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Message: <blockquote data-quote="WJC" data-source="post: 3651801" data-attributes="member: 25191">“Necessarily?” NO. Traditionally? YES.Shorter focal ratio systems CAN be equal to longer ones. However, it usually requires MORE EXOTIC glass types (read: $$), at least one ASPHERIC ELEMENT (read: $$), MORE ELEMENTS (read: extra weight).Those unfamiliar with optical engineering have a proclivity to think that you can correct for optical aberrations onesie-twosie. ‘Tain’t so. You might correct one aberration to a desired level at the expense of increasing another a little and driving two others into the realm of the unreasonable. An engineer tries to design using his “degrees of freedom” to come up with a design that will meet the requirements of the MAJORITY of potential buyers.This is also true of conceptual matters. I designed a Houghton telescope in which all photo-visual wavelengths would fit into the Airy Disc like a pea in the center of the platter. The contrast-destroying central obstruction was about 40%. By simply changing the designated stop from the primary mirror to the secondary mirror, I reduced the central obstruction to 30%. By so doing, the contrast was MUCH better. However, spots and OPDs looked like they had been concocted by a third grader.Physics can be a harsh taskmaster. :cat:Bill</blockquote>