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.

2IThe Celestron EdgeHD
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 astro-
photographers. 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 of-
fers clean, diffraction-limited images for high power observation
of the planets and the Moon. As an aplanatic, flat-field astro-
graph, 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-to-
the-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 compre-
hensive 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 high-
quality images over a much wider field of view than the clas-
sic 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.

The Celestron EdgeHD 3
Edge HD 1400
Edge HD 1100
Edge HD 925
Edge HD 800
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.
EdgeHD Series

4IThe Celestron EdgeHD
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.
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.

The Celestron EdgeHD 5
On-Axis 5.00 mm 10.00 mm 15.00 mm 20.00 mm
100 μm
Off-axis distance (millimeters)
CLASSIC SCT
“COMA-FREE” SCT
EDGEHD
THE OPTICAL PERFORMANCE OF THE EDGEHD COMPARED TO OTHER SCTS
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.

6IThe Celestron EdgeHD
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®professional
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.
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.
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.
Telescope with Field Curvature
Field Curvature
Flat-Field Telescope

The Celestron EdgeHD 7
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 larg-
er illuminated field of view.
To ensure full compatibility with the remarkable Starizona Hyper-
star 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
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.
-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
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.

8IThe Celestron EdgeHD
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 high-
performance 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 high-
power 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.
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, high-
transmission, 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.
14” ƒ/11 Flat-Field EdgeHD14” ƒ/10 Coma-Free SCT
Spot diagrams plotted for 0.0, 5, 10, 15, and 20mm off-axis; showing λ= 0.486, 0.546, and 0.656μm.
-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
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.
FIGURE 6. The mirror clutch mechanism shown in this cross-
section prevents the primary mirror from shifting during the long
exposures used in imaging.

