Improving Refractor Performance With a Star
Diagonal Prism.
One of the most important accessories that refractor owners
use is a star diagonal. It allows observers to comfortably look through
their instruments without craning necks or assuming painful
contortions, when the scope is pointed upwards. Star diagonals come in
two basic types-mirror or prismatic and can range in cost from just a
few dollars to several hundred, depending on quality and size.
Many years ago, star diagonals were typically the prismatic
type. In the 1980's, there was a change in trends and more vendors
began offering mirror-type diagonals. This was largely due to an
increasing number of "fast" optical systems appearing, which can have
problems with a prism-type diagonal.
During the 1980's and '90s, vendors strove to offer flatter,
better made mirror diagonals, with ever more efficient coatings. Today,
we can routinely buy excellent units with 99% reflectivity, thanks to
the use of di-electric materials on the mirror surface. These are very
fine performers, have reflective surfaces which are extremely durable
and have become the most sought-after diagonals for many types of
telescopes.
Indeed, high efficiency dielectric mirror star diagonals can
be the best choice in most instruments. But, there are exceptions and
it can benefit the user to determine if their particular scope might
achieve even better performance using a different type of star
diagonal... namely, the prism type.
(Old Celestron 1.25” diagonal prism used with a Megrez 110ED
refractor)
Over the past few years, an increasing number of relatively
inexpensive ED refractors have come to market. Many of these scopes
have color correction much better than achromatic. They might not be
truly apochromatic, but many observers are happy to say "close enough".
The manufacturers of these ED refractors have chosen an
optical formula which minimizes the appearance of defocused blue/violet
light at the slight expense of red correction. Observers are often
sensitive to worse correction at shorter wavelengths and typically look
for "blue/violet" halos around bright objects as an indication of color
error. Manufacturers have become increasingly aware of this and realize
that the best way to make their optics appear to display less chromatic
error, is to "shift" correction slightly. The result being lenses which
generate less visible defocus at shorter wavelengths, but more blurring
in the red. Unfortunately, this poorer correction at longer wavelengths
is more detrimental to performance than many people might imagine. It
makes it more difficult to achieve sharp visual focus, especially at
higher magnifications. Planetary and lunar detail suffers. Difficult
double stars (especially when the brighter component is reddish) are
harder to discern.
(Celestron 1.25” prism and Baader T-2 prism)
This is where a prism-type star diagonal can help. Since light
passing through a prism is refracted slightly, image correction is
altered. A star diagonal prism shifts color correction. As it happens,
this shift is beneficial to a surprising number of today's ED doublet
and triplet refractors sourced from Taiwan and China. The lens's red
defocus is reduced by the refractive property of the prism. A small
amount of blue/violet blur is generated as a result, but that defect is
far less damaging to image quality.
Another optical characteristic of a diagonal prism is that
it's naturally overcorrected for spherical aberration. Many refractors
are made slightly undercorrected for spherical aberration. Thus, a
prism can neatly (or nearly so) null the spherical correction of an
optical system.
How will you know if a star diagonal prism will work best for
you? It's really a matter of trial and error. Fortunately, a good
quality 1.25" star diagonal prism can be purchased for around $40. So,
you don't have to spend a fortune to find out.
For best results, make sure it’s a traditional “right-angle”
prism, rather than a “correct image” (Amici-type) model. The
right-angle prism leaves the image reversed. The Amici prism corrects
this reversal, but can introduce image-degrading aberrations.
Owners of refractors by Astro-Physics, Takahashi, TeleVue, TMB
and other very premium makes likely won't see significant performance
benefits from using a prism. Those optical systems were designed to
work best with a mirror diagonal. But, as mentioned, it's a cheap
experiment, so might be worth trying even in those cases. From the
results seen by the undersigned and an increasing number of other users
who've tried a star diagonal prism with their ED refractors, there's a
good chance you'll come away very pleased with the view!
Clive Gibbons. |
