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Hi Barry
I saw your post and it got me to thinking. I see the evidence of the Pareto principle in many things, including the use of astronomical instruments.
I note that with SCT telescopes, the sweet spot appears to be about 8" (200mm) aperture. Over that size, the weight goes up, the portability suffers, the cost really goes up, and the field of view gets ever smaller. Under that size, the light collection ability is compromised.
With Dobs, the sweet spot appears to be changing. It used to be 8", but the recent influx of Goto dobs with collapsible truss would appear to be driving the sweet spot nearer the 12" (300mm) mark. Over that, they are getting too heavy, awkward (not portable) and costly.
Now if one were fitting out an observatory, weight and portability would be a lesser concern. If the mount can tolerate the weight, then bigger is better … sort of. The field of view will diminish as the aperture goes up, although the Newtonian mirrors with f4 ratio (or even less) can make the use of a crazy-big scope (think metre class Dob) still worthwhile. There is an article about such a monster scope in AS&T this month.
For an observatory fixed location, I think a 30" (750mm) f/4 Newtonian is probably as big as one could reasonably go before the field of view is compromised too much and the usability starts to be impacted. The club I am a member of has access to a telescope of this size, fitted with ArgoNavis and StellarCat … it takes two people to work it (despite the goto), and while the views are simply stunning, the trouble of operating it means it's as big as I would consider usable.
Regards,
Tony Barry
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