The Celestron EdgeHD 9
We cut corrector blanks from large sheets of the glass, then run
them through a double-sided surfacing machine to grind and
polish both surfaces to an optical finish. The blanks are inspected
and any with defects are discarded.
The Johnson/Celestron method for producing the polynomial
aspheric curve is based on precision “master blocks” with the
exact inverse of the desired curve. We clean the master block
and corrector blank, and then, by applying a vacuum from the
center of the block, pull them into intimate optical contact,
excluding any lint, dust, or air between them, gently bending
the flat corrector blank to match the reverse curve of the block.
We then take the combined master block and corrector blank
and process the top surface of the corrector to a polished
concave spherical surface. With the corrector lens still on the
master block, an optician tests the radius and figure of the new
surface against a precision reference matchplate (also known as
an optical test plate or test glass) using optical interference to
read the Newton’s rings or interference fringes, as shown in
Figure 7. If the surface radius lies within a tolerance of zero to
three fringes (about 1.5 wavelengths of light, or 750nm concave),
and the surface irregularity is less than half of one fringe (¼–
wavelength of light), the corrector is separated from the master
block. The thin glass springs back to its original shape, so that
the side that was against the master block becomes flat and the
polished surface assumes the profile of a Schmidt corrector lens.
The corrector is tested again, this time in a double-pass auto
collimator. Green laser light at 532nm wavelength (green)
enters through an eyepiece, strikes an EdgeHD secondary and
primary mirror, passes through the corrector lens under test,
reflects from a precision optical flat, goes back through the
corrector to reflect again from the mirrors, and finally back to
focus. Because the light passes twice through the Schmidt
corrector lens, any errors are seen doubled. The double-pass
autocollimation test (see Figure 9) ensures that every Schmidt
corrector meets the stringent requirements of an EdgeHD
optical system.
Primary mirrors begin as precision-annealed molded castings of
low-expansion borosilicate glass with a weight-saving conical
back surface and a concave front surface. The molded casting
is edged round, its central hole is cored, and the radius of the
front surface is roughed in. Celestron grinds the front surface of
primary mirrors with a succession of progressively finer diamond
abrasive pellet tools using high-speed spindle machines, then
transfers them to an abrasive free room where they are polished
to a precise spherical surface. Each mirror is checked for both
radius and optical spherical figure against a convex precision
reference matchplate. When the interference fringes indicate
the radius is within ±1 fringe from the nominal radius and the
surface irregularity is less than one fourth of one fringe, the
mirror receives a final check using the classic mirror-maker’s
null test familiar to every professional optican. Afterwards,
every primary mirror is taken to the QA Interferometry Lab—shown
in Figure 10—where the surface irregularity of each mirror is
verified, via interferometer, to be within specification.
The smaller secondary mirrors are also made of low-expansion
borosilicate glass. Like the primaries, the secondaries are edged
and centered, then ground and polished. The secondary is a
convex mirror so during manufacture it is tested against a
concave precision reference matchplate to check both its radius
of curvature and figure. The secondary mirrors are also brought
to the QA Interferometry Lab where the radius and irregularity of
each mirror is verified through interferometric measurement to
assure that each one lies within specification.
When we designed the EdgeHD optical system, we strongly
favored spherical surfaces because a sphere can be tested by
optical interference to high accuracy in a matter of minutes. If we
had specified a hyperboloidal surface for the secondary mirror,
we would have been forced to use slower, less accurate testing
methods that might miss zonal errors. Furthermore, coma-free
SCT designs with hyperboloidal mirrors still suffer from field
curvature—an aberration that we specifically wished to avoid in
the EdgeHD design.
Finally, the sub-aperture corrector lenses are made using the
same manufacturing techniques used for high-performance
refractor objectives. The EdgeHD design specifies optical glass
from Schott AG. The 8-, 9.25-, and 11-inch use N-SK2 and K10
glasses, while the 14-inch uses N-SK2 and N-BALF2 glasses.
To ensure homogeneity, optical glass is made in relatively small
batches, extruded in boules. The raw glass is then diamond-
milled to the correct diameter, thickness, and radius. Each lens
blank is blocked, ground, and polished, then the radius and figure
are compared to matchplates to ensure they meet EdgeHD’s
tight tolerances.
Our assembly workstations resemble the optical benches used
to qualify corrector plates. The primary mirror and corrector plate
slip into kinematic support jigs, and we place the secondary
mirror in its holder. The sub-aperture corrector lenses meet
specifications so reliably that a master set is used in the assem-
bly workstation. Laser light from the focus position passes in
reverse through the optics, reflects from a master autocollimation
flat, then passes back through the optics. Tested in autocollimation,
the optician can see and correct surface errors considerably
smaller than a millionth of an inch.
If the combined optics set shows any slight residual under-or
over-correction, zones, astigmatism, upturned or downturned
edges, holes, or bulges, the optician marks the Foucault test
shadow transitions on the secondary mirror, then removes the
secondary mirror from the test fixture and translates these
markings into a paper pattern. The pattern is pressed against
a pitch polishing tool, and the optician applies corrective polishing
to the secondary mirror—as we show in Figure 11—until the
optical system as a whole displays a perfectly uniform illumination
(no unwanted zones or shadows) under the double-pass
Foucault test and smooth and straight fringes under the double-
pass Ronchi test. The in-focus Airy disk pattern is evaluated for
roundness, a single uniform diffraction ring, and freedom from
scattered light. In addition, the intra- and extra-focal diffraction
pattern must display the same structure and central obscuration
on both sides of focus, and it must appear round and uniform.
FIGURE 7. Matchplates use interference fringes to check
the radius and smoothness of the correction. In this picture, a
corrector blank is attached to a master block. The matchplate
rests on top; interference fringes appear as green and blue
circles. The circular pattern indicates a difference in radius.

