:
  • FITSID – ID is generated from FITS header (see also the FITSID parameter).
  • INDEX – ID is given by an integer as taken from the INDICES parameter. This normally gives the frameset generated from the N’th image in the IN list an ID with index N.
  • SET – ID is given by an integer taken from the Set Index attribute of the CCDPACK Set header of each input file.
A list of images from which framesets are to be extracted. The Current frame of each should normally be the same, and should be a frame in which the different images are correctly registered. The image names may be specified using wildcards, or may be specified using an indirection file (the indirection character is "^").
If IDTYPE is set to INDEX, then this parameter is a list of integers with as many elements as there are images accessed by the IN parameter. It gives the sequence of indices N to be used for generating the ID values. If set null (!) the images will be considered in the order 1,2,3,… which will normally be appropriate unless the images are being presented in an order different from that in which they are likely to be presented to ASTIMP. [!]
Name of the CCDPACK logfile. If a null (!) value is given for this parameter, then no logfile will be written, regardless of the value of the LOGTO parameter.

If the logging system has been initialised using CCDSETUP, then the value specified there will be used. Otherwise, the default is "CCDPACK.LOG". [CCDPACK.LOG]

Every CCDPACK application has the ability to log its output for future reference as well as for display on the terminal. This parameter controls this process, and may be set to any unique abbreviation of the following:
  • TERMINAL – Send output to the terminal only

  • LOGFILE – Send output to the logfile only (see the LOGFILE parameter)

  • BOTH – Send output to both the terminal and the logfile

  • NEITHER – Produce no output at all

If the logging system has been initialised using CCDSETUP then the value specified there will be used. Otherwise, the default is "BOTH". [BOTH]

This parameter gives the name of the new alignment domain for the frames written out to the AST file. It is a good idea to choose a value which is not likely to exist previously in the WCS components of the images to which ASTFILE will be applied. A suitable value might be the name of the instrument from which the images are obtained.

Note that the frames which are written to the AST file are always the Current frames of the images supplied; this parameter only gives the name that the frames will have in the AST file, and consequently the name by which they will be known when the WCS information is imported into other images using ASTIMP or MAKESET.

The name is converted to upper case, and whitespace is removed. [CCD_EXPORT]


astexp reg_data camera.ast idtype=fitsid fitsid=CHIPNUM outdomain=camera
This will save the information about the relative positioning of the images reg_data to the file camera.ast, calling the alignment domain CAMERA. The file camera.ast can later be used by the ASTIMP or MAKESET applications to add the same coordinate information to a different set of images from the same instrument. Before running this, the images reg_data should be correctly aligned in their Current domain. CHIPNUM must be the name of a FITS header keyword present in the FITS extension of each image whose value distinguishes the CCDs from each other (presumably present in the unreduced data). The mappings between the pixel coordinates and Current coordinates of the input images are recorded.
astexp "im1,im2,im3" astfile=camera.ast baseframe=axis title="Focal plane alignment" accept
In this case the OUTDOMAIN parameter takes its default value of CCD_EXPORT, but mappings are between the Current coordinates of the input images and their AXIS coordinates. This could be a good idea if the images had been shrunk using KAPPAs COMPAVE or something similar, which modifies the PIXEL coordinates but leaves the AXIS coordinates unchanged. No suitable FITS header is available to distinguish the different types of image, so the IDTYPE parameter is allowed to assume its default value of INDEX. When camera.ast is used for importing frameset information, the images from the three different chips must be listed in the same order as when this command was invoked. The title of the output Current frame will be as given.
astexp "r10595[2345]" wfc.ast outdomain=wfc idtype=fitsid fitsid=CHIPNAME fitsrot=ROTSKYPA
This exports the alignment information from the four named images to a file wfc.ast. The CHIPNAME FITS header identifies the source CCD for each, and the ROTSKYPA FITS header gives a number of degrees to rotate each frame additional to the relative alignment information.

See also

Re-use of coordinate system information with AST files”, ASTIMP.

AST file format

The AST file is designed to be written by ASTEXP and read by ASTIMP or MAKESET, and the user does not need to understand its format. It is however a text file, and if care is taken it may be edited by hand. Removing entire framesets and modifying ID values or domain names may be done fairly easily, but care should be taken (see SUN/210) if any more involved changes are to be undertaken. The format of the file is explained here.

The AST file consists of the following, in order:

<global modifiers >
(blank line)
<frameset 1 >
<frameset 1 modifiers >
(blank line)
<frameset 2 >
<frameset 2 modifiers >
(blank line)
(end of file)

Characters after a # character are normally ignored. The constituent parts are composed as follows:

Blank line:
A single blank line, which may contain spaces but no comments.

The framesets are written in AST native format, as explained in SUN/210.

Each frameset has an ID, and contains two frames (a Base frame and a Current frame) and a mapping between them. The domains of all the Base frames should normally be the same, and likewise for all the Current frames. For the images to which the file will be applied by ASTIMP, their WCS components should contain frames in the same domain as the AST files Base frame.

The ID of each frameset is used to determine, for each image, which of the framesets in the file should be applied to it. This ID is a string which can assume one of the following forms:

Modifiers describe additional modifications to be made to the framesets on import. They are of the form

USE keyword arguments

Currently the only modifier defined is FITSROT, which defines the name of a FITS header which specifies how many degrees to rotate the image before use. This rotation is carried out after the mapping defined by the frameset itself.

Global modifiers affect all images processed with the AST file. Frameset modifiers affect only those images which correspond to their frameset.

Rigorous error checking of the AST file is not performed, so that unhelpful modifications to the WCS components of the target images may occur if it is not in accordance with these requirements.

Behaviour of parameters

Most parameters retain their current value as default. The "current" value is the value assigned on the last run of the application. If the application has not been run then the "intrinsic" defaults, as shown in the parameter help, apply.

Retaining parameter values has the advantage of allowing you to define the default behaviour of the application but does mean that additional care needs to be taken when re-using the application after a break of sometime. The intrinsic default behaviour of the application may be restored by using the RESET keyword on the command line.

Certain parameters (LOGTO, LOGFILE and NDFNAMES) have global values. These global values will always take precedence, except when an assignment is made on the command line. Global values may be set and reset using the CCDSETUP and CCDCLEAR commands.