Define crude criteria to select F-stars from HSC photometry

'What stars can we use for flux calibration?  Can we use F2V?  F9V?  K2V?' - this is not a trivial question and we need to carefully define a metric of good/bad.  This needs thinking, but for now, we just need a crude selection and perhaps we can define color cuts to select F1V-F9V stars?  Please leave comments if you have any thoughts on this.  We will revisit this issue in the future with a separate ticket (Tanaka is thinking that we can employ an probabilistic approach: e.g., if you have F3V, use it, but if not, accept K1V).