10 IThe Celestron EdgeHD
FIGURE 8. After all the testing is done, the ultimate test is the night sky. This close-up image of the Pelican Nebula testifies to the
EdgeHD’s ability to focus clean, neat, round star images from center to edge. The telescope was a 14-inch EdgeHD on a CGE Pro
Mount; the CCD camera was an Apogee U16m. The mage above shows a 21.5×29.8mm section cropped from the original 36.8mm
square image.
EDGEHD’S CLOSE-UP ON THE PELICAN NEBULA

The Celestron EdgeHD 11
FIGURE 9. In autocollimation testing, light goes through an
optical system, reflects from a plane mirror, and passes through
again. This super-sensitive test method doubles the apparent size
of all errors.
Autocollimation Testing
Precision optical flat
Beamslitter
Green Laser (532 nm)
Eyepiece and Ronchi grating
Telescope being checked
FIGURE 10. We test all of our primary mirrors on an optical
bench by means of laser interferometry. In the picture, stacks of
polished primary mirrors await testing.
After we remove each set of optics from the autocollimator, we
send the components to our in-house coating chamber. Here,
the primary and secondary mirrors receive their high-reflectance
aluminum coatings, and the corrector lens is anti-reflectance
coated. Each set of optics is then installed into an optical tube
assembly (OTA).
Completed OTAs undergo the Visual Acceptance Test. In a
temperature-stabilized optical test tunnel, green laser light at
532nm wavelength is reflected from a precision paraboloidal
mirror to act as an artificial star. With a high-power ocular, a QA
Inspector views the artificial star critically.
To pass the Visual Acceptance Test, an optical tube assembly
must meetthe following requirements:
• The in-focus Airy disk must be round, free of scattered light
around the disk, and display only one bright ring.
• Inside and outside focus, the diffraction patterns must be
round, uniform, and appear similar on both sides of focus.
• Observed with a 150 line-pairs-per-inch Ronchi grating, the
bands must be straight, uniformly spaced, and high in contrast.
Because its optics have been tested and tuned in error-revealing
double-pass mode, and because each assembled OTA has been
tested again and qualified visually, the telescope’s images should
be flawless when observing and imaging the night sky.
7. FINAL ACCEPTANCE TESTING
AND CERTIFICATION
Before it can leave Celestron’s facilities, every EdgeHD must
pass its Final Acceptance Test, or FAT. We conduct the FAT on
an optical test bench in a specially-constructed temperature-
controlled room (Figure 12). Rather than use laser light for this
test, we use white light so that the FAT reproduces the same
conditions an observer would experience while viewing or
photographing the night sky. To avoid placing any heat sources
in the optical path, the light for our artificial star is carried to the
focus of a precision parabolic mirror through a fiber-optic cable.
After striking the parabolic mirror, the parallel rays of light travel
down the optical bench to the EdgeHD under test, through the
telescope, to a full-frame format digital SLR camera placed at
its focus.
Using a set of kinematic test cradles, there is no need to change
the test configuration between different EdgeHD telescopes.
We simply place the telescope in its test cradle on the bench,
and it’s ready for testing. The Final Acceptance Test verifies an
EdgeHD’s ability to form sharp star images in the center and
on the edges of a full-frame (24×36mm format, with a 42mm
diagonal measurement) digital SLR camera. The QA Inspector
attaches the camera to the telescope, focuses carefully, and
takes an on-axis image. The telescope is then pointed so the
artificial star image falls in the corner of the frame, and with-
out refocusing, the inspector takes another image. The process
is repeated for each corner of the camera frame, and another
picture is taken at the center of the frame.
To pass the test, the telescope must form a sharp image at the
center of the field, at each corner of the camera frame, and
again at the center. The images are examined critically. To pass,
every one of the test images must be tight, round, and in perfect
focus. Any EdgeHD that does not pass the FAT is automatically
returned to the assembly room to recheck the collimation and
centering of its corrector plate. No EdgeHD can leave the
factory until it has passed its FAT.
FIGURE 11. To correct any remaining optical errors, the figure of
the secondary mirror is fine-tuned against the entire optical system
in double-pass autocollimation setup. This delicate match process
ensures that every telescope performs to the diffraction limit.

