Colour Delivery in SCT's vs Newts etc.

nick wrote:

VcDy wrote:
Why, would a 16" lx200 be stingy in its color delivery vs. a 17" f/4.5
Coulter (which is great) ? Is there something about the sct design
which
inherently retracts colour delivery vs say larger refractors, newts, and

maksutovs.

Several have suggested its the size of the sct at question...

SCT color error does increase in proportion to its aperture.
For a common commercial configuration (f/2/10, neutral zone
at 0.866 radius) chromatic blur diameter is given approximately by
dD/73 in mm,
with "d" being the difference in refractive index vs. that
for which the corrector is optimized, and D the aperture diameter.
So a 16" SCT has chromatic blurs larger by a factor of 2 vs. an 8".

Also, SCT corrector with the 0.866 radius neutral zone is optimized
for photography, not visual. It brings together best focus for the
optimized wavelength and circles of least confusion of other colors.
While the circle of least confusion is smallest of all blurs between
paraxial and marginal foci, it has twice the wavefront error of the
so called "best" or "diffraction" focus, which is a common
focus for other (than the optimized) colors when the neutral zone is
at 0.707 radius. For instance, a 16" f/2/10 SCT with 0.866 radius
neutral zone, corrector refractive index ~1.5 at the optimized
~550nm, would have ~0.016mm blur diameter in the blue F-line (d~0.003)
and ~0.027mm blur diameter in the red C-line (d~0.005). It translates
into 1/5 wave wavefront error in the blue, and 1/3.1 wavefront error
in the red. Should the neutral zone be placed at 0.707 radius, the
color wavefront errors would be 1/10 wave and 1/6.2 wave, respectively.

Obviously, even at 16" aperture, the color error is still relatively
small, even more so considering lowered eye sensitivity. It may affect
color saturation of smallest resolvable low-contrast details, but not
likely very significantly. More likely culprit for lower color
saturation over an entire object is a higher amount of scattered light

There we go. This is a beautiful analysis Vlad - Thanks. So the obvious
place one can go is to baffling and any ordianry thing one can do to
improve contrast.

Thanks - Jerry


(from rough
surfaces and/or and compromised glass homogeneity of the corrector),
and less efficient baffling.

Vlad