Peter, I've owned one of these for a few years now, got it second hand for a reasonable price. It's a Tamron 500mm f8 and as well as using it for birdlife in our garden, I've chucked it on my Nano Tracker and pointed it at a few astro objects. It does an OK job, a bit tedious to focus but mine holds well once that's achieved. It occasionally gets some strange flaring off bright stars but you can fix that in PS or just crop them out. Below are a couple of examples taken with the mirror lens.
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