Celestron EDGEHD User Manual
A FLEXIBLE IMAGING PLATFORM
AT AN AFFORDABLE PRICE
Superior flat-field, coma-free imaging
by the Celestron Engineering Team
Ver. 04-2013, For release in April 2013.
The Celestron EdgeHD A Flexible
Imaging Platform at an Affordable Price
By the Celestron Engineering Team
ABSTRACT:
The Celestron EdgeHD is an advanced, flat-field, aplanatic
series of telescopes designed for visual observation and imaging
with astronomical CCD cameras and full-frame digital SLR
cameras. This paper describes the development goals and
design decisions behind EdgeHD technology and their practical
realization in 8-, 9.25-, 11-, and 14-inch apertures. We include
cross-sections of the EdgeHD series, a table with visual and
imaging specifications, and comparative spot diagrams for
the EdgeHD and competing “coma-free” Schmidt-Cassegrain
designs. We also outline the construction and testing process for
EdgeHD telescopes and provide instructions for placing sensors
at the optimum back-focus distance for astroimaging.
1. INTRODUCTION
The classic Schmidt-Cassegrain telescope (SCT) manufactured
by Celestron served an entire generation of observers and astrophotographers. With the advent of wide-field and ultra-wide-field
eyepieces, large format CCD cameras, and full frame digital SLR
cameras, the inherent drawbacks of the classic SCT called for a
new design. The EdgeHD is that new design. The EdgeHD offers clean, diffraction-limited images for high power observation
of the planets and the Moon. As an aplanatic, flat-field astrograph, the EdgeHD’s optics provide tight, round, edge-to-edge
star images over a wide, 42mm diameter flat field of view for
stunning color, monochrome, and narrow-band imaging of deep
sky objects.
2. SETTING GOALS FOR THE EDGEHD TELESCOPE
The story of the EdgeHD began with our setting performance
goals, quality goals, and price goals. Like the classic SCT, the
new Celestron optic would need to be light and compact.
Optically, we set twin goals. First, the new telescope had to
be capable of extraordinary wide-field viewing with advanced
eyepiece designs. Second, the optic had to produce sharp-tothe-edge astrophotography with both digital SLR cameras and
astronomical CCD cameras. Finally, we wanted to leverage
Celestron’s proven ability to manufacture high-performance
telescopes at a consumer-friendly price point. In short, we sought
to create a flexible imaging platform at a very affordable price.
Given an unlimited budget, engineering high-performance optics
is not difficult. The challenge Celestron accepted was to
control the price, complexity, and cost of manufacture without
compromising optical performance. We began with a comprehensive review of the classic SCT and possible alternatives.
Our classic SCT has three optical components: a spherical
primary mirror, a spherical secondary mirror, and a corrector plate
with a polynomial curve. As every amateur telescope maker and
professional optician knows, a sphere is the most desirable
optical figure. In polishing a lens or mirror, the work piece moves
over a lap made of optical pitch that slowly conforms to the glass
surface. Geometrically, the only surfaces that can slide freely
against one another are spheres. Any spot that is low relative to
the common spherical surface receives no wear; any spot that is
higher is worn off. Spherical surfaces result almost automatically.
A skilled optician in a well-equipped optical shop can reliably
produce near-perfect spherical surfaces. Furthermore, by
comparing an optical surface against a matchplate—a precision
reference surface—departures in both the radius and sphericity
can be quickly assessed.
In forty years of manufacturing its classic Schmidt-Cassegrain
telescope, Celestron had fully mastered the art of making
large numbers of essentially perfect spherical primary and
secondary mirrors.
In addition, Celestron’s strengths included the production of
Schmidt corrector plates. In the early 1970s, Tom Johnson,
Celestron’s founder, perfected the necessary techniques.
Before Johnson, corrector plates like that on the 48-inch
Schmidt camera on Palomar Mountain required many long
hours of skilled work by master opticians. Johnson’s innovative
production methods made possible the volume production of a
complex and formerly expensive optical component, triggering
the SCT revolution of the 1970s.
