The transform object presumably transforms positions "to" the specified coordinate_system. But what does it transform "from"? What if I want to find out where my object is on the focal plane? Do I need to transform from sky to pixel and then from pixel to focal plane? The AST way to do this would be something like:

transform = wcs.convert( "SKY", "FPLANE" )
(fx,fy) = transform.convert( ra, dec )