Questions and Answers

The Questions:

• Can I do visual deep sky viewing with the Centurion 18?
• Is the Centurion a true robotic telescope?
• Is the 50" focal length long enough to do good deep sky CCD images? Doesn't the Niquest theorem state otherwise?
• If I wanted to use the Centurion @ f/5.6 during exceptional seeing or at a very good site can a short style 2X Barlow lens be used between the Centurion and CCD camera?
• Why isn't the Centurion 18 offered in a more compact alt/az configuration since it is computer controlled?
• What are the pros and cons of an open tube OTA?
• What about putting a CCD or film camera in the optical path? Does this degrade image quality?
• Can the Centurion be disassembled for transport to remote sites?
• How is the Centurion 18 shipped and how big is the crate?
• Some say you can use (33%) focal reducers to get from f/6.3 down to f/2.1 or f/10 to f/3.3 with some SCTs; why pay more for a dedicated 18" f/2.8?
• What about coma corrected Newtonians for CCD images?
• How about high end refractors for CCD imaging?
• Why are most of the Centurion's structural components such as the fork and polar frame made of steel?
• What type of observatory is best for the Centurion 18?
• Can I use the optional mobile platform to image from and not purchase a permanent observatory?
• How often must I clean the 18" primary mirror?
• Will I ever need to have my mirror recoated?
• While we are on the subject of optics, just how good are the Centurion optics?
• Is the Centurion 18 computer control difficult to use?
• How about interfacing to another PC to control telescope through The Sky, etc?
• Is the Centurion hard to recollimate?
• Which CCD imagers work best with the Centurion 18?
• Would the Centurion benefit by using the SBIG AO device?
• Have you considered selling the mount by itself?
• What is needed to do deep sky astrophotography with the Centurion?
• If my latitude is built into the polar base, how can I use it elsewhere if needed?
• Can you offer a smaller Centurion for less money?
• Is there any provision to route additional cabling up to the focal plane such as CCD camera wires?
• What is the prime focus weight-load limit?
• I am interested in doing asteroid searches and need to know what kind of limiting magnitude is possible with the Centurion and ST-8 camera?
• Is the ST-8 enhanced cooling package prewire and gear pump really necessary?
• How do you know all these things?

Q. Can I do visual deep sky viewing with the Centurion 18?

A. An eyepiece may be occasionally positioned in the prime focus adapter for initial finder or guide scope pointing at low declinations. Visual deep sky will become disappointing once the user sees real time on a monitor what the CCD imager sees. With the large 18" aperture @ f/2.8 and 3X binned 1 second exposures, the limiting magnitude and detail are beyond what a scope twice its size (1 meter class) can show visually. Also the CCD has the added benefit of increased spectral sensitivity, yet dramatically limits light pollution effects on deep sky objects. Plus, if you like what you see, click the mouse to save and archive the image with observing notes as done on this image.

M33 @ just 1 sec w/ST-8 non-abg camera

If you require observing in tandem, the telescope's steel trunion box is massive and has 48 each 1/4-20 threaded holes on 6 mounting surfaces to attach various photo/visual instruments as desired.

As an option, we offer machined aluminum mounting plates to attach such payloads. Please refer to price list for a description. These can be easily modified per the weights in the right image. This is sufficient weight to counter poise an 8" SCT.

Q. Is the Centurion a true robotic telescope?

A. No, it is a computer controlled telescope like many observatory telescopes. While it satisfies some requirements for a robotic telescope, there exist many other requirements that become quite involved once one fully considers all of the ramifications. A very thorough description of these requirements has been compiled on the website of Sonoma State University.

Q. Is the 50" focal length long enough to do good deep sky CCD images? Doesn't the Niquest theorem state otherwise?

A. With 6.8 to 9 micron pixel CCDs, the sampling is in the 0.9 to 1.5 arc sec/pixel range which already exceeds most sites' seeing. Look at the Centurion images, compare to others and then wonder about theory.

