At 10:44 1999-01-23 -0800, Jan Steinman wrote:
>.....I got a focal reducer -- opposite of a teleconverter -- for mine,
which makes it a 1280mm
>f6.3.....
>Perhaps it's my imagination, or just because defects are 63 0.000000e+00ss
>noticable, or the viewfinder is 63% brighter, but I like what I get with
>the focal reducer better than when using the telescope at prime focal
>length.
It's normal. The (commercial) Schmidt-Cassegrain telescopes are built with
a *designed* field curvature (to get the long focal length, while make the
tube short and at the same time keep the secondary mirror small). The f6.3
reducer/corrector has an in-built field-flattener - which flattens the field.
The Schmidt-Cassegrain telescope is an all-purpose telescope - somewhat
optimized for high magnification visual observervation. For the wide-field
visual observer - and absolutely for the photographer - the f6.3 reducer
corrector is a "necessity".
>This is counter-intuitive, since more glass should always mean less
>quality.
Naaahhhh.... that is not true. E.g. two (glass) lenses *can* make an
achromatic lens (correct for two colors) - while you *need* at least three
(glass) lenses to make it apochromatic (correction for three colors). If
color correction is what you want. You may loose other things though with
more lenses.
Any optical design is always a compromise - some abberations are brought
down, while the design will suffer more from others. That is true for
telescopes as well as camera lenses.
/Claes
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