Different perspective: I'd prefer to hire the astronomy PhD who knows the basics of the craft over one who doesn't. Reflecting vs refracting is pretty simple - astronomy doesn't really interest me and I'm aware of the differences. It's fundamental to the optics, and understanding the differences helps understanding why real data looks different depending on method of acquisition. Why not penalise the person that doesn't know the basics? You wouldn't forgive a carpenter for not knowing the difference between a hand saw and a table saw, yet they both cut wood and the difference takes all of a few seconds to learn.

For example: I'm a midrange DevOps/Sysadmin/Whatever-it-is-these-days. I will likely never be a senior. The (real) seniors are the folks who go home and mess with hardware and pull new toys apart to see what chips they were made with and so forth. I like to go home and play games. I do my job competently, but I don't have the underlying knowledge that those guys do. If I was up against one of those in a job application, then you'd be crazy to hire me at the same salary as a senior - there'd have to be a pretty big black mark against the senior, like being completely unpersonable or abusive or somesuch. Or the role used some of the other things I'm decent at. Perhaps a colloquial way to put it is that I live these things, but the seniors live and breathe them :)