The Celestron Rowe-Ackermann Schmidt Astrograph (RASA) updates and improves the Celestron Schmidt camera, a premium wide-field imaging instrument of the 1970s that had a loyal following, but a difficult instrument to use that only a really hard-core 35mm film astrophotographer could love.
Today, fast DSLR and CCD electronic imaging has taken the place of slow 35mm film. Home computers can manipulate and improve DSLR and CCD images in ways that 35mm imagers could only dream of. DSLRs and CCD cameras can have sensors as large as (and in some cases larger than) 35mm film.
To compensate for these new large sensors Celestron had to push the boundaries of the Schmidt camera design and make an entirely new type of instrument, the RASA. The Celestron RASA provides an advance in the performance of deep space imaging scopes that lets today’s amateur astrophotographers produce wide field results rivaling those of the best professional observatory photos of only a few short years ago.
The 11” 620mm focal length f/2.22 Celestron Rowe-Ackermann Schmidt Astrograph is an imaging-only optical tube. It has a huge 70mm image circle that can handle full frame DSLRs and the largest sensor size CCD cameras with minimal or no vignetting. The RASA combines a proven Schmidt corrector optical system with a built-in 4-element rare earth corrector lens that keeps the images free of coma, field curvature, and false color. The optical quality and spot size across the entire image circle are unprecedented for an astrograph in this price range – or even that of much more expensive instruments.
Its fast, wide field, f/2.22 optics give you two huge advantages over traditional f/10 catadioptric imaging scopes (even those using an optional f/6.3 or f/7 focal reducer). You get better apparent tracking due to the image scale, plus shorter exposure times due to the speed of the optics. That means you can create better-looking deep space images and mosaics in a fraction of the time it used to take, even without using an autoguider.
The Celestron 11" RASA has a Rayleigh Limit (photographic resolution) of 0.50 arc seconds. It is capable of revealing much finer deep space detail than a similar focal length 4" apo refractor (a scope type often used for wide field imaging), which has a Rayleigh Limit of 1.36 arc seconds. And the RASA will record those more detailed images in a fraction the time of that 4" apo.
The Celestron 11” RASA optical tube includes a custom-engineered linear brass focuser bearing to reduce image shift It is combined with a FeatherTouch Micro Focus Knob to allow you to make the precise fine adjustments you need to capture the perfect image. Mirror locks hold your precise focus. A quiet high-output 12V internal MagLev fan on the rear cell reduces cooldown time and provides optimal airflow through the dust filtered 33” long optical tube.
Naturally, industry leading StarBright XLT optical multicoatings are standard for the highest possible light throughput. 42mm T-thread DSLR and 48mm CCD camera adapters are standard equipment. Two Losmandy-style “D-plate” dovetails are standard, one on the bottom of the RASA for installing it on the CGX-L mount, and one on top for installing accessories (such as on optional photoguide scope). The back focus from the included camera adapters is 55mm.
Naturally, an astrograph as good as the Celestron 11" RASA demands an equally good mount. The Celestron CGX-L is that mount.
The German equatorial mount has long been the favored choice of astronomy buffs and astrophotographers because of its stability and portability. It is more stable because the center of gravity is directly over the center of its base, and more portable because it can be broken down into smaller component parts than a fork-mount telescope for easy storage and transportation.
For astrophotography, the German equatorial mount offers easier balancing; unlimited space at the rear of the telescope tube to mount a long camera equipment train that can’t bump into the drive base, as is the case with many fork-mounted scopes; and whole sky access that many fork-mounted scopes can’t achieve.
Celestron has looked to update the way that people interact with their equipment. This quest has brought out a new EQ Mount Series, the CGX. This particular model in the series is the CGX-L and boasts a 75 pound payload capacity.
Heavy duty mount head: The Celestron CGX-L is a newly-designed high payload capacity/high precision go-to German equatorial mount that epitomizes all of these German equatorial mount virtues. With its full 75 pound payload capacity it will carry Celestron SCT/HD optical tubes up to 14” in aperture, as well as virtually any other optical tube and accessory payload weighing up to 75 pounds.
New features are everywhere in the mount head:
1. Ergonomic handles help you move the 47 pound head with ease. That’s right; the head weighs 47 pounds and will carry 75. It is an incredible load to weight ratio.
