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OPTICAL ABERRATIONS
For those not familiar with the art of optical design, this brief
primer explains what aberrations are and how they appear in a
telescopic image.
OFF-AXIS COMA
Coma is an off-axis aberration that occurs when the rays from
successive zones are displaced outward relative to the principal
(central) ray. A star image with coma appears to have wispy "hair"
or little "wings" extending from the image. In a coma-free optical
system, rays from all zones are centered on the (central) ray, so
stars appear round across the field
FIELD CURVATURE
Field curvature occurs when the best off-axis images in an optical
system focus ahead or behind the focused on-axis image. The
result is that star images in the center of the field of view are
sharp, but off-axis images appear more and more out of focus. A
telescope with no field curvature has a "flat field, " so images are
sharp across the whole field of view.
SPHEROCHROMATISM
In the Schmidt-Cassegrain, spherochromatism is present, but
not deleterious in designs with modest apertures and focal
ratios. It occurs because the optical "power" of the Schmidt
corrector plate varies slightly with wavelength. Only in very large
apertures or fast SCTs does spherochromism become a problem.
3. ENGINEERING A NEW ASTROGRAPH
We did not take lightly the task of improving the classic SCT. Its
two spherical mirrors and our method of making corrector lenses
allowed us to offer a high-quality telescope at a low cost. We
investigated the pros and cons of producing a Ritchey-Chrétien
(R-C) Cassegrain, but the cost and complexity of producing its
hyperbolic mirrors, as well as the long-term disadvantages of
an open-tube telescope, dissuaded us. We also designed and
produced two prototype Corrected Dall-Kirkham (CDK)
telescopes, but the design's ellipsoidal primary mirror led
inevitably to a more expensive instrument. While the R-C and
CDK are fine optical systems, we wanted to produce equally fine
imaging telescopes at a more consumer-friendly price.
As we've already noted, our most important design goal for the
new telescope was to eliminate coma and field curvature over
a field of view large enough to accommodate a top-of-the-
line, full-frame digital SLR camera or larger astronomical CCD
camera. This meant setting the field of view at 42 mm in
diameter. Of course, any design that would satisfy the
full-frame requirement would also work with the less expensive
APS-C digital SLR cameras (under $800) and less expensive
astronomical CCD cameras (under $2,000). There are several
ways to modify the classic SCT to reduce or eliminate coma.
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The Celestron EdgeHD
Unfortunately, these methods do not address the problem of field
curvature. For example, we could replace either the spherical
primary or secondary with an aspheric (i.e., non-spherical)
mirror. Making the smaller secondary mirror into a hyperboloid
was an obvious choice. Although this would have given us a
coma-free design, its uncorrected field curvature would leave
soft star images at the edges of the field. We were also
concerned that by aspherizing the secondary, the resulting
coma-free telescopes would potentially have zones that would
scatter light and compromise the high-power definition that
visual observers expect from an astronomical telescope.
Furthermore, the aspheric secondary mirror places demands
on alignment and centration that often result in difficulty
maintaining collimation.
The inspiration for the EdgeHD optics came from combining
the best features of the CDK with the best features of the
classic SCT. We placed two small lenses in the beam of light
converging toward focus and re-optimized the entire telescope
for center-to-edge performance. In the EdgeHD, the primary and
secondary mirrors retain smooth spherical surfaces, and the
corrector plate remains unchanged. The two small lenses do
the big job of correcting aberrations for a small increment in
cost to the telescope buyer. Furthermore, because it retains key
elements of the classic SCT, the EdgeHD design is compatible
with the popular Starizona Hyperstar accessory.
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