12 IThe Celestron EdgeHD
Throughout the telescope-building process, we maintain a
quality-assurance paper trail for each instrument. All test
images are numbered and cross referenced. Should a telescope
be returned to Celestron for service, we can consult our records
to see how well it performed before it left our facility. Once a tele-
scope has passed the FAT, we apply Loctite®to the set screws
to permanently hold the alignment of the corrector plate. The
instrument is then inspected carefully for cosmetic defects. It is
cleaned and packaged for shipment to our dealers and customers.
8. VISUAL OBSERVING WITH THE EDGEHD
Because both the Celestron EdgeHD and our classic SCTs
are diffraction-limited on-axis, their performance is essentially
the same for high-magnification planetary or lunar viewing,
splitting close double stars, or any other visual observing task
that requires first-rate on-axis image quality. However, the
EdgeHD outshines the classic SCT when it comes to observing
deep-sky objects with the new generation of high-performance
wide-field eyepieces.
The classic SCT exhibits off-axis coma and field curvature
which are absent from the EdgeHD design. Modern wide-field
eyepieces, such as the 23mm Luminos, have an apparent field
of view of 82 degrees, so they show you a larger swatch of
the sky. Gone are the light-robbing radial flares of coma and
annoying, out-of-focus peripheral images so sadly familiar
to observers. With the EdgeHD, stars are crisp and sharp to
the edge.
The back of the EdgeHD 800 features an industry standard
2.00×24 tpi threaded flange. A large retaining ring firmly
attaches the 1¼-inch visual back, and this accepts a 1¼-inch
Star Diagonal that will accept any standard 1¼-inch eyepiece.
The EdgeHD 925, 1100, and 1400 feature a heavy-duty flange
with a 3.290×16 tpi threaded flange. This oversize flange allows
you to attach heavy CCD cameras and digital SLR cameras.
For visual observing, use the adapter plate supplied with each
telescope to attach the Visual Back. The 2-inch Diagonal
(also supplied with these telescopes) accepts eyepieces with
1¼-inch and 2-inch barrels.
To your discerning eye—as an observer with experience—on a
night with steady air and good seeing, a properly cooled EdgeHD
performs exceptionally well on stars. You will see a round, clean
Airy disk, a single well-defined diffraction ring, and symmetrical
images inside and outside of focus. Every EdgeHD should
resolve double stars to the Dawes limit, reveal subtle shadings
in the belts of Jupiter, and reveal the Cassini Division in Saturn’s
rings. On deep-sky objects viewed with a high-quality eyepiece,
star images appear sharp and well defined to the edge of the
field of view. The EdgeHD reveals faint nebular details as fine as
the sky quality at the observing site will allow.
FIGURE 12. In the Final Acceptance Test, the EdgeHD optics
must demonstrate the ability to form sharp images at the center
and in the corners of a full-frame digital SLR camera, with a
sensor that measures 42mm corner-to-corner.
FIGURE 13. EdgeHD telescopes are designed to provide
good images across a flat 42mm diameter field of view.
Compare this with the size of a variety of image sensor formats.
The popular APS-C digital SLR format fits easily. The full-frame
DSLR format is fully covered. EdgeHD even covers the 36.8mm
square KAF-16803 format remarkably well.
KAF-16803
KAF-8300
APS-C DSLR Full-Frame DSLR
KAI-10002
KAF-3200
42 mm ∅
EdgeHD Field of View