For more than forty years, the SCT satisfied the needs of
visual observers and astrophotographers. Its performance
resulted from a blend of smooth spherical surfaces and
Johnson’s unique method of producing the complex curve
on the corrector with the same ease as producing spherical
surfaces. As the 21st century began, two emerging technologies
—wide-field eyepieces and CCD cameras—demanded highquality images over a much wider field of view than the classic SCT could provide. Why? The classic SCT is well-corrected
optically for aberrations on the optical axis, that is, in the exact
center of the field of view. Away from the optical axis, however,
its images suffer from two aberrations: coma and field curvature.
Coma causes off-axis star images to flare outward; field curvature
causes images to become progressively out of focus away from
the optical axis. As wide-field eyepieces grew in popularity, and
as observers equipped themselves with advanced CCD cameras,
the classic SCT proved inadequate. To meet the requirements of
observers, we wanted the new Celestron optic to be both free of
coma and to have virtually zero field curvature.
2 I The Celestron EdgeHD
EdgeHD Series
Edge HD 800
Edge HD 925
Edge HD 1100
Edge HD 1400
FIGURE 1. Celestron’s EdgeHD series consists of four aplanatic telescopes with 8-, 9.25-, 11-, 14-inch apertures. The optical design
of each instrument has been individually optimized to provide a flat, coma-free focal plane. Each EdgeHD optic produces sharp images
to the edge of the view with minimal vignetting.
The Celestron EdgeHD 3
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-theline, 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.
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.
4 I The Celestron EdgeHD
THE OPTICAL PERFORMANCE OF THE EDGEHD COMPARED TO OTHER SCTS
CLASSIC SCT
“COMA-FREE” SCT
EDGEHD
100 μm
On-Axis 5.00 mm 10.00 mm 15.00 mm 20.00 mm
Off-axis distance (millimeters)
FIGURE 2. Matrix spot diagrams compare the center-to-edge optical performance of the classic SCT, “coma-free” SCT, and EdgeHD.
The EdgeHD clearly outperforms the other optical systems. The classic SCT shows prominent coma. The “coma-free” SCT is indeed
free of coma, but field curvature causes its off-axis images to become diffuse and out of focus. In comparison, the EdgeHD’s spot pattern
is tight, concentrated, and remains small from on-axis to the edge of the field.
The Celestron EdgeHD 5
4. OPTICAL PERFORMANCE OF THE EDGEHD
Optical design involves complex trade-offs between optical
performance, mechanical tolerances, cost, manufacturability, and
customer needs. In designing the EdgeHD, we prioritized optical
performance first: the instrument would be diffraction-limited on
axis, it would be entirely coma-free, and the field would be flat
to the very edge. (Indeed, the name EdgeHD derives from our
edge-of-field requirements.)
Figure 2 shows ray-traced spot diagrams for the 14-inch
aperture classic SCT, coma-free SCT, and EdgeHD. All three are
14-inch aperture telescopes. We used ZEMAX
optical ray-trace software to design the EdgeHD and produce
these ray-trace data for you.
Each spot pattern combines spots at three wavelengths: red
(0.656µm), green (0.546µm), and blue (0.486µm) for five field
positions: on-axis, 5mm, 10mm, 15mm, and 20mm off-axis
distance. The field of view portrayed has diameter of 40mm—
just under the full 42mm image circle of the EdgeHD—and
the wavelengths span the range seen by the dark-adapted
human eye and the wavelengths most often used in deep-sky
astronomical imaging.
