   By default,  residuals are shown in a very 'traditional' way : in
arcseconds in RA and dec.  There are cases where one might want the
residuals shown in two other ways.

   The 'along-track in seconds,  cross-track in arcseconds' format is
mostly useful for fast-moving objects,  where the residual in the
direction of motion can show the effects of timing error.  If an object
is moving at,  say,  15" per second,  and your timing is bad by a third
of a second, you will get a 5" error in the direction of motion,  even
if your astrometry was perfect.  (Such errors can happen even if your
timing is near-perfect, due to difficulties in measuring trailed
objects.)  Residuals shown in this manner can confirm that the data in
such cases are "probably good",  but poorly defined in the direction
of the object motion.

   Find_Orb treats such cases by recognizing that the uncertainty ellipse
for a particular observation will be 'stretched' in the direction of
apparent motion.  The observation will then have a residual in the
direction of motion and another in the direction perpendicular to the
motion (the "cross-track") direction.  You can have these residuals
shown in normalized form,  in sigmas.  It is the sum of the squares of
these normalized residuals that Find_Orb attempts to minimize.
