If images have been grouped into Sets for alignment purposes by using MAKESET, and the USESET parameter is true, then the program will treat each Set of images as a single image to be aligned.
The graphical interface consists of two parts: a chooser which allows you to nominate pairs of images to be aligned, and an aligner which allows you to move the pair around the screen until they are registered, and to mark points in the overlapping region where the same centroidable features exist on both images.
Operation is as follows. You must first use the chooser window to select a pair of images which have a region in common (if you only have two images this step may be skipped). Use the tabs at either side of the screen to pick the image to appear on that side. You can use the "Show FITS" button to select one or more FITS headers to be displayed alongside each image if this will make it easier to identify which is which. You can use the "Display cutoff" menu to select the percentiles controlling the brightness of each pixel; alignment is easier if the same features are of a similar brightness in different images. The images are displayed resampled into their Current coordinates, so that their orientation will be the same as in the aligner. You can only align them using this program if a simple offset (translation) maps one onto another in these coordinates (or very nearly does so). If that is not the case, you will have to set their Current coordinate system to a different value (see WCSEDIT) or align them using a different method. The whole of each image will be displayed in the chooser window, select a pair with an overlapping region which you wish to align, and click the "Use this pair" button. The aligner window will then appear, displaying the two images which you have selected. The chooser window can normally be resized in the normal way to make the images bigger or smaller. However there is currently a bug which causes this to crash in some window managers which use continuous resizing. In this case you must use the PREVX and PREVY parameters to change the image size.
In the aligner window you can drag either of these images around the display region by holding down mouse button 1 (usually the left one) as you move the mouse; the easiest way to align the pair is to "pick up" one image by an identifiable feature and "drop" it on the same feature in the other image. Where the images overlap their pixels will be averaged. If they are not correctly positioned, you can move them again. Once you are happy that they are aligned about right, then click in the overlap region to mark features which appear in both images. During this part you mark points by clicking with mouse button 1 (usually the left one) and you can remove them by clicking with button 3 (usually the right one).
When you add a point by clicking it will be centroided on both images, and two markers plotted, one for each centroided position. If a centroidable object near that point cannot be identified on both images the program will not allow you to mark a point there. However, note that the centroiding algorithm is capable of locating spurious objects from noise, so the fact that a point can be marked does not prove that a real feature exists on both images. By looking at the two markers it should be possible to see whether a real feature has been located. Though the two markers do not need to be exactly concentric (REGISTER can take care of that later), the offset between them should be similar to that of other marked objects nearby in the overlap region. If you do not think the same object has been identified in both images, you should remove this point (with mouse button 3).
The aligner window can be resized, the magnification changed using the "Zoom" control, the display region scrolled using the scrollbars, and the shape and colour of the point markers selected. When you have aligned the images and marked shared features, or if you decide that the pair cannot be satisfactorily registered, click the "Done" button.
You will then be returned to the chooser window to select another pair and repeat the process. After the first time however, you will only be allowed to select a pair of images to align if at least one of them has already been aligned. Those which have already been done are marked with a `' sign on their selection tabs.
Once you have made enough pairings to register the whole set, the graphical windows will disappear and the program will complete the global matching up of positions without any further user interaction.
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]
In this case, any images for which matching was not achieved will have their associated position lists removed from their .MORE.CCDPACK extensions. Thus after running PAIRNDF with OVERRIDE set to TRUE, any position list associated with an image is guaranteed to be one which has been matched, and not just one left over from the previously associated unmatched list. [TRUE]
These may be specified as list of comma separated names, using indirection if required, OR, as a single modification element (of the input names). The simplest modification element is the asterisk "" which means call each of the output lists the same name as the corresponding input images (but without the ".sdf" extension). So, IN OUTLIST signifies that all the images in the current directory should be used and the output lists should have the same names.
Other types of modification can also occur, such as, OUTLIST _objs.dat which means call the position lists the same as the input images but put "_objs.dat" after the names. Replacement of a specified string with another in the output file names can also be used, OUTLIST _debias_images.dat this replaces the string "_debias" with "_images.dat" in any of the output names.
If wildcarded names for the input images are used then it is recommended that wildcards are also used for the position list names as the correspondence between these may be confusing. [.DAT]
If the input images have no Set headers, or if they have no Set alignment coordinate system (one with a Domain of CCD_SET) the setting of USESET will make no difference.
If a global value for this parameter has been set using CCDSETUP then that value will be used. [FALSE]
In all cases, the coordinates in position lists are pixel coordinates.
Retaining parameter values has the advantage of allowing you to define the default behaviour of the application. 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.
Some of the parameters (MARKSTYLE1, MARKSTYLE2, MAXCANV, PERCENTILES, PREVX, PREVY, WINX, WINY) give initial values for quantities which can be modified while the program is running. Although these may be specified on the command line, it is normally easier to start the program up and modify them using the graphical user interface. If the program exits normally, their values at the end of the run will be used as defaults next time the program starts up.