Goal: To design, engineer, and manufacture the finest large aperture optical system possible for both imaging and visual observation without any design or cost constraints.

Achievement: The C20 Astrograph is the first telescope system of its kind ever offered by a large commercial telescope manufacturer.

After evaluating numerous design options, Celestron's solution to the design of this instrument is a 20" (1/2 meter) diameter f/6.8 coma-correct flat-field Dall-Kirkham Cassegrian (corrected Dall-Kirkham) optical assembly that is mounted on a newly designed, massive observatory-class computerized German Equatorial mount.

Revolutionary Optical System Celestron’s Modified Corrected Dall-Kirkham design exceeds the off-axis performance of even the Ritchey-Chrétien design. RMS spot sizes at the edge of a 35mm frame remain smaller than a single pixel on the most advanced CCD cameras available to amateurs today. The optical system also features a conical primary mirror with low thermal mass and three cooling fans to minimize the cool down time. This no-compromise design provides the superb resolving power Dall- Kirkham telescopes are known for, but without the coma and field curvature that has previously limited their wide-field performance. Because of its flat-field design, you'll find it a joy to use for imaging as well as visual use. The C20 optical system will come standard with Celestron’s advanced StarBright® XLT High- Performance Optical Coatings. BELOW: 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.

The CGE Pro Mount The tracking accuracy of this observatory-class mount is better than 5 arcseconds peak to valley withoutenabling the PEC function. The C20 Astrograph mount separates into three components for easier transporting and includes fully integrated electronics as well as Celestron’s renowned and proven NexStar® software. It is also supplied with NexRemote® Pro, a software package that creates models of the mount which yields observatory class pointing accuracy. In A Class Of Its Own Other high-end telescopes that are in this category, such as the Ritchey- Chrétien, cost many thousands of dollars more and require that the optical tube, mount, and tripod be purchased separately. In contrast, Celestron is offering the C20 Astrograph as a complete package. The C20 system includes the optical assembly, a carbon-fiber truss system which minimizes refocusing due to temperature changes, high performance computerized German Equatorial mount, and professional class pier/tripod. The pier will be available in both portable and permanent versions. An Engineering Triumph The C20 is without a doubt the most sophisticated telescope that Celestron has ever designed and manufactured. The C20 is a system that meets the highest standards Celestron sets for technical innovation, design and engineering superiority, precision performance, and uncompromising optical quality. Telescope Features       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. Telescope Specifications       OTA Specifications Design: Coma-correct flat-field Dall-Kirkham Cassegrian (corrected Dall-Kirkham) Aperture: 20" (0.51 m) Focal Length: 3454 mm System Focal Ratio: 6.8 Central Obstruction: 39% Optical Performance: Flat field over 42mm film plane, 6 micron RMS spot size 21mm off axis Weight: 140 lb (est.) Back focus: 8.9” (from the Focuser mounting surface of the back plate) Primary Diameter: 20.25” Aperture: 20” F-ratio: 3 Material: Precision Annealed Pyrex Shape: Prolate Ellipsoid Coating: StarBright XLT Multilayer Reflective Coating Secondary Diameter: 7.5” Magnification ratio: 2.3x Material: Annealed Pyrex Coating: StarBright XLT Multilayer Reflective Coating Lens Group Coma correcting, field flattening, doublet lens group Lens Diameter: 90mm Coating: StarBright XLT High Transmission Coating Coatings Reflective: StarBright XLT reflective coatings peak at 95% reflectivity at 520nm. The coatings are made from precise layers of Aluminum (Al), SiO2 (quartz), TiO2 (Titanium Dioxide), and Si02. Reflectivity is fairly flat across the spectrum, optimizing it for both CCD imaging and visual use. AR Coatings: StarBright XLT anti-reflectant coatings reach a peak transmission of 99% between 500 and 590nm. The coatings are made from precise layers of MgF2 (Magnesium Fluoride), and HfO2 (Hafnium Dioxide). A rare element costing nearly $2000 per kilogram, Hafnium gives us a wider band pass than Titanium, used in competing coatings.

Mount Specifications       General Specifications Type: Computerized German Equatorial Instrument: Capacity 180 lb RA Gear Diameter: 11.25" Anodized Aluminum RA Gear tooth count: 360 RA Worm: 1.0” diameter stainless steel RA Shaft Diameter: 2.5” aluminum to match the thermal expansion of the RA bearing housing RA Axis Bearings: Contains 4 separate bearings, a 9.8” thrust bearing, 8.16” thrust bearing, 4.33” ball bearing, and 4.33” tapered roller bearing DEC Gear Diameter: 9" DEC Gear tooth count: 360 DEC Worm Diameter: .745” diameter stainless steel DEC Shaft Diameter: 2.5” aluminum to match the thermal expansion of the DEC bearing housing DEC Axis Bearings Contains: 4 separate bearings, a 6.7” thrust bearing, 6.1” thrust bearing, 4.33” ball bearing, and a 4.33” tapered roller bearing Latitude Range: 25 - 60 deg (optional latitude wedge for 0 - 90 deg) Tracking Accuracy: 5 arcseconds or less without Periodic Error Correction enabled Weight: 156 lb without counterweight shaft RA Axis + Latitude Base: 90 lb Dec Axis with dovetail: 66 lb RA Axis: 64 lb Counterweight shaft: 24 lb Counterweight, individual: 22.5 lb Electronics Hand Control: NexStar Computerized Hand Control with 40,000+ objects and flash upgradable Software: NexRemote Precision Telescope Guidance Software (for PC Control) Max Power Required: 12V DC, 5 amps, sidereal tracking = .33 amps, slew dual axis = 2 amps Observatory Clearance Specifications C20 clearance specification Permanent Pier Dimensions