FIT_LIMB will find the center and radius of an image, even images with
a partial limb.
FIT_LIMB is a modified version of SXT_CENTER which will also work on
non-SXT images. For non-SXT images, the pixel size must be known.
The algorithm requires a VERY good initial guess to remove bad limb
points.
Some sample calls are:
IDL> fit_limb, image, x, y, r, pixel_size=2.5
IDL> fit_limb, image, x, y, r, pixel_size=3.97, /interactive
IDL> fit_limb, image, x, y, r, pixel_size=2.5, initial=[434,512,403]
IDL> fit_limb, image, x, y, r, index=index
where the input is image which is the solar image. The outputs
and 
are the E/W and N/S position
of the sun and 
is the solar radius.
The and locations are in full resolution pixels if it
is an SXT image. It is possible to return the location in the
summed pixels (half resolution or quarter resolution) by using
the option /summed_pixels.
The keyword index=index
should be used for SXT image, and the keyword pixel_size should
be used for non-SXT images. For non-SXT images, the user should specify
an initial guess using the keyword initial or the /interactive
option which will allow the user to click on the sun center location.
There are other options and output (like the oblateness and the phase of the
2nd harmonic) which is described in more detail in the documentation header.