The Centurion 28
F/3.1
wickedly fast prime focus imaging system
Units now in stockUpdated 5/08/2010 C-28 during test, original C18 in background 2240mm f/3.1 with focal length of 88 inches Primary Mirror: 28.3" nominal diameter x 4" thick. Light weight spun cast Pyrex cellular for rapid thermal response throughout the evening. Weight is just 90 pounds. The spin cast technology actually has
the molten hot mirror blank rotating at a speed sufficient to mold a uniform thickness front surface at the required curve with ~.5 inch sagitta depth. All molded rib sections are nearly equal in thickness to allow a fast temperature response and will deliver maximum opto-mechanical rigidity. With this novel process, over 100 pounds of undesired glass has been eliminated over a solid equivalent. Mirrors are ground and figured to the necessary hyperbola conic constant to 1/10 wave surface error. They are aluminized and 1/4 wave SiO overcoated for durability. There is a scribed laser alignment target ring on the optical axis at the very center of the mirror. Mirror support is via an 18 point Wangsnessonian flotation.
Beautiful spun cast structure of uncoated mirror along with its test image taken at f/3.1 Note: This is a non-visual telescope.
Field corrector: The multi-element lens group has a clear aperture of 85mm (3.3 inches). The lenses are air-spaced and anti-reflection coated. The lens group is designed with moderately curved surfaces to minimize multiple reflections from brighter stars and is magnesium fluoride AR coated on all surfaces
. They are sized to illuminate a 50 mm (1.2 degree) circle with negligible vignetting. A longer back focus distance of 5 inches allows for almost all types of CCD imagers to be used. Focusing is accomplished by simple longitudinal movement of the corrector lens cell per below description. Primary mirror and camera are fixed.
Optical design performance: By slowing down the speed from f/2.8 of the C-18 to f/3.1 and using newer corrector lens glasses, a further gain in polychromatic sharpness is realized over the full field of view. At almost twice the image scale of the C-18 (50" fl.), the C-28 @ an 88" focal length has even smaller spot sizes on-axis and more superior off-axis sharpness as judged by the theoretical focused spot sizes. As an example, for a 9 micron square pixel imager, the image resolution will be pixel limited full field for a large 4K X 4K array @ .8 arc seconds per pixel sampling. And this is at a very fast f/3.1 imaging speed! Shown is an additional spot size for 70mm image circle which is extreme corner of a 50mm square imaging chip. This is a 1.8 degree field. On-axis RMS spot is just 1.4micron.
The .0088mm width is an approximation of what a 9 micron wide pixel will see. Also note that 8.8 microns equals a star size diameter of 0.8 arc seconds at the C-28's 88 inch focal length at prime focus. Y=field radius is shown.
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 powder coat painted (baked on) all surfaces. Most fasteners are stainless steel. Removable light shrouds at primary are low reflection textured material. Simple collimation adjustments are very accessible. The CNC stamped trunion box cage has 12 each 1/4-20 threaded inserts on 6 sides for simple yet rigid payload mounting. These 72 holes are CNC stamped on an exact bolt pattern.
Construction image of C-28 trunion box assembly, and mirror support frame casting. Trunion box is 33 inches wide between flats.
The optical tube truss assembly is .10" wall carbon fiber tube for superior stiffness and thermal stability. Since these are hollow, there are thru hole provisions at the end mounts for each of the 8 longitudinal tubes. This is to allow routing of wires inside these up to the prime focus.
The prime focus central obscuration ratio is limited by the upper spider hub boss diameter which is 7.5" of the 28" clear aperture. This is just a 27% aperture or about 7% by area, however the selected CCD imager may increase this. A typical STL 11000M camera has very little change.
Primary baffle has 5 aperture stops to help minimize stray light.
The large end has a 7" nominal diameter that represents a 6% obscuration by area. Shown before painting.
Focusing is via small DC stepper motor with absolute position control via Technical Innovation Robo Focus controller and software. The miniature gear reduced Nippon Pulse motor consumes very little power and space. Focus control is ASCOM compliant.
Mirror fans are included standard. Their purpose is not so much for mirror cooling as to effectively break up the thermal boundary layer at the front surface of the large mirror. There are 2 fans with variable speed control. These will help control local seeing beginning at the mirror. They are not shown in above image.
Mirror handling is safely done with an included balance bracket. This is visible to lower left in above image and is removable after use. It allows for careful lifiting of the 200 pound mirror/cell assembly during assembly or recoating.
