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22
Model Item # Aperture Type Focal Length Pier/Tripod
C20 Astrograph 11095 0.51m (20”) Corrected Dall-Kirkham 3454mm f/6.8 Professional Class Portable
C20 Astrograph 11096 0.51m (20”) Corrected Dall-Kirkham 3454mm f/6.8 Professional Class Permanent
C20 OTA 91095 0.51m (20”) Corrected Dall-Kirkham 3454mm f/6.8 N/A
CGE Pro Mount 91520 N/A Computerized German EQ N/A N/A
CGE Pro Tripod 93490 N/A Heavy Duty Field Tripod N/A Professional Class Portable
CGE Pro Pier 93491 N/A Heavy Duty Pier N/A Professional Class Permanent
For complete specifications and product information, visit: www.celestron.com
C20 ASTROGRAPH FEATURES AT A GLANCE
C20 • Whirlpool Galaxy • Image courtesy of Richard Hedrick
C20 ASTROGRAPH SPECIFICATIONS
OTA Features
• Coma-Corrected Flat-Field Dall-Kirkham Cassegrain (Corrected Dall-Kirkham) - The
C20 is called an Astrograph because it is designed for a coma free flat field which
gives pinpoint stars across a large CCD chip or a wide field eyepiece.
• 20” (Half meter) aperture - Large aperture for imaging faint objects with ultra fine
resolution.
• F/6.8 Focal Ratio - A fast system f-ratio for shorter image times and a wider field
of view. This is 28% faster than f/8 and 54% faster than f/10.
• Carbon Fiber Dual Truss Design - both the lower primary mirror cage and the upper
cage are supported by a truss design made of carbon fiber tubes; this minimizes
expansion and contraction due to temperature change, and therefore limits
the change in focus.
• Carbon Fiber Light Shrouds - Carbon fiber light shrouds are available for the C20
OTA; the lower light shroud comes standard while the upper light shroud is optional
and removable. The shrouds physically protect the mirror and block unwanted stray
light.
• Filtered Cooling Fans - Three fans blow filtered air onto the back of the primary
mirror to minimize cool-down times.
• Fixed Conical Primary Mirror - The primary is fixed at Celestron to ensure that the
focal plane of the optics is true to the focal plane of the mechanics; the primary
is molded in a conical shape to reduce thermal mass (for quick cooling) and
minimize flexure of the optics.
• Spherical Secondary - A spherical secondary is much more forgiving to centering
errors compared to ellipsoidal or hyperbolic secondaries used in competing high-end
telescope designs; the result is that with the C20 you can easily align the system
to get the performance you paid for. With aspheric secondaries, you may never
achieve the performance you paid for.
• Dovetail Expansion Joint - the dovetail is made of aluminum and thus expands and
contracts due to temperature changes; the C20 dovetail has a built-in expansion joint
which allows it to expand and contract without inducing stress to the optical tube.
• 3.5” Heavy Load Focuser - This Crayford style focuser has been modified to run
on a lead screw rather than a friction surface; the result is a focuser that can
handle heavy imaging equipment without the focus shifting due to changes
in the telescope’s orientation.
• Temperature Sensors - The C20 has built-in temperature sensors; with the optional
control electronics, the sensors detect the temperature of the primary mirror and
the ambient air, and automatically control the fans to minimize any temperature
differential that may exist.
Mount Features
• Heavy Duty Computerized CGE Pro Mount - Weighing over 150 pounds the mount
is massive enough to hold payloads of 180 pounds.
• Meridian Limits - This mount can track more than 30 degrees beyond the meridian;
this is more than any other German Equatorial mount in the high-end market.
• Internal Wiring - For a permanent setup, the mount has an internal path through
which cable can be routed; for a CCD Camera for instance, the control cable,
the Autoguider cable and the power cable can be routed through the mount
to minimize dangling wires.
• Internal Switches - Built-in optical switches on both axes give the mount a very
accurate home position and will prevent the mount from tracking or slewing
to the point where the Dec axis hits the RA axis.
• Clutch System - Four set screws, orthogonal to the RA and Dec axes, lock the axes
so there is no slipping; when the set screws are loosened, the axes move freely
to facilitate precise balancing of the system.
• NexStar Computerized Control System - The mount comes standard with the latest
NexStar Computerized Control System; both the hand control and motor control
firmware are upgradeable with the latest firmware revisions which can be
downloaded from Celestron’s website.
• NexRemote Pro: Software for controlling the C20 from a personal computer;
NexRemote Pro models the mount to the sky using multiple alignment stars in order
to dramatically increase pointing accuracy. NexRemote Pro has a built-in polar
alignment algorithm accurate enough for a permanent observatory and quick enough
for a one night observing run; this alignment algorithm does not require sighting
on Polaris. Also included is a Finder Screen that acts like a mini-planetarium
program, a Mount Model Screen that displays the mount errors and the expected
pointing accuracy, and a virtual hand control that has most of the features of the
NexStar hand control unit — all displayed on the user’s computer screen.
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C20 Astrograph
Rear View
Optical performance comparison of the
C20 astrograph with an equivalent
Ritchey-Chrétien (RC) system.
Grid squares are 9 microns and represent the pixel size of the
KAF-6303 CCD. The RC has a curved field and requires a field
flattener to achieve optimum imaging performance. The C20
has an inherently flat field. Simulated wavelength is 550 nm.
Note: The C20 optical performance is the same whether using the system
visually or using the system for imaging since it has a flat field. The RC
design has a curved field. This means that the system will perform
visually like the example labeled RC curved field because the eye can
accommodate the curved field of the design. When imaging, the design
will perform like the example labeled RC flat field.
C20 Astrograph RC Curved Field RC Flat Field
On-axis
Corner of
KAF-6303
CCD (17mm
off-axis)
C20 ASTROGRAPH
NexRemote Pro Finder Screen
NexRemote Pro Mount Model Screen
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