xform

Overview:

xform takes FITS files containing EUV image data (such as those exported by euv_imtool or obtained from the online EUV FITS archive) and takes into account EUV position and perspective effects to reproject the data as a magnetic-polar projection. For each input file, xform produces two 600x600 FITS files, one of which is the actual projection and the other is a shadow mask, indicating which pixels in the resultant image are valid and which are on the seams, in the shadow, or not represented by the original FITS file. The output from xform is may be read into the xftool software for display or further processing.

How to use:

xform does not have an interactive graphical interface. Instead, it is simply invoked from the IDL command line. The syntax is as follows:

xform, center = [cx, cy], mxl = 8, /mlt, /all

• center = [cx, cy] specifies that the center of the FITS files should be taken to be (cx, cy) where cx and cy are pixel offsets from the bottom-left corner of the image. The default is (142, 152). If the center is not specified, a centering circle will be displayed on the image, which you may use to define the center by positioning the circle and clicking the mouse.
• mxl = 8 specifies the maximum distance (in earth radii) that you want represented in the output files. The default is 8.
• /mlt forces xform to plot the output file with respect to magnetic local time. By default the plot is made with respect to magnetic longitude.
• /all tells xform to perform the operation on all FITS files in the current directory. Without this flag set, xform will bring up a dialog for the user to specify which files to use.

Details:

xform partially corrects perspective effects by mapping points in the image to the plane of the magnetic equator along magnetic field lines. The procedure that it uses begins with the line of sight corresponding to each pixel in the image. Using the known position of IMAGE relative to Earth, and the direction of the chosen line of sight, the mapping algorithm analytically solves for the corresponding dipole field line, among all those touching the line of sight, that has the minimum value of L. The algorithm reports the position of this field line in coordinates of L and magnetic longitude (or magnetic local time), and xform generates its output image by plotting the input pixel's value at the position corresponding to these L and maglon/mlt coordinates.

Notes:

Generating the matrices for the transformation can be quite time-consuming, particularly on slower computers. For this reason, xform saves this matrix as a base.mtx file, where base is the base of the filename of the input FITS file. If the .mtx file exists, xform will simply use this data rather than regenerate the matrix. Because this matrix is dependent upon several of the options above, changes to the centering or max-L arguments may not take effect unless the .mtx file is deleted.