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Thanks to everyone for their comments.
My question was prompted by eggy stars in unguided images. I was surprised that I was unable to take round stars using my PMX. I suspected that it was poor PA. I had a PA report suggesting that I was 8 arcminutes too low. But as I had a 150 point TPoint model, I could slew quite accurately despite this. However, with poor PA, I would still get eggy stars.
Turning on guiding, I had a much better result.
So Peter's first point in his post is what I had anticipated.
To illustrate, attached are screenshots from the central portion of 2 images. Both have integration times of 120 seconds. The scope is an RC8 with a QSI583 binned x2. This gives me a plate scale of about 1.39 arc seconds/pixel. The first is unguided, the second guided. You can see the difference.
I'm sure that collimation is also at play here so its not just the mount/PA.
The eggy stars are across the field so I don't think its field curvature.
I've since refined my PA - the PA report suggests that I can lower my mount 1.8 arc minutes while az is excellent. This gives me Sky RMS of 92.2. This is with a 20 point model which I used to smarten up the PA. he attached pics were taken before I did this.
The next step is to obtain a bigger model, turn on Protrack and then check the PE.
Out of interest, is anyone surprised by this performance? The scope has a focal length of 1604mm which is not exactly long and I'd have hoped to have longer subs than 2 minutes without eggy stars - but that may be explained by the PA.
Pete
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