[fitsbits] polar coordinates

[Walter Jaffe]

I find nothing in the FITS standard document about representing the sky
in polar coordinates.  One of my clients wishes to represent a
theoretical model of disk emission in polar logarithmic coordinates.
In other words CTYPE1 is LOG(RADIUS) (common log of
radius in degrees on the sky) and 
CTYPE2 is PA (or POSITION ANGLE), (degrees E from North).

Has this been discussed before?  Is there a straightforward
convention?

Walter