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.

• Suggestion? - avecfort - May 22, 2001 - 13:00 UTC
  • Diadonals, etc - walter - May 22, 2001 - 17:01 UTC
    • 24% of diameter obstruction by the secondary? - avecfort - May 24, 2001 - 07:40 UTC
      • Program for calculating contrast degredation by secondary size?? - avecfort - May 24, 2001 - 07:41 UTC
      • Secondary Obstruction - walter - May 24, 2001 - 20:52 UTC
        • Me thinks you're right :) - avecfort - May 25, 2001 - 05:03 UTC
          • Good Luck Building! - walter - May 25, 2001 - 12:03 UTC
            • Halmstad Sweden - avecfort - May 25, 2001 - 15:27 UTC