The Kepler 4040 takes some getting used to, and this is the first image where I tried to use Howard Trottier's technique of matching the flat mean pixel value to that of the lights. It was quite good. but I found the low intensity flats didn't adequately correct for the transfer function and I had to use far more gradient correction that I normally do to remove vignetting.

There is also an annoying artefact throughout: all stars have a slight east-west elongation. This seems to be due to mistracking. I don't use guiding on exposures up to 5 minutes, relying on TheSkyX's ProTrack. I had just installed the 64-bit update of TheSkyX a few days before and got it all dialled in. When I came to this set of subs, the wretched Windows 10 had decided to upgrade and restart (such bad behaviour doesn't happen on macOS). I hadn’t saved any setting on TSX, so it reverted almost to factory settings. In my hurry to set up again I must have forgotten to engage ProTrack. Result: slight east-west elongation.

Messier 13, the Hercules Globular Cluster, is a globular cluster of about 300 000 stars and is almost 150 lightyears across. Like other globular clusters, it it in orbit around our Milky way galaxy, and is about 25 100 lightyears away from us.

In addition to M13, at the edge of our galaxy there are 24 other galaxies visible in this image. Most are pretty small but NGC 6207 to the upper left, discovered by William Hershel in 1787, is very obvious.