Centurion 18 Specifications

Last updated January, 2005

1999 prototype image


Primary Mirror 18.2" nominal diameter. Molded /generated light weight tapering Pyrex for rapid cool-down. Weight ~ 40 pounds. Hyperbolic asphere yielding a 50" focal length in conjunction with field corrector lens group. Null figured to 1/4 wave or better. Aluminized and 1/4 wave silicon monoxide overcoated. A central target for laser alignment is permanently etched on surface. The mirror /rear cell is attached to the trunion box by 4 nuts and can be removed without affecting collimation and then placed in supplied hinging anti-static padded carrying case.

Field corrector lens group is designed with moderately curved surfaces to minimize multiple reflections from brighter stars and is magnesium fluoride AR coated on all surfaces. It is sized to illuminate a 35mm format to corners with negligible vignetting. The optical design is balanced to optimize the central 20mm for CCD performance and still deliver ideal 35mm film imagery. Back focus distance from lens cell to focal plane is a maximum of 60mm OPD. Optional enlarged rotatable T-ring adapters can interface 35mm SLRs.

Optical Tube Assembly is a combination of formed steel (trunion box and spider vanes), aluminum castings and milled bar, and carbon graphite truss tubes. Metal parts are either black anodized or non-glare texture powder coat painted (baked on) all surfaces. Most fasteners are stainless steel. Removable light shrouds at primary and prime focus are painted with low reflection textured powder coat. Full collimation adjustments are accessible.

Main trunion box assembly, less primary mirror and its 3 point collimation fasteners.

Focusing is via tiny DC gear motor with absolute position numeric LCD display on a remote hand box that is battery operated with auto-off feature. A standard cable length of 10 feet is supplied and extensions to add another 50 feet are available. Focus is achieved by moving the corrective lens group inside the spider hub casting via a backlash free drive screw. Focus is positive and repeatable. The CCD camera is rigidly affixed to the spider hub casting.


Central obscuration The prime focus baffle has a nominal O.D. which is 4.5" or ~ 6% by area in relation to the primary mirror diameter. The truss tubes are outside the entrance pupil and do not interfere. Spider vanes are stamped thin steel. With the ST-237 imager, the focus baffle diameter is still the limiting obscuration size. With the ST-8/10/2k imagers there is a nominal increase to ~5" diameter (7.7%) obscuration with a slight offset (see Q&A image).


35 mm SLR attached at prime focus




Fork: CNC stamped, formed and TIG welded steel. Two pairs of opposing tapered roller bearings are preloaded for thrust and radial control of central trunion box. Declination axles are precision centerless ground and threaded stainless steel. Fork has a massive 6" x 12" cross-section at its base, tapering continuously up toward the dec shaft bearing housings. Internal ribs and shear plates as well as true one piece construction make this assembly extremely stiff, yet light. The fork and trunion box have through holes to allow for use of the included polar alignment scope even while the entire telescope is operational. The fork and polar base are designed to follow the now popular Stealthlike angular aesthetics.


Polar Base:
Two massive tapered roller bearings, separated by ~18", support the precision centerless ground and threaded hollow steel axle. Rear threads allow attachment of the illuminated reticle pattern polar alignment scope. The fork is coupled through a massive drive disk casting (anodized) which has minimal overhang and a near zero torque moment distance. Six each 3/4" stainless steel bolts couple the fork to the drive flange.



Polar housing prior to welding

The main structure is welded tubing sized for minimal weight with maximum rigidity. The base has adjustable side mount feet bolted to the user's latitude range and has leveling bolts and through holes for hold down. The entire telescope is designed to balance on a flat surface before being bolted down. Steel side panels enclose the base to offer a pyramidal form. All structural parts are 100% powder coat painted (baked on) excepting non-exposed critical machined surfaces.

The polar axle roller release operates by turning the black lever.


Drive System:

Both of the anodized cast aluminum drive disks are directly driven independent of any possible torque flexure from their supporting axles. This allows for precise response and maximum rigidity. The declination drive roller is a hardened and precision ground stainless steel output shaft. originating directly from the custom made planetary gear reducer / stepper motor drive modules. This drive module contains 6 roller and 5 ball bearing units which result in a powerful high speed, quiet slew that is energy efficient with minimal backlash. The RA drive is a new worm reducer type roller drive(2004 and later). These are hand lapped for near zero backlash performance.

Lightened Dec drive disk casting



The dec drive module has a quick release tension mechanism to allow for precise balancing of payloads and also safe
transporting. A preset slippage tension helps to minimize any accidental slew-to damage. Heavy gauge spun aluminum covers protect the drive disks.

A simple one half turn of the arrowed lever will disengage the declination roller for exact balancing.


Assembly calibrations:

Squareness of the declination axis to the polar axis is set to + 15 arc seconds or better.

Squareness of optical axis to the declination axis is set to 40 arc seconds or better. Note that this is also subject to primary mirror collimation adjustment in the field.