Q. If I wanted to use the Centurion @ f/5.6 during exceptional seeing or at a very good site can a short style 2X Barlow lens be used between the Centurion and CCD camera?

A. Yes, for certain high brightness objects but keep in mind as the focal length doubles, the exposure time is 4 times as long for a given image density on the CCD. Also simple Barlow lenses are not optimized for working in a very fast light cone such as the Centurion's f/2.8. In a pinch they can be used with varying degrees of success.

M57 taken with a Celestron short 2X Barlow and the PixCel 237(ST-237). The Centurion was @ approximately a 100" efl.

Compare this image to July 2000 S&T page 32 Hubble image.

Q. Why isn't the Centurion 18 offered in a more compact alt/az configuration since it is computer controlled?

A. The best area to image in is near the zenith and such systems have severe limitations tracking accurately through that area. Also, the complexity and support of the necessary image derotator that can handle an f/2.8 light cone, color filter wheels and such, preclude that option which is best suited to slower Cassegrain designs.

Q. What are the pros and cons of an open tube telescope?

A. The positives are light weight, minimal wind buffeting, good thermal tracking and aesthetic appeal. The negatives are a greater propensity toward mirror dewing and more stray light paths. The latter is handled well in the Centurion by a partial truss beginning not at the primary, but one-quarter the way up the optical path with a light shield area around the primary. Also, its prime focus is fitted with a tapering light shroud of optimal size.Obviously the aesthetics of a nicely proportioned Surrier truss are hard to match. It is a timeless classic telescope look repeated in all the major observatories.
Dewing can be countered by limiting the subtended angle of the cooler (night sky's) black body radiation incident on the mirror. An observatory is one good way of doing this "like the pros". Also, using a resistive element heat wrap tape of low wattage can work well (dew caps with dew heaters work similarly for SCTs). As an option we offer a sewn cloth light shroud described in the latest series web page.

Q. What about putting a CCD or film camera in the optical path? Does this degrade image quality?

A. The 4 meter Mayall telescope on Kitt Peak makes its wide field images in that manner with the Wynn camera configuration 158" dia. f/2.7! See 4 meter article in July 1973 Sky and Telescope page 10. More recently the CFH 3.6 meter on Mauna Kea is producing spectacular prime focus images with a huge 12K X 8K CCD array. See May 2000 S&T page 54-55 for image. Field of view is similar to the Centurion 18.

A CCD camera like the ST-237, at just 3 1/4" diameter is smaller in size than most 10"-16" scopes' secondary mirrors. It also benefits from engineered and tested spiral laminar flow cooling fins with an inverted conical air diffuser that is fan powered. Can't ask for a more optimal match. 3 1/4" is only ~3% of the 18" primary's area! Actually, the Centurion prime focus light shroud is larger at 4 1/2" (~6%). By comparison most SCTs must have an 11-25% area effective obscuration (including secondary mirror shroud).

The ST 7/8 CCD package is slightly offset from center, and performs very well with our internal liquid cooling /prewire option (advisable for warm climates). The optical path is in open air surrounding the CCD camera. The outline of the 18" primary mirror is added in yellow. Taken from mirror skyward. Note that all cabling including enhanced cooling is in place and wrapped to spider vanes for this image. Both CCD chips can just be seen.

The 4 spider support vanes are slotted to allow air to pass through. Not shown are 4 holes along the vanes' edge for black plastic wire clamps which can be used to secure wiring and/or plumbling lines to the secondary hub.

Q. Can the Centurion be disassembled for transport to remote sites?

A. No, since there must be critically adjusted relationships in a computer controlled telescope, such as hub and cone errors etc., which are preset during final assembly. Centurions are shipped fully assembled less primary mirror optics and plug-in electronics.

Q. How is the Centurion 18 shipped and how big is the crate?

A. The Centurion 18 is shipped in a crate that is 42"wide x 67" high X 74" long. The crated weight is approximately 800 pounds. The 350 pound telescope is fully assembled excepting the primary mirror and cell is removed and packed in a padded case for obvious reasons. The crate can be removed from a lift gate equipped truck trailer with a pallet jack (usually brought along by the delivery service).

