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-0.8 mm
Spot diagrams plotted for 0.0, 5, 10, 15, and 20mm off-axis; showing λ = 0.486, 0.546, and 0.656
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 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.
8
I
The Celestron EdgeHD
14" ƒ/10 Coma-Free SCT
-0.4 mm
0.0 mm
+0.4 mm
+0.8 mm
14" ƒ/11 Flat-Field EdgeHD
-0.8 mm
-0.4 mm
0.0 mm
On-axis
5 mm
off-axis
10 mm
off-axis
15 mm
off-axis
20 mm
off-axis
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, 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.
+0.4 mm
+0.8 mm
m
.
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