I'm sure I read somewhere that there is an argument for having a smaller aperture for solar observation. I recall it goes something like - you have buckets of light, so there is no need for a larger aperture to collect light. When observing the sun, it is normally in a heated atmosphere (ie daytime
) and hence stability of the air column between scope and upper atmosphere is important for resolution. The maths apparently suggests that a smaller aperture will provide better resolution that a larger aperture under such conditions. Well I think that is what I read.
Hence the many offaxis small aperture solar filters available - it's not just for lowering the cost.