Q. Some say you can use (33%) focal reducers to get from f/6.3 down to f/2.1 or f/10 to f/3.3 with some SCTs; why pay more for a dedicated 18" f/2.8?

A. Just look at the representative images. Sure, it is possible to put a V8 in a go-cart and a 2" focuser on a Newtonian having a 1" minor axis secondary mirror, but who's fooling who? The now popular f/6.3 reducers, first introduced by Celestron, takes that optical application to its maximum limits and also minimally corrects some off-axis aberrations. This is at the cost of less light than f/6.3 since the primary mirror in those systems must be refocused so far forward that field illumination suffers as well as actual T# (f/ratio). This occurs when the secondary mirror is overfilled by the primary's reflection and the full aperture spills over, effectively reducing light while increasing relative secondary obscuration ratios. A similar situation arises with f/3.3 accessory reducers which will not even begin to fill a KAF 1600 (ST-8) format. A definite compromise is made, especially when attempting tri-color CCD or film with these approaches. Such systems were never bottom/up designed for optimal film or CCD imaging, (film Schmidt cameras excepted). Those original SCT designs are from the 60s and 70s and were not designed for recent CCD accessory "add-ons" such as focal reducers. CCDs didn't go mainstream in the amateur astronomy community until the mid 90s and are still not acknowledged by some telescope manufacturers as being viable on their scopes!

If all this were not enough, the resulting vignetting from using some focal reducers on SCTs with any of the larger chip size imagers is horrendous and so bad that flat fielding is of little help.

Q. What about coma corrected Newtonians for deep sky CCD images?

A. Nothing wrong with the optics as long as one operates at f/6 or longer and doesn't mind throwing away pixel count through binning. Coma correctors for faster parabolas work to an extent, however on-axis sharpness is usually sacrificed for any off-axis field sharpening benefit from this type of system. This effect is not too objectionable on film, but really shows a lack of detail on CCDs. An 8" f/4 of very high quality was tested with and without two different brand coma correctors on both film and an ST-8 CCD for this observation by me.

Q. How about high end refractors for deep sky CCD imaging?

A. They are typically too slow and small in aperture. With the new enhanced blue CCDs becoming the norm, acceptable color aberrations may be exceeding the design limits of these high dollar refractors. Be prepared for multi-hour duration exposures with such systems.

Q. Why are most of the Centurion's structural components such as the fork and polar frame made of steel?

A. Strength to weight, fabrication cost and even thermal characteristics made steel the logical choice. Steel has one-half the thermal expansion of aluminum. Our fork is a single TIG welded assembly made tip to tip with very lightweight CNC stamped and formed sheet with internal stiffeners. More massive reinforced plates are at the base of the fork for rock solid attachment with six 3/4" stainless bolts. The fork is short, wide, and amazingly stiff. The 8-sided central truss box is the main structure of the OTA and also the dec drive. CNC stamped and fabricated steel is the material of choice there. Even the polar base side panels are paint-lock steel sheet with non-glare powder coat paint on all surfaces. Close thermal tracking is maximized by keeping all structural sections as thin as allowed.

Q. What type of observatory is best for the Centurion 18?

A. Since the Centurion 18 slews very quickly, a rotating dome's speed and complexity can be a limiting factor. A clamshell opening design is more ideal since it exposes the entire sky. Some open air drawbacks of this design such as wind loading/buffeting are minimized since the open frame truss tube poses minimal wind resistance and the massive mount just isn't affected by "observable winds". The clamshell doesn't need to rotate and the individual shell folds can be strategically oriented to block out any winds or light sources. A 12 ft. size is needed for the Centurion. A simple roll off roof type observatory is fine and also a 10 ft. Home Dome will work.We also offer an option for a roll-out Centurion when fitted with our new rolling base. This novel solution works very well and can be used in existing structures.

