This is a bit confusing.  You have three variables that all read basically the same: `flux_untruncated`, `_flux_untruncated`, and `flux_untrunc`.  At this point they are all equal, since `_flux_untruncated = flux_untruncated` in the above initialization.  And now you set `flux_untrunc = _flux_untruncated`.

I think you should at least reuse the first one here, rather than create the new variable.  But later when you let it diverge from the `_flux_untrunctated` value, you should explain its new meaning -- that it now refers to whether SersicInfo should adjust the value of re to account for the flux.

Even better, I think it might be worth pulling the flux_untruncated stuff out of SersicInfo and put it here instead.  Then just always provide SersicInfo with the correct re and flux coefficient that it needs to build the lookup table.  I think that would provide for a cleaner code.