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As Barry mentioned, a top tier mirror is costly (actually over $10K landed or $12K if you want the quartz 20F4).
The Skywatcher secondary is listed as having a major axis of 136mm which translates to a minor axis of about 3.78" or 96mm which is not surprising for a 20" F4.0 scope.
I don't know anything about the quality of these mirror but can understand that they, like many astro manufacturers, don't want to publish any specs for fear of people, in places like the USA, then trying to handpick scopes.
In reality these are cheap scopes compared to custom ones.
@Barry, that Quartz 18F4 is very tempting !!!!
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