Best PA for guiding

I believe that this is now done, and tested via the OT and a test case.
(We may wish to add more test cases for this?)
Here we can be sure that all the candidates are reachable at some position angle (?) and bright enough but in `selectUnbounded()` we need to validate them with the `CandidateValidator`?  Is there a difference in the candidate pool that justifies this distinction or perhaps I'm just confused?