Optional rolling base set for N-S

Q. Can I use a mobile platform to image from and not purchase a permanent observatory?

A. If you have the garage space, the scope can be wheeled in and out by hand as well as driven to remote locations. All of the Centurion's images on this website (prior to Jan. 2000) were done by wheeling in and out of a garage each night with marks on concrete and a tape measure to polar align! On hot days the concrete driveway was hosed down at sunset to precool. If your driveway isn't very level (hillside home) don't try this technique. Gravel or brick could also work with extra effort.

This observing platform is very rigid and can be stood on if needed during observing sessions (between exposures) without affecting pointing/alignment. It also has a minimum footprint for easy access to telescope. The main platform is just 45" wide x 72" long to encompass the Centurion instrument width/length envelope. Width across the fenders is less than 6 feet and overall length is ~ 9' 6". Total height is ~ 6 feet. Plans are free to any Centurion customer. Not shown is the 3 point leg support and south leg jacking screw. This trailer is used to test every Centurion from.

Q. How often must I clean the 18" primary mirror?

A. This depends upon use and the individual. The standard mirror coating is hard overcoated with 1/4 wave of silicon monoxide. The telescope is designed for simple cleaning of the mirror in place without its removal. A pint of distilled water, cotton balls, liquid detergent, and a small plastic dishpan to catch the drainage are easily obtainable. The main mirror baffle lifts forward to expose the mirror for inspection and cleaning. Some may only wish to perform this yearly while others more often.

Q. Will I ever need to have my mirror recoated?

A. Yes, eventually depending upon how often it is cleaned. Recoating will be less inconvenient with our supplied mirror case. Most coaters need 3 weeks turnaround to strip and recoat an 18" mirror. We can recommend coaters when that time comes.

This case has high density anti-static cut foam inserts and is shipped in an outer cardboard container. The mirror's face is not in contact with the foam, but is suspended above it by the mirror cell casting. Nominal case size is 26x22x10 inches.

Example case, actual may vary.

Q. While we are on the subject of optics, just how good are the Centurion optics?

A. They are only as good as the total integrated design allows and dictates. Our goal of 80% encircled energy (multi wavelength) focused within a 9 micron flat field spot dictates the aspheric primary should be smoothly corrected within 1/4 wave to match the corrective lens group specs (which are made to domestic refractor lens quality). The bottom line is in terms of imaging performance. With the ST-237 imager, FWHM on the smaller stars is in the low 2 arc second range for well focused and guided images on average deep sky seeing nights. The ST-8 imager is pixel limited to the corners.

Q. Is the Centurion 18 computer control difficult to use?

A. As simple as turning on the unit, using the hand box to center a known star in the finder, scrolling down a star list and locking on. No other input is required. It is basically plug and play without software to load. Close polar alignment, although simple, is probably more difficult and also reading the manual helps. There are some cords with connectors to plug in. The standard slew speed of 900X allows for about a 30 second maximum acquisition time to center objects and for most, less than 10 seconds is needed.

Q. How about interfacing to another PC to control telescope through The Sky, etc?

A. Simple. Plug in supplied serial cord between Centurion CPU and your PC, launch The Sky, click on green telescope icon to lock-on and you will be shown exactly where your scope is pointing (providing you previously set up The Sky for your observing site and selected a reference star). Once synched, you can control the telescope from a PC with slew-to and keyboard hand pad buttons. Horizon limits can be preset in The Sky to prevent accidental slew-to commands from being carried out for objects too low.The Centurion hand control also works in parallel for positioning/ centering and manual (button) slewing, with The Sky's screen updating current position. The hand control cord length is limited to ~50 feet from the scope controller with optional extensions.

A laptop PC can be used to obtain images with the SBIG brand CCD imagers and run The Sky software. All SBIG cameras have a feature of not requiring the user to operate from a full size PC (necessary to accept a plug-in card to operate the other cameras with faster communication lines). Currently the Astro Works Centurion 18 is remotely controlled from inside a room connected by 50 foot long parallel and serial cables which allow communication to CCD cameras and The Sky for pointing. An optional extension cable is fitted to the remote focuser.

