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 imaging-only optical tube has a huge 70mm image circle that can handle full frame DSLRs and the largest sensor size CCD cameras with minimal vignetting. It 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 a much more expensive instrument.
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.
With a Rayleigh Limit (photographic resolution) of 0.50 arc seconds, the Celestron 11" RASA 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.
Of course 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 CGEM DX 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 RASA demands an equally good mount. The Celestron CGX is that mount.
The Celestron CGX is culmination of years of engineering by the USA R & D division of Celestron. The CGX isn’t just a pretty face, the mechanics and ergonomics are upgraded as well. Celestron has taken their equatorial mounts to a higher level than ever before.
The new CGX looks like a cross between the CGEM II and the CGE Pro mount from Celestron. The ergonomically-designed CGX has a fresh, attractive, bold appearance and easier set-up of than earlier EQ mounts. It has been uprated with mechanical and electronic changes to a 55 pound payload capacity. The new design allows for a lower profile EQ head with a better balance point. Celestron has also added a integrated handles to assist with setup and transportation.
The CGX mount head contains dual-axis slewing/tracking motors on each axis for go-to computer control. This is where the real upgrades start. The mount uses spring-loaded worm gears with belt drives. The belt drives offer smooth, quiet operation and precision slewing and tracking. The motor armatures are skewed to minimize cogging for precise jitter-free low speed tracking. The motor drives and their wiring are all internal, keeping them snag-free and free from dust and debris.
Redesigned electronics deliver constant regulated power to the motors, making them capable of driving the telescope even when the mount is not perfectly balanced. This allows the CGX to have the payload capacity of much larger mounts without sacrificing smooth tracking motion and pointing accuracy across the entire sky. The mount of course supports the SkyPortal WiFi (for your smart devices) as well as the StarSense AutoAlign putting cutting edge control and setup at your disposal.
The mount requires a minimum 3.5 amp 12 VDC power supply to operate. The maximum 3.5 amp power draw happens only briefly when accelerating to the high speed slewing mode from a standing start, or decelerating from the high speed mode when approaching a computer-chosen object. Normal power draw with a well-balanced payload is generally 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 12V 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.
To make a casual polar alignment for visual use quick and easy, there’s a latitude scale with large ergonomically-friendly altitude and azimuth adjustment knobs. No polar alignment finderscope is available to aid in the precise polar alignment required for long exposure astrophotography, nor is any needed. A unique Celestron All-Star Polar Alignment program built into the NexStar hand control helps you do a very precise computerized polar alignment in mere minutes, using any bright star, even when Polaris is hidden by trees or buildings. It works in both northern and southern hemispheres. This technology basically gets rid of the need for a polar alignment scope.
The supplied 17 pound counterweight is locked in place with a single hand-tighten knob, making it easy to rebalance a scope if you add heavy photographic accessories. If needed, optional counterweights are available to balance very heavy loads.
For observing or imaging objects near the meridian (the imaginary line passing directly overhead from North to South), the CGX is designed to track 20° past the meridian for uninterrupted imaging through the most ideal part of the sky.
There are nine motor drive speeds: 0.5x and 1x sidereal for guiding; 4x, 8x, 16x, and 64x for centering; and 1°, 2°, and a fast 5° per second for slewing. Preset tracking rates include sidereal, solar, and lunar. Equatorial tracking modes are available for both northern and southern hemispheres.
New for the CGX mount is a telescope control software that has been developed in conjunction with Planewave Instruments. Planewave Instruments produces some of the most sophisticated imaging systems on the planet. The features of the included software are extremely robust. They include multi-point goto modeling, focusing, and plate solving. You will no longer need a third party advanced PC control software as this has it all.
The supplied NexStar computer hand control has a built-in database of more than 40,000 stars and objects. 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; selected variable stars; non-planetary solar system objects; and asterisms.