In the matrix of spots, examine the left hand column. These are
the on-axis spots. The black circle in each one represents the
diameter of the Airy disk. If the majority of the rays fall within
the circle representing the Airy disk, a star image viewed at
high power will be limited almost entirely by diffraction, and is
therefore said to be diffraction-limited. By this standard, all three
SCT designs are diffraction-limited on the optical axis. In each
case, the Schmidt corrector removes spherical aberration for
green light. Because the index of refraction of the glass used in
the corrector plate varies with wavelength, the Schmidt corrector
allows a small amount of spherical aberration to remain in red
and blue light. This aberration is called spherochromatism, that
is, spherical aberration resulting from the color of the light. While
the green rays converge to a near-perfect point, the red and
blue spot patterns fill or slightly overfill the Airy disk. Numerically,
the radius of the Airy disk is 7.2µm, (14.4µm diameter) while the
root-mean-square radius of the spots at all three wavelengths is
5.3µm (10.6µm diameter). Because the human eye is considerably
more sensitive to green light than it is to red or blue, images in
the eyepiece appear nearly perfect even to a skilled observer.
Spherochromatism depends on the amount of correction, or
the refractive strength, of the Schmidt lens. To minimize
spherochromatism, high-performance SCTs have traditionally
been ƒ/10 or slower. When pushed to focal ratios faster than
ƒ/10 (that is, when pushed to ƒ/8, ƒ/6, etc.) spherochromatism
increases undesirably.
Next, comparing the EdgeHD with the classic SCT and the
“coma-free” SCT, you can see that off-axis images in the classic
SCT images are strongly affected by coma. As expected, the
images in the coma-free design do not show the characteristic
comatic flare, but off-axis they do become quite enlarged. This is
the result of field curvature.
Figure 3 illustrates how field curvature affects off-axis images.
In an imaging telescope, we expect on-axis and off-axis rays
to focus on the flat surface of a CCD or digital SLR image
sensor. But unfortunately, with field curvature, off-axis rays come
to sharp focus on a curved surface. In a “coma-free” SCT, your
off-axis star images are in focus ahead of the CCD.
At the edge of a 40mm field, the “coma-free” telescope’s stars
have swelled to more than 100µm in diameter. Edge-of-field star
images appear large, soft, and out of focus.
®
professional
Meanwhile, at the edge of its 40mm field, the EdgeHD’s
images have enlarged only slightly, to a root-mean-square
radius of 10.5µm (21µm diameter). But because the green rays
are concentrated strongly toward the center, and because every
ray, including the faint “wings” of red light, lie inside a circle only
50µm in diameter, the images in the EdgeHD have proven to be
quite acceptable in the very corners of the image captured by a
full-frame digital SLR camera.
Field curvature negatively impacts imaging when you want
high-quality images across the entire field of view. Figures 4 and
5 clearly demonstrate the effects of field curvature in 8- and
14-inch telescopes. Note how the spot patterns change with
off-axis distance and focus. A negative focus distance means
closer to the telescope; a positive distance mean focusing
outward. In the EdgeHD, the smallest spots all fall at the same
focus position. If you focus on a star at the center of the field,
stars across the entire field of view will be in focus.
In comparison, the sharpest star images at the edge of the
field in the “coma-free” telescope come to focus in front of the
on-axis best focus. If you focus for the center of the image, star
images become progressively enlarged at greater distances. The
best you can do is focus at a compromise off-axis distance, and
accept slightly out-of-focus stars both on-axis and at the edge
of the field.
Any optical designer with the requisite skills and optical ray-tracing
software can, in theory, replicate and verify these results. The
data show that eliminating coma alone is not enough to guarantee
good images across the field of view. For high-performance
imaging, an imaging telescope must be diffraction-limited
on-axis and corrected for both coma and field curvature off-axis.
That’s what you get with the EdgeHD, at a very affordable price.
Field Curvature
Telescope with Field Curvature
Flat-Field Telescope
FIGURE 3. In an optical system with field curvature, objects
are not sharply focused on a flat surface. Instead, off-axis rays
focus behind or ahead of the focus point of the on-axis rays at
the center of the field. As a result, the off-axis star images are
enlarged by being slightly out of focus.