The Celestron EdgeHD 13
9. IMAGING WITH THE EDGEHD
The Celestron EdgeHD was designed and optimized for imaging
with astronomical CCD cameras, digital SLR cameras, video
astronomy sensors, electronic eyepieces, and webcams. We
designed the EdgeHD 800 to deliver the best images 5.25
inches (133.35mm) behind the surface of the telescope’s rear
cell 2.00×24 tpi threaded baffle tube lock nut. The EdgeHD
925, 1100, and 1400 form their best images 5.75 inches
(146.05mm) behind the telescope’s rear cell 3.290×16 tpi
threaded baffle tube lock nut. For best results, the image sensor
should be located within ±0.5mm of this back-focus distance.
It is easy to place a digital SLR (DSLR) camera at the proper
distance using the Small T-Adapter (item #93644) for the
EdgeHD 800, or the Large T-Adapter (item #93646) for the
EdgeHD 925, 1100, and 1400. The small adapter is 78.35mm
long while the large adapter adds 91.05mm, in both cases
placing the best focus 55mm behind the T-Adapter. Because
55mm is the industry standard T-mount to sensor distance, add
a T-Ring adapter (T-Ring for Canon EOS, item #93419; T-Ring
for Nikon, item #93402) and attach your camera to it. That’s
all there is to placing your digital SLR camera at the correct
back-focus location. (By the way, if you’ve never heard of the
T-mount system, you need to know about it. The T-mount is a
set of industry standard sizes and distances for camera lenses.
A standard T-mount thread (M42×0.75) is available for most
astronomical CCD cameras. The standard T-mount flange-to-
sensor distance is 55mm.)
The T-mount system also makes spacing an astronomical CCD
camera easy. Consult your CCD camera’s documentation to find
the flange-to-sensor distance for your CCD camera. Attaching
the Celestron T-Adapter to your EdgeHD gives you the standard
55mm spacing. If your CCD’s front flange-to-sensor distance
is 35mm, you need an additional 20mm distance. A 20mm
T-mount Extension Tube, available from many astronomy
retailers, will help you achieve the correct back focus distance. If
you require a more complex optical train for your CCD camera,
consult a trusted astronomy retailer.
For imaging, we recommend using T-system components
because threaded connections place your CCD camera or
digital SLR at the correct back focus distance for optimum
performance. Not only are they strong, but they also hold your
camera perfectly square to the light path.
To mount a high-performance video camera, add the T-Adapter
plus a T-to-C adapter. (Like the T-mount system, the C-mount
system is an industry standard. It uses 1×32 tpi threads with a
back-focus distance of17.5mm.)
For consumer video systems such as electronic eyepieces,
planetary cameras, and webcams that attach to the telescope
using a standard 1.25-inch eyepiece barrel, simply use the same
components that you use for visual observing. Just remove the
eyepiece from the telescope and replace it with the camera.
Many imaging programs allow you to shoot short exposures
through the telescope. On a solid, polar-aligned equatorial
mounting, you may be able to expose for 30 seconds or more.
With such exposure times, you can capture wonderful images of
the Moon, planets, eclipses, bright star clusters, and objects like
the Orion Nebula.
However, for long exposures on deep-sky objects, you will need
to guide the telescope. The days of guiding by eye are over;
electronic auto-guiders are the new standard. A functional and
relatively inexpensive autoguiding setup consists of a small
refractor mounted piggyback on your EdgeHD telescope. You
will need a dovetail bar attached to the EdgeHD tube. Celestron
offers an 80mm guide telescope package (item #52309) to
be used with the NexGuide Autoguider (item #93713). For
sub-exposures exceeding 10 minutes or so, piggybacked guide
telescopes potentially suffer from differential flexure; for such
imaging, consider the Off-Axis Guider (item #93648).
Imaging with Celestron EdgeHD Telescopes
EdgeHD Aperture
Focal Ratio
Focal Length
Secondary Ø
Obstruction1
Back Focus
Distance
Adapter
Thread Size
Image Circle
Linear Ø
Angular Ø
Airy Disk
Angular Ø
Linear Ø
Rayleigh2
Image Scale
arcsec/pixel
(6.4 μm
pixel)
EdgeHD
800
203.2mm
ƒ/10.456
2125mm
68.6mm
34 %
133.35mm
2.00”-24 tpi
42mm Ø
68.0 arcmin
1.36” Ø
14.0μm Ø
0.68”
0.62”/pix
EdgeHD
925
235mm
ƒ/9.878
2321mm
85.1mm
36%
146.05mm
3.29”-16 tpi
42mm Ø
62.2 arcmin
1.18” Ø
13.2μm Ø
0.59”
0.57”/pix
EdgeHD
110 0
279.4mm
ƒ/9.978
2788mm
92.3mm
33%
146.05mm
3.29”-16 tpi
42mm Ø
51.8 arcmin
0.99” Ø
13.3μm Ø
0.50”
0.47”/pix
EdgeHD
1400
355.6mm
ƒ/10.846
3857mm
114.3mm
32%
146.05mm
3.29”-16 tpi
42mm Ø
37.4 arcmin
0.78” Ø
14.4μm Ø
0.39”
0.34”/pix
1. The Ø symbol means diameter. Central obscuration is given as a percentage of the aperture.
2. The Rayleigh Limit for resolving doubles with equally bright components. The “ symbol means arcseconds.