2. Additional accessory ports built into the dovetail saddle plate of the equatorial head. There are two auxillary ports as well as an autoguider port. The internal cabling of the CGX-L mount works hand in hand with these new ports on the saddle helping the imager in all of us manage our cables.
3. The dovetail saddle plate is longer for added stability and will also accept both Vixen/AVX and Losmandy/Celestron D Style dovetails.
4. There are built in home and limit sensors. The home sensor will take the scope back to its home position at any time and the limit sensor will prevent the mount from slewing past a certain point.
5. There are also built-in mechanical hard stops to prevent an instrument from damage should you forget to lock the mount.
Mount drive system: The CGX-L mount head contains 144mm worm wheels in both Right Ascension and Declination. The improved drive system uses a belt-driven worm gear that is spring loaded. The significance of this is it allows the drive to maintain optimum gear mesh for smoother tracking and less backlash.
The mount requires 2.5 amp 12 VDC power to operate. The maximum 2.5 amp power draw happens only briefly when accelerating to the high speed slewing mode from a standing start. Normal power draw with a well-balanced payload is generally considerably less than half the maximum draw. The mount comes with a car battery cord to operate from the cigarette lighter plug of your car or from a rechargeable 12VDC battery pack. The 17 amp hour capacity Celestron Power Tank #4517V is recommended and will operate the mount all night long without danger of running out of power.
Specially designed power management electronics deliver constant regulated power to the motor so that it is capable of driving the mount even when your scope not perfectly balanced. This allows the CGX-L to have the payload capacity of that of much larger (and expensive) mounts without sacrificing smooth tracking motion and pointing accuracy across the entire sky.
Polar alignment: To make a casual no-tool polar alignment for visual use quick and easy, there’s a latitude scale with large ergonomically-friendly altitude and azimuth adjustment knobs. If serious long exposure astrophotography is in your plans, the special All-Star Polar Alignment software described below allows you to choose any bright alignment star for a software-assisted alignment of the mount’s polar axis, even if you can't see the North Star. The mount also has upgraded adjustments for Latitude and Azimuth. Large one hand knobs make moving a full loaded mount head a dream with a simple twist of the wrist.
Counterweight: The CGX-L mount comes with one 22 pound counterweight. The counterweight locks in place on the 5 pound steel counterweight shaft with a single hand-tighten knob, making it easy to rebalance your scope in right ascension if you add heavy photographic accessories. If needed, optional counterweights are available to balance very heavy loads.
Heavy duty tripod: The CGX-L mount’s adjustable height Super HD tripod has 2.75” diameter stainless steel legs. The height adjustment leg locks are on the inside of the tripod legs to keep them from snagging on clothing in the dark. The tripod uses a dual leg support for maximum rigidity, with an upper leg brace to provide an outward preload and a lower leg brace providing inward tension. The upper leg brace forms a convenient tray that holds eyepieces and accessories, as well as a slot to hold your smart phone or tablet, to keep them up out of the dew-soaked grass. This tray is a new design that will allow you to keep it attached while you fold the legs up.
Mounting an optical tube: Optical tubes are installed on the CGX-L mount using a slot on the mount head that accepts a Celestron CGE-style/Losmandy D-plate dovetail or a Celestron AVX/CG5/Vixen slide bar. This allows the optical tube to be quickly and precisely balanced fore and aft on the mount in declination, eliminating the need for an extra counterweight to balance a camera or other accessories. Setup and takedown times are exceptionally fast, with large hand-tighten knobs holding the optical tube in place.
Mount weight: The CGE Pro mount weighs in at 121 pounds. The equatorial head is 47 pounds. The tripod weighs 47 pounds. The supplied counterweight weighs 22 pounds, while the thread-in counterweight shaft weighs 5 pounds. The new ergonomic handles makes it much easier for a single person to move and break down the mount in the field with ease.
NexStar computer hand control: The NexStar computer hand control of the CGX-L mount has a built-in database of more than 40,000 stars, deep space objects, and solar system objects it can locate for you. These include the complete RNGC, Messier, Caldwell, IC, and Abell catalogs; selected SAO stars; the planets, the Moon, and others. The custom database lists of all the most famous deep-sky objects by name and catalog number; the most beautiful double, triple and quadruple stars; variable stars; non-planetary solar system objects; and asterisms. It contains enough fascinating objects to keep you busy observing for the rest of your life.