Fork Mount
Fork: These are CAD designed on Solid Works. 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 8X16 inch cross-section from 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. The fork alone is a 300 pound component and part of an effort to minimize weight and thermal mass in the vicinity of the optical path. The forks are sandblasted and heat treated to normalize during the welding process. The large fork arms have 36" space and are 54" wide at their elbows.These are 5 times as heavy as the C-18 fork.
Comparison of C-18 and an unfinished C-28 fork
Polar Base: has two massive tapered roller bearings, separated by 24'" to support the precision centerless ground and threaded hollow steel axle. The fork is coupled through a massive 16 inch diameter drive disk casting (not shown) 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.
The main structure is welded tubing sized for minimal weight with maximum rigidity, and minimum thermal mass. 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 any non-exposed critical machined surfaces.
Drive System: Both of the anodized cast aluminum drive disks are directly driven independently of any possible torque flexure from their supporting axles. The disks are phosphor bronzed faced for durability. The drive rollers are hardened and precision ground stainless steel output shafts originating directly from the custom made worm gear reducer /servo motor drive modules. These offer a powerful high speed, quiet slew that is energy efficient with very low backlash and increased drive accuracy. The C-28 drive disks are increased to 16 inch diameter as compared the 12 inch diameter for previous C-18s. This allows for precise response and maximum rigidity.
The drive modules have a quick release tension mechanism (circled below) 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 circled lever will disengage the declination roller for balancing. Stepper motor drive shown.
Assembly calibrations for minimal mechanical hub and optical cone errors
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.
Both of these relationships and many others can be later optimized with control system modeling of mount and telescope using Point XP. This will maximize both pointing and unguided tracking accuracy.
We can offer DC servo or stepper systems based on customer application. Price to be quoted. Telescope can be delivered complete as stepper type or as a customer add-on in the case of DC servo. We have mechanical interface hardware in stock for both types. The FS-2 stepper has 1 arc minute GOTO and the DC Servo has 5 arc sec GOTO. Accurate mount/scope modeling with T-Point or equivalent is needed to reach these specs. Both types of systems track very well, allowing for short unguided imaging. As with the C-18s, remote/robotic operation of this telescope is up to the customer and not Astro Works. We can recommend other resources for this endeavor.
OTHER SPECIFICATIONS AND FEATURES
Two 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 are included for simple visual and basic CCD coupling or laser collimation checking. A 12 piece set of screw-on counterpoise weights are supplied for precise balancing in the declination and RA axes. A 1X illuminated star pointer/finder is included for simple 1 star startup initialization. An anodized machined aluminum mounting plate for an auxiliary instrument is included. Both can be placed as desired on the trunion box. Manuals covering setup and operation are included. The control system manual is downloadable. . Support via phone and email is included. Total telescope weight is approximately 1,000 lbs. There will be an additional crating and packing charge which will be quoted based on customer logistics. On site set-up/installation is available.
Observatory ideas: Overall instrument dimensions are 84" high, 54" wide x 116" long OTA. This is for a nominal latitude range of 34 degrees and telescope parked level in fork at horizon. Following are 4 examples that were tried and proven on the C18s and apply to the C28.
Roll-off Roof: So if for example you have a roll-off roof observatory with an 85" high or higher inside, you are OK.
Dome: In this latitude range, the spring line height from floor to dec axle centerline is ~ 53" and end of tube is at a 91" radius minimum to dome wall (requiring 16 ft diameter dome).
Roll away enclosure: Also to be considered is a double wall insulated roll away observatory. Scope is permanent and observatory rolls off to north or south on simple angle iron rails. This is much cheaper and easier than a dome, but a perimeter wind fence would be a helpful feature.
Roll-out telescope: Then there is the last resort which we use to test the C28 telescopes, an 80 foot long track that the entire telescope is rolled onto at sundown for evening testing. Only try this at home. Example at C18 rolling base pictures. resort to page back arrow to return here.
A happy telescope builder/astrophotograper at 8800 foot elevation in the 80s. Note the 4X5 large format sensor film back.
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 28. We perform 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.).
Options: CCD camera adapters are the only option available. Price by quotation.
Note: Prices and specifications subject to change without notice.
Note: A limited run of 3 are built.
Contact: Jim Riffle, astrowk@tabletoptelephone.com
