Compressor design optimization for efficiency
 

I've been looking at a paper on the optimization of isentropic efficiency for a centrifugal compressor.

Here's the paper I'm referring to - http://www.dynardo.de/fileadmin/Mate..._Prognosis.pdf

In that work, the compressor design has been optimized considering the operating point and the choking condition, could someone please let me know if this is a usual practice to consider also the choking condition for such optimization study.

Could some one please refer to some material that explains the procedure to determine the operating point of a centrifugal compressor.

In case of turbo machinery, is it always the efficiency alone that serves as the objective of optimization