Can you make some of these units more complex (like in http://bpaste.net/show/7xPU9U41CK6P65mXLa63/).
Maybe at least `mag * arcsec**(-2)` and `pixel**2`.  There was some discussion in #1093 about units.  I don't think we need to really worry about the units having the same string representation but they should end up being equivalent.