Celestron 8" RASA on CGEM II Equatorial Mount
- Flat image field free of optical aberrations like field curvature, coma, astigmatism, and chromatic aberration across an entire APS-C sensor
- NEW Ultra-Stable Focus System – six precision sealed ball bearings virtually eliminate image shift
- Integrated air cooling system –12V DC MagLev fan reduces cooldown time and provides optimal air flow while filtering out dust
- Internal filter mount – seamlessly accommodates a Light Pollution Imaging Filter into the optical path
- CGE dovetail mounting rail
- Performs over a wider spectral range than most telescopes, from 390-800 nm, so more of the light passing through the astrograph is in sharp focus
- Wider spectral range means brighter image results with your digital camera
- The CGX, with its 55 lb load capacity, is Celestron’s most innovative, state-of-the-art, computerized equatorial mount
- This CGX mount provides a very sturdy and rigid platform for the fully-loaded RASA 8
- The drive system utilizes low-cog DC Servo motors with integrated optical encoders, belt and pulley motor connection, spring-loaded steel worm gears supported by precision sealed bearings, and brass worm wheels for smooth, accurate slewing and tracking performance
- Fully remote imaging capable; it has optical limit switches, home sensors, through-mount cabling, and hard stops
- Included Celestron PWI software allows direct computer control via USB 2.0 connection
- Proven NexStar+ computerized telescope control technology supports functions to address imagers’ needs, such as All-Star Polar Alignment, Permanent Periodic Error Correction (PPEC), and adjustable tracking through the meridian
The 8" Rowe-Ackermann Schmidt Astrograph (RASA) from Celestron, is a cost-effective optical system for wide field astroimaging. It offers unprecedented value in aperture, speed, field of view, and optical performance. The RASA design has a convenient external prime-focus image capture location with a flat focal plane, providing small spot sizes to the edge of a wide field. The result is images that are free of optical defects like field curvature, off-axis coma, or astigmatism. RASA 8" has an ample back-focus distance of 25 mm which accommodates a wide variety of imaging sensors.
Some Information About the RASA Technology:
Celestron's 8" Rowe-Ackermann Schmidt Astrograph (RASA’s) optical design is patented (US 2016/0299331 A1). The design consists of a Schmidt corrector, spherical primary mirror, lens group, and optical window. The lens group contains 4 elements and utilizes rare-earth elements. Unlike many telescopes that only perform well over the visible spectrum (400 nm-700 nm), the 8” RASA’s optics are designed to perform over a wider spectral range, from 390 nm-800 nm. This allows more of the light emitted from the astronomical object to be sharply focused in the image. Also better spectral matching to silicon array sensors.
With many imaging systems, adding an extra piece of flat glass such as a filter may not affect the optical performance. However, that is not the case with super-fast optical systems like the RASA. Celestron's engineers addressed this concern by designing the RASA with a removeable optical window. This means you can maintain peak optical performance if a filter is added or if a camera has its own optical window. Simply remove the flat window and replace it with a Celestron filter. Celestron offers a Light Pollution Imaging Filter designed specifically for the 8” RASA, which replaces that optical window.
All refractive optical surfaces are coated with StarBright XLT coatings, while the primary mirror uses enhanced aluminum coatings. This maintains high light transmission through the entire optical system. When compared with a “Hyperstar SCT” system, the RASA 8 provides better optical performance and field illumination.
8" Astrograph Scope Description:
Capture spectacular wide-field deep sky images in seconds with Celestron’s new portable astrograph, the 8” Rowe-Ackermann Schmidt Astrograph (RASA). This incredibly fast f/2.0 system is the perfect companion to today’s color astronomical CMOS cameras, smaller CCD cameras, and mirrorless cameras. Thanks to its fast focal ratio and patented optical design, you can produce sharp, detailed images and often, you can eliminate the need for using an autoguider completely because of the fast exposures. Weighing in at just 17 pounds, it’s easy to transport your 8” RASA from Celestron to the most remote dark sky locations.
The 8” RASA is an imaging telescope that delivers a flat field without optical aberrations for razor sharp stars across a wide field of view. This astrograph can capture stunning deep-sky astronomical images without the challenges typically presented by longer focal length instruments at a fraction of the cost of those systems. The latest addition to the RASA family, this 8” version has many of the same thoughtfully designed features as its “big brother” the RASA 11", including the integrated air-cooling system, internal filter mount, and sturdy CGE dovetail mounting bar.
