Aligns images using multiple coordinate systems


This application takes a set of images which have World Coordinate System (WCS) components, and tries to align them all according to a given list of coordinate system domains (labels). If successful, it adds a new coordinate frame to the WCS component of each within which they are all aligned. The TRANLIST or TRANNDF applications can then be used on the resulting images.

This can be of use when different kinds of alignment information are available between different members of a group of images. By supplying an ordered list of coordinate systems within which to align, the best alignment available can be made between different members of the group, falling back on second or third choices of alignment types where first choices are not available.

The application operates on a set of images, IN. A list of domains DOMAINS within which to align, in order of preference, is specified, and a reference image is denoted by REFPOS. On successful completion, a new coordinate frame (which becomes Current), with a domain given by OUTDOMAIN (default CCD_WCSREG) is added to each of the images in the input set. Any previously existing frames with this domain will be removed.

The new coordinate system is a copy of the pixel coordinate system of the reference image, so for the reference image there is a unit mapping between its pixel and new Current coordinates. For each other image, the program attempts to find a mapping from the reference image to it. If it and the reference image do not share frames in any of the domains given by the DOMAINS parameter, it will try to use the WCS components of intermediate images to find a path between them; this path is a subgraph of a graph in which the nodes are the images and an edge exists between two nodes if the images share a domain in the given list. The shortest available path which connects a pair is chosen, and if there is more than one which meets this criterion, one which uses domains near the head of the list is preferred.

If the USESET parameter is true, then WCSREG will take account of alignment information stored in the CCDPACK Set header; this means that the alignment implied when images were previously grouped into a Set can be guaranteed to be retained.

If the graph is not fully connected, a list of the existing subgraphs is output, and the program will normally terminate, however it can be made to continue with registration of the connected images by setting the OVERRIDE parameter.


WCSREG in domains


This parameter should be a list of frame domains, in order of preference for achieving alignment. Alignment paths between images are selected by shortness of path, but in case of a tie, those using domains nearest the start of this list are used by preference. You should not normally include the CCD_SET domain in this list; for details of how this domain is treated specially, see the USESET parameter.

Note that this parameter is an array of strings, so that either the whole list should be surrounded by square brackets, or each element should be surrounded by double quotes. The whole thing may need to be protected from the Unix shell by using, e.g., single quotes.

Supplying the null value (!) is equivalent to specifying the current domain of the reference image. The effect of this is to retain the alignment already given by the Current coordinates of each image, but to ensure that the pixels are aligned with the pixels of the reference image. This will result in the images being aligned in a coordinate system suitable for resampling with TRANNDF. [!]

A list of the names of the images which are to be aligned. The names should be separated by commas and may include wildcards. They may alternatively be specified using an indirection file (the indirection character is "^").

If the application is successful, a new frame with a domain determined by the OUTDOMAIN parameter will be added to each of the IN files containing the alignment information. This frame will be made the new Current frame.

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]

The name of an output file in which to record all the images to which new coordinate systems were successfully added. This may not be the same as the IN list if OVERRIDE is set true. [wcsreg.lis]
This gives the lable of the new coordinate system to be attached to the images on successful completion. If any coordinate systems with the same label previously exist they are removed. The name is converted to upper case, and whitespace is removed. [CCD_WCSREG]
If not all the images can be aligned using the domains given in DOMAINS then the application will report on which sets of images form connectable subsets of the IN list. In this case, if this parameter is set FALSE, then the application will exit with an error message. If it is set TRUE however, it will continue and insert new frames in those images which can be reached from the one indicated by REFPOS, making no change to the others, except to remove any frames in the domain OUTDOMAIN which already exist.

The NAMELIST parameter can be used to record which images were successfully registered when OVERRIDE is true (if OVERRIDE is false, then it will be the same as IN unless the program fails). [FALSE]

The position within the IN list which corresponds to the reference image. The registration frame is a copy of (and unitmapped to) the pixel frame of the reference image, and for each other image the program tries to find a path from it to the reference image going from one image to another only when they both have frames in the same one of the entries in the DOMAINS list. [1]
This parameter governs whether Set-based alignment information in the images, if it exists, should be used. If it is set to true, then coordinate frames with the domain CCD_SET will take precedence over all the ones named in the DOMAINS parameter. In this case, if two of the images both have a CCD_SET coordinate frame and also share the same Set Name attribute, the connection will be made in CCD_SET frame. If no CCD_SET frames are present, this parameter has no effect.

If a global value for this parameter has been set using CCDSETUP then that value will be used. [FALSE]


wcsreg [ccd_reg,sky]
In this example all the images in the current directory are being aligned. All have an attached SKY coordinate sysetm with approximate information about the pointing, added by the telescope system at observation time. Some of the images however overlap, and have been run through the REGISTER program which has added a CCD_REG coordinate system containing more accurate alignment information derived from matching objects between different images. Where two of the images have CCD_REG coordinates, these will be used to align them, but where they do not, the program will fall back on the less accurate SKY coordinates for alignment. The new coordinate frame added will be given the default name CCD_WCSREG.

After this process, the images can be presented to TRANNDF for resampling prior to making a mosaic.

wcsreg "obs1_,obs2_" outdomain=final domains=[ccd_reg,inst_obs1,inst_obs2]
In this example images with names starting obs1_ and obs2_ are aligned. Where they share CCD_REG coordinates this will be used for alignment, but otherwise the INST_OBS1 and INST_OBS2 coordinate systems will be used. These perhaps contain information about the relative alignment of CCDs on the focal plane of the instrument, and may have been added to the WCS component using the ASTIMP application. The name FINAL is used for the new domain added to the WCS component.
wcsreg "skyfr1,skyfr2,skyfr3,skyfr4" refpos=2 domains=!
Here wcsreg is being used with a somewhat different intent. The images named are already fully aligned in their Current coordinates, but executing this command has the effect of aligning them in a new coordinate system which is a copy of the pixel coordinate system of skyfr2. Since this has units which are the size of pixels, the resulting image files are suitable for resampling using TRANNDF. Supposing that they were originally aligned in SKY coordinates they could not have been resampled by TRANNDF in their initial state, since the SKY coordinates have units of radians, which are much too large compared to pixels.

See also

Combining coordinate systems”.

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 USESET) 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.