Q. Is the Centurion difficult to recollimate?

A. No. It is a snap with an optional laser collimator. Plug a laser into the prime focus adapter, verify the beam hits the center ring target on primary, adjust spider if needed. Next, look at the primary's retro-reflection back to the laser and adjust primary if needed to recenter retro-reflection. That's it. Additionally, very close collimation can be done viewing CCD imaged stars real time in focus mode and adjusting the primary collimation very slightly. Focal plane squareness is automatic with a laser technique, since a secondary mirror is not used in the Centurion design.

Q. Which CCD imagers work best with the Centurion 18?

A. As of this writing, the SBIG brand ST-7, -8, -9, -10, ST-237 (PixCel 237) , ST-2000 & Apogee AP-8 ,9,&16 have been tested and all work well. We have standardized adapters available for them as well as a custom folded connector prewire/plumbing package for the ST-7-10 series which have enhanced cooling. This is offered to minimize those camera's cordage footprint.

The ABG (anti-blooming gate) versions of the ST-7/8 are recommended to help minimize blooming since @ f/2.8 stars get very bright fast. The slight decrease in sensitivity over the non-ABG is more than offset by the fast f/2.8 speed. All the above mentioned SBIG imagers work well with color filter wheels, however the ST-8 filters vignette slightly which flat fielding takes care of. The open position on that filter wheel doesn't show that effect. The ST-7 and ST-237 images show no full frame illumination variations requiring flats, unless filter dust is an issue. Only a clear filter flat was used for the ST-8 LRGB images found on this site. Of course, any other camera with a tolerable footprint and user friendly "cordage" can work and will probably be tried as long as the OPD back focus is within the available 55-60mm range. We can test a prospective customer's camera for compatibility purposes or fabricate adapters.

PixCel 237 image (7.4u pixel) 1/100 sec.
The Centurion is capable of high resolution @ 1.2 arc seconds per pixel.

Click for a comparison of the same object captured with an ST-8 and also an ST-237. Please refer to the various CCD camera images in the Gallery section to draw your own conclusion. Since we are a dealer for SBIG, we can offer package pricing on a Centurion and CCD cameras as a service to our telescope customers.

Q. Would the Centurion benefit by using the SBIG AO device?

A. No. Little advantage would be gained from the added complexity. With the Centurion's 50 inch focal length,the AO would not close the control/imaging loop any tighter and improve "seeing". Actually, SBIG's CCDSHARP or Maxim's Digital Development image processing software would be much more effective for enhancing image detail and sharpness. Some of this website's images have benefited from this software.

Q. Have you considered selling the mount by itself?

A. Popular question with "no" as an answer. For years we sold just mounts and later combined them with custom instruments. In almost every case, each mount becomes custom and costs increase. This particular combination of telescope and mount is highly tuned as a total system and will have to stay that way. We will, however, help and support our Centurion 18 customers with their additional telescope mechanical interface needs.

Q. What is needed to do deep sky astrophotography with the Centurion?

A. A 35mm SLR, custom T-ring adapter (see price list), a guide scope (see price list), and a 12" or longer locking cable release. Field of view is ~1.6 x 1 degree. Kodak's PPF color film is a good film to try with 1 hour or regular processing. At f/2.8, exposure times of 10-15 minutes should be striking. You can then use a 35mm film scanner as a digital darkroom to get your images into cyberspace or to a printer. There is a slight trick to accurate focusing @ f/2.8 I can share with those customers. If desired, masking down to a 16" f/3 helps eliminate corner vignetting caused by 35 mm SLR camera openings.

Full frame 35 mm exposure of 8 minutes on standard ASA 400 PPF film. Negative was scanned at 1200 dpi (6.4 megabyte) and reduced to just 64 kilobyte above. Original film image is much better.

Q. If my latitude is built into the polar base, how can I use it elsewhere if needed?

A. The side leg feet are both adjustable and removable.The mount comes with standard shorter adjustable feet as shown. These feet are massively constructed in 3/8" steel plate. Note: Feet are preset to your latitude and then fine adjusted with the vertical leveling bolts during polar alignment. Through holes 7/8" diameter are for hold down bolts.