14 IThe Celestron EdgeHD
FIGURE 14. It is easy to position your digital SLR camera, an astronomical CCD camera, a high-performance video camera, or an
inexpensive webcam at the focus plane of your EdgeHD telescope. For the sharpest wide-field imaging, your goal is to place the
sensor 5.25 inches behind the rear flange of the EdgeHD 800, or 5.75 inches behind the rear flange of the EdgeHD 925, 1100,
and1400.
5.75 inches
146.05±0.5 mm
5.25 inches
133.35±0.5 mm
Celestron’s EdgeHD: The Versatile Imaging Platform
EdgeHD
800
EdgeHD
925, 1100,
and 1400
Small T-Adapter
Camera Adapter
Digital SLR
Reducing Ring
Small T-Adapter
T-to-1.25” Adapter
Webcam
Large T-Adapter
T-Ring Adapter
Digital SLR
Large T-Adapter
T-to-C Adapter
Astro Video Camera
Large T-Adapter
T-system Spacer
CCD Camera
For those who wish to make images with a faster focal ratio
than EdgeHD 1100’s ƒ/10 or the EdgeHD 1400’s ƒ/11, we
designed a five-element 0.7× reducer lens for each of these
EdgeHD telescopes. (For more information, see Appendix B.)
The Reducer Lens 0.7× for the EdgeHD 1100 is item #94241;
for the 14-inch, item #94240. (The newly-released EdgeHD
800 Focal Reducer is item #94242.)
The reducer lens attaches directly to the 3.290×16 tpi threaded
baffle tube lock nut on the back of the telescope. Since the back
focus distance for the 1100 and 1400 reducer lens is 5.75 inches
(146.05mm), you can use the same T-Adapter and camera
T-Ring you would use for imaging at the ƒ/10 or ƒ/11 focus. The
linear field of view is still 42mm diameter, but the angular field is
43% larger, and exposure times drop by a factor of two.
For super-fast, super-wide imaging, the EdgeHD telescope
series supports Starizona’s Hyperstar lens. Mounted on the
corrector plate in place of the secondary mirror, the Hyperstar
provides an ƒ/1.9 focal ratio on the EdgeHD 1400, and ƒ/2.0 or
ƒ/2.1 on the 800, 925, and 1100. Covering a 27mm diameter
field of view, the Hyperstar is a perfect match for APS-C format
digital SLR cameras. Because of the short focal length and
fast focal ratio, sub-exposures are just a minutes. With a solid,
polar-aligned equatorial mount, guiding seldom necessary.
Of course, the focal length of any EdgeHD telescope can be
extended using a Barlow lens such as the Celestron 2x X-Cel
LX (item #93529) or 3x X-Cel LX (item #93428). Using a
Barlow, imagers can reach the desirable f/22 to f/32 range for
ultra-high-resolution lunar and planetary imaging.
In summary, the Celestron EdgeHD telescopes provide a flexible
platform for imaging. You can work at the normal ƒ/10 or ƒ/11
Cassegrain focus for seeing-limited deep-sky images or add
the reducer lens for wider fields and shorter exposure times.
With a Hyperstar, you can grab wide-field, deep-sky images
in mere minutes. And finally, you can extend the focus to capture
fine lunar and planetary images with a quality Barlow lens.
When you buy an EdgeHD telescope, you’re getting an imaging
platform that covers all the bases, from fast, wide-field imaging to
high-resolution imaging of the moon and planets.
Celestron's EdgeHD: The Versatile Imaging Platform

