That's a pretty nice result for less than half an hour in light-polluted skies. However, by the shapes of the stars in the corners, it would appear that the lens would give even better results if stopped down a bit; by at least one stop to f/4. That is typical even for expensive lenses. Of course, that does mean the exposure time must be doubled (assuming f/4) to achieve the same signal-to-noise ratio. I routinely do this with all of my lenses, none of which were expensive (most are vintage Pentax lenses from the 70's that a I bought used on eBay). You can see the results
here. (Ignore the prime-focus telescope shots there.)
To Hereticuk (heretic in the UK?), the least expensive thing you can do to go forward from where you are, is to build yourself a motorized barn-door tracker. Yes, you can continue to attach your camera to your telescope but do you really want to lug that out every time you want to take a photo through a lens? Myself, I certainly did not. Hence, many years ago I built my own motorized barn-door tracker (see it
here) and it served me well until I got the K-3II with Astrotracer.
Which brings me to the second least-expensive thing you can do, which is to buy an O-GPS1 to get Astrotracer. As you can see from my Astrobin gallery that I linked to, all but one of the K-3II photos through lenses I posted there were taken using Astrotracer. It does take some patience if you intend to stack many images as Julio did (I am assuming he also used Astrotracer, but he didn't say), because you need to keep re-aiming the camera to follow the target. But it is really nice IMO not to need to fiddle with setting up and aiming a tracker.
