Beg to differ with Clive's analysis.
With imaging it comes down to field correction, spot size and seeing. A well made Riccardi-Honders delivers 5 micron stars across around a 70mm flat field. Most other systems have spot sizes 2x larger on axis, and
degrade to *way more* off axis due to field curvature, astigmatism etc.
Other factors such as thermal and mechanical stability also come into play, and literally shift focus as an exposure is being taken. Exacting focusers and thermally stable materials usually come at a cost. My experience so far has been, you get what you pay for.