John:

Thank you for the "Most Excellent" post. Your information has caused me to close
my wallet and think through this issue.


Thank you,


art



art



> Darned good question, and the answer depends on what the transmission of the
> lens is, and the transmission of the SCT corrector and reflectivity of its
> mirrors. These in turn all depend upon how high-tech the coatings are, both
> on the lenses and mirrors.
>
> My semi-educated guess is that the average modern fluorite refractor might
> lose 2% per lens element. The SCT might lose 12% due to secondary
> obstruction, 2% at the corrector plate, another 10-20% at the primary, and 4%
> to 20% at the secondary. I don't know of any production SCTs using 99%
> dielectic coatings even on the secondary, but I might be wrong.
>
> Then both would take another loss at the diagonal, under 1% or the best
> dielectric mirror diagonals, maybe 4% for "enhanced coatings" and maybe
> 10-20% again for a typical mirror.
>
> Remember to multiply the tranmission amounts, for example
> 0.88x0.98x0.90x0.90x0.90 with all 90% reflective mirrors and diagonal or
> 0.98x0.98x0.99 for a fluorite doublet with MaxBright or Everbright diagonal.
>
> That's apples to oranges on the diagonals, but hopefully accurately
> represents how they are purchased nowadays.
>
> In this example an 8" SCT with 90% diagonal would pass 62.9% of its
> theoretical capacity, and a 4" doublet would pass 95.1% of its. The SCT
> would still pass 400%*(62.9/95.1) = 264.6% the light of the 4" refractor.
> With apples to apples on the diagonals, it becomes 291%. To match the 8" SCT
> light gathering with equal diagonals using my example numbers would take a
> 6.8" refractor, and to match the light gathering of the 4" refractor would
> only take a 4.7" SCT.
>
> With higher-end reflector coatings, the differences become even smaller,
> approaching the loss due to the obstruction.
>
> So refractor owners need to either get out their checkbook in a very big way,
> or be prepared to lose light-gathering comparisons, or both. A good
> compromise is to get one of each type of scope and use each for what it is
> best at!
>
> John
>
> In a message dated 08/27/2000 9:51:05 AM Eastern Daylight Time,
>
> << Is there a direct comparison between SCT and fluorite glass refractors
> as to their light gathering capability? It there some qualitative
> measurement or a “Rule of Thumb”? An example would be that a 4"
> fluorite glass refractor gathers as much light as an 8” SCT.
>
>
> Thank you,
>
>
> Art >>

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