First attempt at starfields

[MarkH]

Closing the aperture down a couple of stops will increase the depth of feild and focusing at that distance will be much easier.

[MarkS]

I've just read that inceasing the aperture a couple of stops from its lowest value can help reduce coma. Is that something you've seen?

Yes, stopping the lens down i.e. decreasing the aperture size i.e. increasing the f-ratio will reduce coma and other aberrations near the edges and corners.  It will also help near the centre of the image.

Mark