You can also store and edit the right ascension and declination of 100 objects of your own choosing, such as the comet and asteroid coordinates published monthly in Astronomy and Sky & Telescope magazines. The computer control can quickly find any of those objects at your command, and track them with high accuracy for visual observing or astrophotography.
All of the database and scope operation information is displayed on a double line, 16-character, liquid crystal display on the hand control. This display leads you through the steps necessary to line up the scope on the sky, locate objects, control scope functions like the brightness of the hand control display, and much more. It shows you basic information about the object being viewed (such as the object’s name, catalog designation, type, magnitude, and so forth). In addition to this basic information, there is enhanced information on over 200 of the most note-worthy objects. The display can also show you the right ascension and declination coordinates at which the scope is aimed.
Alignment on the sky and go-to accuracy: No polar alignment finderscope is available for the CGX-L mount, nor is any needed. A unique Celestron All-Star Polar Alignment program built into the NexStar hand control helps you do a very precise polar alignment in mere minutes for long exposure astrophotography. It works in both northern and southern hemispheres.
To accurately polar align your scope, first do a reasonably-accurate mechanical alignment of the mount on the celestial pole by eye. To make this initial alignment quick and easy, there’s a latitude scale and fine adjustment controls in both altitude and azimuth. Next, use the mount’s standard two-star alignment method to align your scope on the sky. Select a suitable bright star from the NexStar hand control’s data base and slew your telescope to the star. A star near the meridian and close to the celestial equator will give the most accurate alignment results. Then, press the Align button on the hand control and select Polar Align: Align Mount from the hand control menu. Your telescope will re-slew to the alignment star and ask you to center it in the eyepiece in order to “Sync” on the star. After you sync on the star, your telescope will automatically slew to the position that the star should be at if your mount was accurately polar aligned. Use the mount’s altitude and azimuth adjustment knobs to place the star in the center of the eyepiece and press the Align button. That’s all there is to it! Your CGE Pro mount is now accurately aligned on the celestial pole.
Although the telescope’s tracking may be very good after alignment, pointing accuracy may need to be improved (particularly if you are trying to located small objects on a CCD chip), depending on how close your initial mechanical alignment was on the pole, and by much the mount had to be moved during polar alignment. If that’s the case, you can easily update your telescope’s star alignment in a minute or two if necessary.
Once polar aligned, several different sky alignment methods are built into the NexStar computer to line up the scope on the celestial sphere, allowing you to choose a level of computer accuracy in automatically finding objects with which you are comfortable. These include 2-Star Align, Solar System Align, Last Alignment, Quick Align, and 1-Star Align,
With just a standard hand control alignment, the CGE Pro computer has the ability to center a star in your telescope’s eyepiece or on a CCD chip to within 5 arc minutes. Using the NexStar computer's advanced pointing features (such as Calibration Stars, Sync, and Precise Go-To) further improves the pointing accuracy to as low as 1 arc minute in the desired region of the sky. The high precision pointing subroutine (Precise Go-To) in the computer lets you point accurately at objects that you want to photograph that are too dim to be seen though the scope. Other software features include: database filter limits, a mount hibernate mode, and user-defined slew limits.
GPS compatible: In addition, the NexStar computer hand control is GPS-compatible (using an optional inexpensive CN16 GPS module) for full GPS (Global Positioning System) satellite accuracy. Once the scope is approximately polar aligned, the 16-channel CN16 GPS system uses signals from government satellites to calculate the scope’s location on earth with an accuracy measured in meters. The system also calculates the current time based on the split second accuracy of the GPS time signals. After the CN16 quickly completes these calculations and enters the information into the computer control for you automatically, the computer then orients the scope with the sky, slews to a pair of guide stars, asks you to confirm that the stars are in the center of the field (and center them if they’re not precisely aligned), and then starts finding and tracking over 40,000 objects for you at your command. With the CGX-L go-to mount and the CN16, orienting the scope in time and space on earth and aligning your telescope on the sky becomes almost as easy a task as simply turning on your CGX-L mount.
The complete RASA/CGX-L mount combination weighs 169 pounds (85.9 kg). The tripod/electronics pier is the heaviest component, at 47 pounds (23.6 kg). The equatorial head weighs 47 pounds (34.1 kg). The supplied counterweight weighs 22 pounds (10 kg) each, and the counterweight shaft 5 pounds (2.3 kg). The 11” RASA optical tube weighs 35 pounds (15.9 kg). The image shown above includes an optional CCD camera mounted on the RASA OTA.