Hi Colin,
It's a good photo
I think you're probably right though, it does look a little soft.
I use my Canon 600D (un-modified) and generally I've been very happy with the focus (with and without a lens). Previously when I've used the 18-55 and 55-250mm lens I've always focused at the focal length I plan on using (I use the x5 or x10 live-view zoom feature, usually at max ISO so I can find a relevant star - easier said than done mind you
).
Suggest you try again but this time try focusing at the 24mm instead?
Cheers, Evan