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.

ASTFILE = LITERAL (Read)

BASEEPOCH = _DOUBLE (Read)

BASEFRAME = LITERAL (Read)

FITSID = LITERAL (Read)

FITSROT = LITERAL (Read)

IDTYPE = LITERAL (Read)

IN = LITERAL (Read)

INDICES( ) = _INTEGER (Read)

LOGFILE = FILENAME (Read)

LOGTO = LITERAL (Read)

OUTDOMAIN = LITERAL (Read)

astexp reg_data$\ast$ camera.ast idtype=fitsid fitsid=CHIPNUM outdomain=camera

This will save the information about the relative positioning of the images ’reg_data$\ast$’ 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$\ast$’ 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 KAPPA’s 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.

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

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:

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.

Frameset:
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 file’s 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:

• "FITSID KEY VALUE" — This will match an image if the first FITS header card with the keyword KEY has the value VALUE. If the value is of type CHARACTER it must be in single quotes. KEY may be compound (of the form keyword1.keyword2 etc) to permit reading of hierarchical keywords.

• "INDEX N" — This associates a frameset with an integer N. Usually N will take the values 1,2,3,... for the framesets in the file. Typically the N’th image in a list will match the one with an ID of "INDEX N".

• "SET N" — This will match an image if the Set Index attribute iin its CCDPACK Set header is equal to the integer N.

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

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.

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.