4.8.2 Determining the Coordinates Using DSK_LOCB

If you have a full disk image with well a defined limb, you can use the routine DSK_LOCB. A sample call is:
IDL> dsk_locb,img,horzcnt=x0,vertcnt=y0,horzrad=rz,vertrad=ry
 
IDL> corner2 = [-x0, -y0]

See the Reference Guide or the documentation header for more details on DSK_LOCB.