Q. Can I do visual deep sky viewing with the Centurion
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?
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
Q. Is the 50" focal length long enough to do
good deep sky CCD images? Doesn't the Niquest theorem state otherwise?
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
NGC 604 in M33's spiral
arms. Cropped portion of ST-8 image
(also see NGC 604 in gallery with 237 imager)
also see M51 and Saturn together for
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.
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?
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
Q. What are the pros and cons of an open tube telescope?
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?
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).
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.
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?
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?
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?
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?
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
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?
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
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
Q. Can I use a mobile platform to image from and
not purchase a permanent observatory?
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
Q. How often must I clean the 18" primary mirror?
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
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
Example case, actual may vary.
Q. While we are on the subject of optics, just how
good are the Centurion optics?
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
Q. Is the Centurion 18 computer control difficult
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
Q. How about interfacing to another PC to control
telescope through The Sky, etc?
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
Q. Is the Centurion difficult to recollimate?
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
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.
ST-8 ABG image of
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
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?
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?
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 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?
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.
This base is preset for
Q. Can you offer a smaller Centurion for less money?
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".
Q. Is there any provision to route additional cabling
up to the focal plane such as CCD camera wires?
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?
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?
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
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
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?
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)
Home Page | History | Specifications
| Q & A | Gallery