You can also store and edit the right ascension and declination of 100 additional user-defined objects of your own choosing, such as the comet and asteroid coordinates published monthly in Astronomy and Sky & Telescope magazines. The computer control can find any of those objects with only a few keystrokes, and track them with high accuracy for visual or photographic applications. There is enhanced information on over 200 of the most note-worthy objects.
The NexStar computer control of the smooth and precise CGX go-to mount makes finding over 40,000 stars and objects easy and automatic, so you can spend more time looking at them, rather than looking for them.
All of the database and mount operation information is displayed on a double line, 16-character, liquid crystal display on the hand control. There are 19 fiber optic backlit LED buttons to make it easy for you to control the NexStar computer without affecting your dark-adapted vision. A USB 2.0 communication port on the hand control allows you to operate the telescope via a personal computer using a USB cable and NexRemote software CD-ROM.
The NexStar hand control provides a constant digital r. a. and dec readout of the scope’s position on the sky. This provides much greater accuracy than conventional manual setting circles for star-hopping or locating objects by their celestial coordinates alone, should you want to observe the old-fashioned way without using the computer to find objects for you.
The software built into the NexStar hand control includes mount calibration, database filter limits, hibernate, five alignment procedures, user-defined slew limits, a new "All-Star" polar alignment routine that uses any bright star for a quick and accurate polar alignment, and more. Built-in adjustable backlash compensation permits precise corrections during astrophotography and when observing visually at high powers. A high precision pointing subroutine in the computer lets you point accurately at objects that you want to photograph that are too dim to be seen though the scope.
The software includes permanent programmable periodic error correction (PEC) to correct for the minor periodic tracking errors inherent to all worm drives that might otherwise mar your long exposure photographs. The CGX mount retains the PEC recordings when it is powered down. The hand control and motor control software is flash-upgradeable to allow you to download the latest product updates over the Internet to keep your CGX mount always at the cutting edge of technology.
An autoguider port is located on the electronic pier for long exposure astrophotography. The autoguider port can use a six-pin RJ-12 modular jack ST-4 compatible CCD autoguider to automatically control the drive motors during long exposure astrophotography.
Five different alignment methods are built into the Celestron mount’s NexStar computer – 2-star align, quick align, 1-star align, last alignment, and solar system align. This allows you to choose a level of computer accuracy in automatically finding objects with which you are comfortable. Alignment is easily accomplished in only a few short minutes. You can be observing in less than 15 minutes after you first take your CGX-mounted scope outdoors.
In addition, the NexStar computer hand control is GPS-compatible (using an optional inexpensive SkySync GPS module) for full GPS (Global Positioning System) accuracy. Adding GPS to the computer is something no competitively-priced mount can do. Once the scope is approximately polar aligned, the 16-channel SkySync 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 SkySync 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 go-to mount and the SkySync, orienting your scope in time and space on earth and aligning it on the sky becomes almost as easy a task as simply turning the mount on.
The new, 2”diameter adjustable height tripod has graduated markings on the legs to help with quick leveling. The center leg brace is drilled to form a convenient accessory tray that holds 1.25” and 2” eyepieces to keep them up out of the dew-soaked grass.
Optical tubes are installed on the CGX mount using a Dual-Slot on the mount head that accepts a Celestron CGE-style or Losmandy D-plate dovetail slide bar as well as the narrower Vixen Style dovetail. This allows the optical tube to be quickly and precisely balanced fore and aft on the mount, eliminating the need for an extra counterweight to balance a camera or other accessories. Setup and takedown times are exceptionally fast, as a single large hand-tighten knob holds the optical tube in place. A second lock knob prevents the tube from sliding off the mount should the hand-tighten knob loosen.
The complete CGX mount weighs 81 pounds (36.7 kg). The tripod is weighs in at 20 pounds (9.1 kg). The equatorial head weighs 44 pounds (19.9 kg). The single supplied counterweight weighs 17 pounds (7.7 kg). An optional 17 pound counterweight is available to balance heavy payloads.