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Well done! I had a positive experience with PHD last night too - doing a 20 minute guided shot of NGC 6093 - and finding the stars are beautifully round!
So to me 20 minutes at 2.3 metre focal length is impressive. Some nights the mount see-saws or yo-yos when PHD drives into over-correct mode ever 3 seconds - some night its spot on - some nights variations kick in by hour 3-4 of imaging - go figure.
I have two hints for you:
1. Tune "minimum pixel's before guide pulse sent". I upped it from its default 0.25 of a pixel to between 0.70 to 1.1 of a pixel and this improved the consistency of my results dramatically.
2. If your mount slews a long, long way - consider re-calibrating PHD - press the brain and tick force calibration. I notice after a few hours PHD can suddenly invert and when it tries to centre a star it actually moves the mount in 180 degrees the wrong direction = whoopsie! Then its time to re-calibrate.
With city glow I can see 20 minutes is really pushing things - but 4 * 8 minute shots combined may give me a much nicer result, its just a pleasure seeing how fine your tracking and PE can be if you finally get everything working well!
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