Here is a DXF file for a Carey mask for a refractor (no central obstruction).
http://www.btinternet.com/~gfcarey/m...o_obstruct.dxf
Using Niels Noordhoek's simulation software it can be seen that a focus error of only 20 microns is clearly visible using a Carey mask.

The gap between the right hand spikes is larger than that at the left.