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Posted by Walter Hamler on May 22, 2001 01:36:20 UTC

Another way of looking at it is that with a smaller secondary than "normal" you will be losing effective aperature with objects beyond the 100% cone. As far as the percentage of central obstruction, 20% is a rule of thumb. f/8 scopes are normally around 16~18% with proper size diagonals and f/4~f/5 scopes are at the 20% and higher figure. I't because of the steep angle the cone of light from the primary exibits. A recent S&T article (about 5 or 6 months ago) talked this topic to great length leaving one to conclude the contrast degredation wasn't really as bad as we have been led to believe over the years! I recently upgraded a diagonal in a 10" f/6.8 from a smaller than required to the "proper size" and experienced an actual improvement in image quality with my longer fl eyepieces, exactly what would be expected.

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