The afw.::image::Wcs class is not a well-written base class for Wcs, but is instead a thin layer on wcslib. Furthermore it contains a number of methods that I think can be eliminated or made free functions, including:

• translateReferencePixel
• rotateImageBy90
• flipImage
• isFlipped
  (note that the middle two of these mutate the Wcs yet are marked "const".)

Here are concrete suggestions:

• Change Wcs to an abstract base class for Wcs that knows nothing about wcslib
• Make Wcs and its subclasses immutable
• Think carefully about which methods we really need. Make other required behavior a free function.

• Make a new subclass of Wcs that wraps wcslib
• Rename TanWcs to TanSipWcs, for clarity. The current name is very confusing to those not familiar with the code.