True, but a smaller secondary would also mean a taller ladder. As built, the observing position when pointed at the zenith is only about as high as you'd need to go with the 22" that Brent built. Reducing the secondary diameter also only gains a very small percentage of the total area of the 70" mirror. And the diffraction effects from a 40% obstruction won't be an issue with deep-sky objects, and should be offset by the increased resolution of a massive 70-inch aperture. There are always trade-offs. On Tue, Oct 29, 2013 at 12:45 PM, Erik Hansen <erikhansen@thebluezone.net>wrote:
of course the draw back is you get a shorter tube but a 40% obstruction of the primary, probably could have a used a smaller mirror, to reduce tube length, and get similar light gathering.