Given a source 
and a curve 
,
pick a point on 
and find its tangent 
.
Then the locus of reflections of 
about tangents 
is the orthotomic curve (also known as the secondary caustic).
The evolute of the orthotomic is the catacaustic.
For a parametric curve 
with respect to the point 
,
the orthotomic is
 |  | ![x_0-(2g^'[f^'(g-y_0)-g^'(f-x_0)])/(f^('2)+g^('2))](/images/equations/Orthotomic/Inline11.svg) |
(1)
|
 |  | ![y_0+(2f^'[f^'(g-y_0)-g^'(f-x_0)])/(f^('2)+g^('2)).](/images/equations/Orthotomic/Inline14.svg) |
(2)
|