6 I The Celestron EdgeHD
5. MECHANICAL DESIGN IMPROVEMENTS
To ensure that the completed EdgeHD telescope delivers the
full potential of the optical design, we also redesigned key
mechanical components. With classic SCT designs, for example,
an observer could bring the optical system to focus at different
back focus distances behind the optical tube assembly,
changing effective focal length of the telescope. This caused
on-axis spherical aberration and increased the off-axis
aberration. In the EdgeHD series, the back focus distance is
optimized and set for one specific distance. Every EdgeHD
comes equipped with a visual back that places the eyepiece
at the correct back focus distance, and our Large T-Adapter
accessory automatically places digital SLR cameras at the
optimum back focus position.
As part of the optical redesign, we placed the primary and
secondary mirrors closer than they had been in the classic SCT,
and designed new baffle tubes for both mirrors that allow a larger illuminated field of view.
To ensure full compatibility with the remarkable Starizona Hyperstar accessory that enables imaging at ƒ/1.9 in the EdgeHD 800
and ƒ/2.0 in the EdgeHD 925, 1100, and 1400, all EdgeHDs
have a removable secondary mirror.
Because it covers a wide field of view, the optical elements of
the EdgeHD must meet centering and alignment tolerances
considerably tighter than those of the classic SCT design. For
example, because the corrector plate must remain precisely
centered, we secure it with alignment screws tipped with soft
Nylon plastic. The screws are set on the optical bench during
assembly while we center the corrector plate. Once this
adjustment is perfect, the screws are tightened and sealed with
®
Loctite
to secure the corrector in position. This seemingly small
mechanical change ensures that the corrector plate and the
secondary mirror mounted on the corrector plate stay in
permanent optical alignment.
Centering the primary mirror is even more demanding. In
the classic SCT, the primary mirror is attached to a sliding
“focus” tube. When you focus the telescope, the focus knob
moves the primary mirror longitudinally. When you reverse the
direction of focus travel, the focus tube that carries the primary
can “rock” slightly on the baffle tube, causing the image to shift.
In the classic SCT, the shift does not significantly affect on-axis
image quality. However, in the EdgeHD, off-axis images could
be affected. Because the baffle tube carries the sub-aperture
corrector inside and the primary mirror on the outside, we
manufacture it to an extremely tight diametric tolerance.
The tube that supports the primary was redesigned with a
centering alignment flange, which contacts the optical (front)
surface of the primary mirror. When the primary mirror is assembled
onto the focus tube and secured with RTV adhesive, this small
mechanical change guarantees precise optical centration.
Following assembly, the focus tube carrying the primary is placed
in a test jig. We rotate the mirror and verify that the primary is
precisely squared-on to ensure that the image quality expected
from the optics is maintained.
In any optical system with a moveable primary mirror, focus
shift—movement of the image when the observer changes
focusing direction—has been an annoyance. In Celestron’s SCT
and EdgeHD telescopes, we tightened the tolerances. During
assembly and testing, we measure the focus shift; any unit with
more than 30 arcseconds focus shift is rejected and returned to
an earlier stage of assembly for rework.
In the classic SCT, astrophotographers sometimes experience
an image shift as the telescope tracks across the meridian. The
focus mechanism serves as one support point for the mirror. In
the EdgeHD, we added two stainless steel rods to the back of
the cell that supports the primary mirror. When the two mirror
clutches at the back of the optical tube assembly are engaged,
aluminum pins press against the stainless steel rods, creating
two additional stabilizing support points (see Figure 6).
8” ƒ/10 Flat-Field EdgeHD8” ƒ/10 Coma-Free SCT
-0.8 mm -0.4 mm 0.0 mm +0.4 mm +0.8 mm-0.8 mm -0.4 mm 0.0 mm +0.4 mm +0.8 mm
On-axis
3.5 mm
off-axis
7 mm
off-axis
10.5 mm
off-axis
14 mm
off-axis
Spot diagrams plotted for 0.0, 3.5, 7, 10.5, and 14 mm off-axis; showing λ = 0.486, 0.546, and 0.656 μm.
