Solar Coordinates

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If the solar position and the scale are known for an image, this widget
can be used to draw a Stonyhurst latitude/longitude grid on the image.
The accurancy of this grid is determined by the accurancy of the
position of course. For TRACE, both the position and scale are still
not perfectly determined resulting in errors of several arc seconds.
This can be most evident at the limb. The relative positions of the
TRACE images are accounted for using an offset table. The reference is
the white light image with a table value of (0,0). The latitude/
longitude grids can therefore be used as fiducials for aligning TRACE
images in different wavelengths.

Note that these lines are drawn in the image itself with a line width
of 1 pixel and a color red. If the image is de-zoomed (say to 1/2),
the color is not preserved because the pixel value displayed is a true
average. The lines are still generally visible. For zooms > 1, there
is no such effect.

The auto option can be used to always draw the grid whenever a new
image is read in. Otherwise a new image or a redraw will wipe out
the grid.