The Celestron EdgeHD 15
10. CONCLUSION
The classic Schmidt-Cassegrain telescope introduced tens of
thousands of observers and imagers to astronomy and nurtured
their appreciation for the wonder of the night sky. Today, observers
and imagers want a more capable telescope, a telescope that
provides sharp close-ups as well as high-quality images all the
way across a wide, flat field of view. But, consumers want this
advanced optical technology at an affordable price.
Celestron has designed the EdgeHD to meet customers’ needs.
The EdgeHD is not only coma-free, but it also provides a flat
field so that stars are sharp to the very edge of the field of view.
In this brief technical white paper, we have shown you the inner
workings of our new design. We also demonstrated the care we
exert as we build and test each EdgeHD telescope. We trust that
we have proven that an EdgeHD is the right telescope for you.
You may find the following resources to be useful in
learning about optical design, fabrication, and testing:
11. REFERENCES
DeVany, Arthur S., Master Optical Techniques. John Wiley
and Sons, New York, 1981.
Fischer, Robert E.; Biljana Tadic-Galeb; and Paul R. Yoder,
Optical System Design. McGraw Hill, New York, 2008.
Geary, Joseph M., Introduction to Lens Design.
Willmann-Bell, Richmond, 2002.
Malacara, Daniel, ed., Optical Shop Testing. John Wiley
and Sons, New York, 1978.
Rutten, Harrie, and Martin van Venrooij, Telescope Optics:
A Comprehensive Manual for Amateur Astronomers.
Willmann-Bell, Richmond, 1999.
Smith, Gregory Hallock, Practical Computer-Aided Lens Design.
Willmann-Bell, Richmond, 1998.
Smith, Gregory Hallock; Roger Ceragioli; Richard Berry,
Telescopes, Eyepieces, and Astrographs: Design, Analysis, and
Performance of Modern Astronomical Optics. Willmann-Bell,
Richmond, 2012.
Wikipedia. Search references to specific topics.
See: http://en.wikipedia.org/wiki/Optical_lens_design
and many associated links.
Wikipedia. Search references to T-mount.
See: http://en.wikipedia.org/wiki/T-mount and associated
camera system links.
Wilson, R. N., Reflecting Telescope Optics I and II.
Springer-Verlag, Berlin, 1996.
ZEMAX®Optical Design Program, User’s Guide.
Radiant Zemax LLC, Tucson, 2012.

16 IThe Celestron EdgeHD
Appendix A:
Technical Profiles of EdgeHD Telescopes
When evaluating astronomical telescopes, astroimagers must
bear in mind the many factors that influence image quality. The
major factors at play are:
• The image formed by the telescope
• The sampling by pixels of the image sensor
• The diffraction pattern of the telescope
• The “seeing” quality during exposure
• The guiding accuracy during exposure
To aid astroimagers, this Appendix presents a spot matrix plot
for each of the telescopes in the EdgeHD series. To determine
the size of the images that you observe in your exposures, these
must be compounded with the other factors that affect your images.
In the spot matrix plots we have provided, each large gray box
is 64µm on a side, and consists of a ten small boxes 6.4µm
representing a pixel in a “typical” modern CCD camera. The
black circle represents the diameter of the Airy disk to the first
dark ring. It is immediately clear that for each of the EdgeHDs,
two 6.4µm pixels roughly match the diameter of the Airy disk.
This means that under ideal conditions, a CCD camera with
pixels of this size will capture most of the detail present in the
telescopic image. Referring to Figure A1, the left column shows
the Airy disk for a telescope with a central obstruction of 34%.
Because the light in the Airy disk is concentrated into a smaller
area in the center, capturing all of the image detail in a planetary
or lunar image requires using a 2x or 3x Barlow lens to further
enlarge the Airy disk.
Unfortunately, ideal conditions are fleeting. During a typical CCD
exposure, atmospheric turbulence enlarges the image of all
stars, and furthermore, it causes the images to wander. On the
steadiest nights, the “seeing” effect may be as small as 1 arcsecond.
In Figure A1, the “superb seeing” column shows blurs with a
FWHM (full-width half-maximum) of 1 arcsecond. The next column
shows excellent seeing (1.5”), and the right column shows 2”
seeing blurs, typical of many nights at most observing sites. It
is important to note that as the focal length of the telescope
increases, the diameter of the seeing blur increases in proportion.
With a small telescope, seeing plays a smaller role. With the large
apertures and long focal lengths of the EdgeHD series, nights of
good seeing become particularly valuable.
FIGURE A1. Shown at the same scale as the matrix spot
diagrams are the Airy disk and the point-spread-function
of seeing disks for average (2.0”), excellent (1.5”), and superb
(1.0”) seeing.
Airy Disk and Seeing Blurs
Airy
Disk
Superb
Seeing
Average
Seeing
Excellent
Seeing
14-inch 8-inch11-inch 9.25-inch