FIGURE 4. Compare star images formed by a 8-inch coma-free SCT with those formed by an EdgeHD. The sharpest star images in
the coma-free SCT follow the gray curve, coming to focus approximately 0.6mm in front of the focal plane. In the EdgeHD, small, tight
star images are focused at the focal plane across the field of view, meaning that your images will be crisp and sharp to the very edge.
The Celestron EdgeHD 7
14” ƒ/11 Flat-Field EdgeHD14” ƒ/10 Coma-Free SCT
-0.8 mm -0.4 mm 0.0 mm +0.4 mm +0.8 mm-0.8 mm -0.4 mm 0.0 mm +0.4 mm +0.8 mm
On-axis
5 mm
off-axis
10 mm
off-axis
15 mm
off-axis
20 mm
off-axis
Spot diagrams plotted for 0.0, 5, 10, 15, and 20mm off-axis; showing λ = 0.486, 0.546, and 0.656μm.
FIGURE 5. In a 14-inch coma-free SCT, the smallest off-axis star images lie on the curved focal surface indicated by the gray line.
Since CCD or digital SLR camera sensors are flat, so star images at the edge of the field will be enlarged. In the aplanatic EdgeHD
design, the smallest off-axis images lie on a flat surface. Stars are small and sharp to the edge of the field.
Telescope tubes must “breathe” not only to enable cooling, but
also to prevent the build-up of moisture and possible condensation
inside the tube. In the classic SCT, air can enter through the open
baffle tube. In the EdgeHD, the sub-aperture lenses effectively
close the tube. To promote air exchange, we added ventilation
ports with 60µm stainless steel mesh that keeps out dust but
allows the free passage of air.
In a telescope designed for imaging, users expect to
attach heavy filter wheels, digital SLRs, and astronomical CCD
cameras. We designed the rear threads of the EdgeHD 925,
1100, and 1400 telescopes with a heavy-duty 3.290×16 tpi
thread, and we set the back focus distance to a generous 5.75
inches from the flat rear surface of the baffle tube locking nut.
The rear thread on the EdgeHD 800 remains the standard
2.00×24 tpi, and the back-focus distance is 5.25 inches.
Many suppliers offer precision focusers, rotators, filter wheels,
and camera packages that are fully compatible with the
heavy-duty rear thread and back focus distance of the EdgeHD.
6. MANUFACTURING THE EDGEHD OPTICS
Each EdgeHD has five optical elements: an aspheric Schmidt
corrector plate, a spherical primary mirror, a spherical secondary
mirror, and two sub-aperture corrector lenses. Each element is
manufactured to meet tight tolerances demanded by a highperformance optical design. Celestron applies more than forty
years of experience in shaping, polishing, and testing astronomical
telescope optics to every one of the components in each EdgeHD
telescope. Our tight specifications and repeated, careful testing
guarantee that the telescope will not only perform well for highpower planetary viewing, but will also cover a wide-angle field
for superb edge-to-edge imaging. Nevertheless, we don’t take
this on faith; both before and after assembly, we test and tune
each set of optics.
FIGURE 6. The mirror clutch mechanism shown in this cross-
section prevents the primary mirror from shifting during the long
exposures used in imaging.
Celestron’s founder, Tom Johnson, invented the breakthrough
process used to make Celestron’s corrector plates. Over the
years, his original process has been developed and refined. At
present, we manufacture corrector plates with the same level of
ease, certainty, and repeatability that opticians expect when they
are producing spherical surfaces.
Each corrector plate begins life as a sheet of water-white, hightransmission, low-iron, soda-lime float glass. In manufacturing
float glass, molten glass is extruded onto a tank of molten tin,
where the glass floats on the dense molten metal. The molten
tin surface is very nearly flat (its radius of curvature is the radius
of planet Earth!), and float glass is equally flat.
8 I The Celestron EdgeHD
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