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Very good question! We often hear it said that aperture is king, so why would a smaller scope for similar or more $$$ be used.
The answer is complex and has a number of facets. Firstly, why is a 100 refractor more expensive than a 250mm reflector? Well a lense has 2 optical surfaces that need figuring, while a mirror only has 1, so each lense needs a minimum of twice the time to make. Also small faults in the glass of smaller mirrors will not affect the optical performance unless the are on the surface, while for a lense the glass must be near perfect right through. Also modern refractors use multiple elements which increases the surfaces to be figured. AND refractors have to be collimated in factory while a manufacturer of a reflector leaves that to the end user.
A high quality refractor can hive advantages over a reflector over and above light grasp. Not having a central obstruction improves contrast, they don't need collimation each time they are used, coma is usually not an issue, thought other aberations, particularly colour fringing etc can be evident in non apochromatic refractors of shorter focal length.
Malcolm
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