The Celestron EdgeHD 17
On-axis, the spots show that the 8-inch EdgeHD is diffraction-
limited in both green (for visual observing) and red (for imaging).
And because blue rays are strongly concentrated inside the
Airy disk, the 8-inch EdgeHD is diffraction-limited in blue light.
Off-axis, its images remain diffraction-limited over a field larger
than the full Moon.
For an imager using an APS-C digital SLR camera, relative
illumination falls to 84% at the extreme corners of the image.
Although for bright subjects this minor falloff would pass
unnoticed, for imaging faint objects we recommend making and
applying flat-field images for the best results. For CCD imaging,
we always recommend making flat field images.
Portability and affordability are the hallmarks of the EdgeHD
800. Although the 8-inch covers a 42mm image circle, we
optimized its optics for the central 28mm area, the size of an
APS-C chip in many popular digital SLR cameras.
Celestron EdgeHD 800
Celestron EdgeHD 800

18 IThe Celestron EdgeHD
Celestron EdgeHD 925
The spot matrix shows that on-axis images are diffraction
limited at all three wavelengths, and remain diffraction-limited
over the central 15mm. While blue and red are slightly enlarged,
in green light images are fully diffraction-limited over a 38mm
image circle. The size of the off-axis blue and red spots remain
nicely balanced.
On a night of average seeing, stars will display a FWHM of
23µm, comparable in size to the spot pattern at the very edge
of a 42mm field. Relative illumination in the EdgeHD 925 is
excellent. The central 12mm is completely free of vignetting,
while field edges receive 90% relative illumination. For most
imaging applications, flat fielding would be optional.
For full-field imaging on a tight budget, the EdgeHD 925 is an
excellent choice. It offers nearperfect on-axis performance and
outstanding images over a full 42mm image circle.
.Celestron EdgeHD 925

The Celestron EdgeHD 19
Celestron EdgeHD 1100
The 11-inch EdgeHD is optimized to produce its sharpest
images in green and red; at these wavelengths it is diffraction-
limited over roughly two-thirds of the full 42mm image circle.
The relative illumination remains 100% across the central
16mm, then falls slowly to 83% at the very edge of a 42mm
image circle.
For pictorial images with an APS-C digital SLR camera, flats are
unnecessary. For monochrome imaging with an astronomical
CCD camera, we always recommend making flat-field images.
On nights when the seeing achieves 1.5 arcseconds FWHM,
star images shrink to 18µm at the focal plane. On such nights,
the EdgeHD 1100 delivers fine images over a 30mm image
circle, and well-defined stars over the full 42mm field.
The EdgeHD 1100 is a serious telescope. Its long focal length
and large image scale give it the ability to capture stunning
wide-field images of deep-sky objects with a large-format
CCD camera.
Celestron EdgeHD 1100

20 IThe Celestron EdgeHD
Celestron EdgeHD 1400
In the matrix spot diagrams, note the tight cluster of rays in
green light, and the well balanced spherochromatism in the
blue and red. These spots are far better than spots from a fine
apochromatic refractor of the same aperture.
In green light, the EdgeHD 1400 is diffraction-limited over a
28mm image circle, although atmospheric seeing enables it
to display its full resolution only on the finest nights. Relative
illumination is 100% across the central 16mm, and falls
slowly to 83% in the extreme corners of a full-frame 35mm
image sensor. We have seen excellent results when the
14-inch EdgeHD is used with a KAF-16803 CCD camera over
a 50mm circle.
The EdgeHD 1400 is a massive telescope, well suited to
a backyard observatory or well-planned away-from-home
expeditions. Its long focal length and large image scale offer
skilled imagers the opportunity to make images not possible
with smaller, less capable telescopes.
Celestron EdgeHD 1400