Q. Can you offer a smaller Centurion for less money?

A. Not really since there isn't always economy in scale. Just the main mirror would cost less (in quantities) and the remaining parts costs would be similar. The only other dedicated prime focus hyperbolic systems we know of that are smaller are the Takahashi Epsilon series. The 10" f/3.4 OTA is $16K with a finder (no mount) and a 14" f/3.6 (with nearly identical focal length/image scale to the Centurion) is priced at $30K with finder (less mount). In all fairness, they may cover a larger film format than 35 mm for those willing to pay a little more for it. Also, you can "look through them".

A. Yes. There are .28 inch diameter holes predrilled in the end of every truss tube mounting block. One hole set is used for the focuser control and digital display. This leaves 7 truss tubes for later routing of up to 1/4" nominal diameter wires or tubing. Keep in mind that end connectors may have to be reconnected at one end. Also, split black plastic conduit can be used effectively to neatly wrap flat ribbon wire around the carbon fiber truss tubes as required.

Q. What is the prime focus weight-load limit?

A. An ST-8 with color filter wheel and associated wiring weighs about 4 pounds and up to 4 ea. 20 ounce weights are attached to the Centurion 18 rear cell for proper balance with that imager setup. A camera of 6 pounds should be possible with addition of more or different weights.

Q. I am interested in doing asteroid searches and need to know what kind of limiting magnitude is possible with the Centurion and ST-8 camera?

A. Two visiting astronomers from Tucson stated the following in a letter following a 4 hour observing run at Astro Works in early April 2000 " We reached a limiting magnitude of approximately 20 in the 90 second exposures we took. We have detected at least 28 asteroids and comets in our images and have been able to link 18 of those with known objects. This amounts to detecting almost 50% more objects that we thought we would be imaging."

"Having seen how the scope works, there is no doubt that it is perfectly well suited to our survey project." G.E. (They brought their own computer and older ST-8 non-abg lacking the newer Enhanced chip for this Centurion checkout). Gil has established the following link that displays his asteroid search images from that evening at: http://www.psi.edu/~esquerdo.html. Some of these interesting images are loops that show actual movement of multiple asteroids.

Q. Is the ST-8 enhanced cooling prewire and gear pump really necessary?

A. This option is recommended for all SBIG ST-series since it incorporates the necessary extension cables with connectors for the parallel port communications, the 6 pin power cable, the 6 pin modular auto guider cable and short serial CFW signal cable as well as the 2 stage power supply cord and two aquarium tube sizes cooling lines.

The following test was done on a warm 90 F evening. With just the single stage cooling on, the camera reached a minimum temperature of -3 C and had images with hot pixels after dark subtracting. With the second stage power supply plugged in, the camera cooled to -8C. When the liquid cooling was connected the camera reached -15C, but before the pump was turned on, the camera's fan was blowing noticeably warm air and after the pump was running, the fan air felt much cooler. Our gear pump is designed to move sufficient water through relatively small/lightweight tubing without any imbalance problem. The reservoir is compact, has a lid, quick connect fittings, and a convenient power plug at the gear pump. 3 of the 4 cables involved with the ST-8 prewire are routed within the carbon graphite truss tubes with connectors reattached. A flat ribbon parallel communication cable is wrapped around a truss tube within slit plastic conduit and doesn't even show. This is really a clean set-up.

Also included in the package is installing smaller hose barbs to the customer's camera and boring out your SBIG T to T thread adapter (for less vignetting with a CFW) and adding internal glare reduction threads to that part. This part is then used to couple the CFW to our optional CCD camera coupler.

Q. How do you know all these things?

A. Astro Works has been in business for over 25 years serving serious amateur astronomers and institutional customers. To learn more about the history of Astro Works and the owner's role in the company, you can go to the following link.

NGC 891 Andromeda with ST- 237 (lrgb exposure)