Who's done a masters thats completely unrelated to their undergrad?

Sorry if this is out if topic but I'm actually in a similar situation where I want to study something different from my Bachelors. I'm starting my last year in Mechanical Engineering this September but I want to study a discipline in Physics, most notably Astronomy. I have a 2:1 for the first year and a first class for my second year. It seems that many universities tell me that this will be difficult so I am wondering whether I should apply for a Postgraduate Diploma instead of a normal Postgraduate degree. The thing that I am afraid of is that I feel that a Postgraduate Diploma doesn't seem like a reputable degree type. I am aware that the only difference is that the research paper is absent in a Diploma. I'm sure you understand what I mean by this; the Diploma seems like something inferior to a normal Masters. I would like some advice on this.

The problem will be the amount of Math in your Engineering degree and the amount on theoretical stuff you did. Furthermore what do you want to achieve with it? For your own joy, a postgraduate diploma seems find, but without research experience I can't really see you getting accepted to a PHD (concerning competition) and a PHD is the normal route into Astronomy. The other question is, wether doing a related Engineering Master allowing you to get skills, needed for e.g. telescopes will help you already enough to get a bit more directed in your area of interest.

Another possibility might be to transfer into a Physic undegraduate degree or to study a second undergraduate as an affiliated student, but I don't know, what possibilites there are to accelerate a BSc if there are any.

???

Yes. What about you?