Typical opto-mechanical system flexure over a 120 degree declination motion is ~ 60 arc seconds per T- Point modeling corrections.



Mechanical: A closed loop design utilizing encoder feedback was found to be the most reliable and also allows for manual positioning. It would be simpler to just count motor steps (which our system also allows), however the gear coupled high resolution encoders deliver a 1 arc minute pointing position with little error and can easily follow counts at the maximum slew rate. With such a design, there is no need to re-sync if a clutch or drive slips, a stepper burps, or if you simply desire to grab and point the telescope toward an object. The stepper motors are specially designed to run in a very efficient/accurate micro-step mode. Sidereal tracking step sizes are less than 1/10 arc sec. Both axes encoder drive modules are enclosed in stamped aluminum boxes for protection.

Physical: The user friendly hand control is compact at 2 1/2 x 4 inches and has just 5 buttons and a 2 line variable brightness red LED display. The default display parameter shows the current position coordinates displayed as RA: hour - minute - .1 minute on top line, and DEC: degrees - minutes on next line. A 5 or 10 meter cord with connector at one end attaches to the micro-processor controller. The other cable connector positions at the controller chassis are for motors and encoders, autoguider input (6 pin modular and 15 pin SUB D), serial communication to optional host computer, and 12 VDC input (banana jack). A size matched 13.8 VDC regulated power supply with 110 VAC input is physically joined and electrically attached to the controller chassis by a plug set. The controller draws about 3 amps maximum @ 12 volts and is contained in a heat fin cooled chassis of ~4x6x2 inches.

There are 5 available speed slots that can accept any chosen speed between .05X to 1600X sidereal. Settings of .5X-2X-20X-100X-900X (~4 deg/sec. slew-to) are preset and easy to change as desired. .5X in the first slot is the maximum speed in autoguiding/selfguide modes, while the last slot establishes the fast slew-to speed. No software or computers are required for stand alone operation of the Centurion. All firmware is in EPROM which is easily upgradable as new firmware features appear.

Firmware features: Lunar, solar, sidereal, and non-tracking rates - 2 axis programmable drift correction for comet tracking- spiral search- push-button logic flips- timer- red light- English/German language- de/acceleration ramps- 2 axis variable backlash- fine tracking speed adjust- internal library look-up/scroll-to/GOTO- one star initialization- fool proof guest mode lockout- 24 hour-on energy saving mode that preserves nightly syncronization- direct RA-DEC input, southern hemisphere operation, etc. The stand alone memory has about 4000 objects such as Messier, NGC, IC, stars, solar system objects.

Optional: Once interfaced to The Sky etc. through the user"s PC, the object library number can dramatically increase by 5000 fold. That interface is seamless and as simple as plugging in a serial cord (included). It is also useful for operating the Centurion at distances greater than the hand control cord allows. The Sky presents a scope pad interface which serves as a second hand control box. The Centurion hand control still works in parallel when The Sky is interfaced and synched, e.g. the scope can be moved by either control and also by hand without losing sync. The computer interface emulates most of the standard LX protocol. When using The Sky on a remote computer from distances further than 4 meters, it may be necessary to also extend cables for CCD control and the electric focuser. 50 feet is the current limit for hard wire operation. Beyond that a network connection with two PCs is necessary.

An optional 24 volt power supply and control module will allow higher torque for up to 1400X slewing speeds and is listed on the price list.




The main mirror cover is spun aluminum, powder coat painted and can protect the primary mirror when not in use. Rear light shrouds allow for air flow around the primary (removable for in-place mirror cleaning). The DEC and RA drive modules are enclosed. A precision 2" I.D. focus coupler with mating 1 1/4" reducer allow for simple visual and basic CCD coupling. Exact focus spacing information is supplied for optimal performance. One set of screw-on counterpoise weights are supplied for precise balancing in the declination and RA axes. Manuals covering setup, operation and the control system are included. Support via phone and fax is included. The fully assembled Centurion is palletized for motor freight shipment, with primary mirror in its own padded container and computer control system packed separately. Total telescope weight is approximately 350 lbs. (less crate). The complete telescope fits in an envelope of ~56" high by 46" wide by 72" long @ a 36 degree latitude. Northern latitudes will add to the height dimension.

Guarantee: Every telescope is set up and 100% functionally operated and CCD performance is verified with actual images. Each telescope will be capable of delivering images equal to or better than any displayed on our website. User sky quality and technique will vary, so some results may be better while some worse. As new CCD cameras become available, some may work very well with the Centurion and others may not. We rely upon the reputations of our optical fabricators to maintain high quality standards. Our opticians stand behind their respective work as do we with regards to fabrication and integration of the entire system that makes up the Centurion 18. Our tires meet the pavement with 100% performance testing of every unit for optimal results. This is our guarantee. The drive electronics are warranted for one year from date of purchase excepting acts of nature (rain, electrical storm, snow) and abuse (using a battery charger to power scope, etc.).