As a typical astrograph design, this scope was purely designed for imaging, hence the 8” RASA cannot be used visually. The prime focus focal plane is located near the front aperture of the optical system, so it cannot accommodate a traditional eyepiece. Since it is an F/2.0 optical system, imagers can use shorter exposure times to capture detail in faint objects. With sensitive cameras and the proper “live stacking” software, the 8” RASA can provide an almost real-time observing experience. View images on a computer instantly that are brighter and more detailed than can be seen in much bigger telescopes with the naked eye.
Because shorter exposure times are possible, your equatorial mount won’t need to accurately track over extended periods. The 8” RASA’s relatively short 400 mm focal length also lessens the demand for equatorial tracking accuracy. In many cases, autoguiding will not be required. Weighing only 17 lb, this optical tube pairs perfectly with a wide variety of mounts. Celestron has unveiled a new focuser design that mitigates lateral movement of the primary mirror when focusing, slewing, or tracking with the astrograph. Focusing is easier, more accurate, and more stable than ever. This Ultra-Stable Focus System uses two sets of precision bearings that are precisely aligned and tested during the assembly process to ensure optimal focus results.
The CGX is the culmination of Celestron engineers’ decades of experience in equatorial mount design, incorporating feedback from imagers worldwide. Combined with the RASA 8, it is a state-of-the-art deep-sky astronomical imaging system. The CGX is an incredibly sturdy equatorial mount which rigidly supports the RASA 8. The mount was designed more compactly, so the instrument load is held closer to the R.A. axis, thus reducing torque from the load. It was also designed using FEA (Finite Element Analysis) software, which helped to optimize the stout structure of the CGX. A 55 lb load capacity means it can easily handle the RASA 8, which weighs only 17 lb, along with a camera and any accessories. Expect a vibration-free setup, even in somewhat windy conditions. The RASA 8 connects directly to the CGX with its CGE dovetail mounting bar.
The heavy-duty adjustable height tripod has 2” diameter steel tripod legs with an extra-wide stance for improved stability. Graduated markings on the tripod legs make it easy to extend them to the same height and level the tripod. The mount is packed with functionality and conveniences that make imaging with the 8” RASA easy. The fully computerized mount will automatically “GoTo” any of the 40,000+ astronomical objects selected from the NexStar+ hand controller’s database and automatically track that object.
Designed with imaging in mind, the CGX motor drive system builds upon learnings from previous generations of mounts. It has low-cog DC servo motors with integrated optical encoders. The low-cog design helps the motors run smoothly, and the integrated optical encoders provide accuracy. The motors connect to the worm gears with a belt-and-pulley system (instead of spur gears) to reduce gear backlash when the motor changes directions. The steel worm gears are supported by precision bearings and spring-loaded against a brass worm wheel, which also helps ensure a smoothly operating drive system. This results in minimal backlash and no binding. To track objects in the night sky, the mount must first be polar-aligned. Celestron makes this process easier than ever with the innovative All-Star Polar Alignment procedure. No extra equipment, such as a polar finder scope, is required. (An optional polar alignment finder scope is available if desired.) Best of all, the alignment can usually be completed in about 15 minutes, even without a clear view of Polaris.
All mounts have some amount of “periodic tracking error” inherent in them caused by tiny gear imperfections. To mitigate this, the CGX has Permanent Periodic Error Correction (PPEC). This allows you to run a routine to record the periodic errors only once; the recording is kept in memory, and subsequently corrects for the mount’s periodic tracking error. If an even higher level of tracking accuracy is desired, the CGX also has a dedicated autoguider port to connect an autoguiding camera directly to the mount.
Astroimaging is best done when celestial objects are at their highest point in the sky with the darkest background, which is called the meridian. But some German equatorial mounts have trouble tracking near the meridian because they need to perform a “meridian flip,” readjusting their position to avoid the tube striking the mount. CGX does not suffer from this problem, imaging through the meridian with ease and not requiring a meridian flip until the object is a full 20° past the meridian.
Transporting the entire system to a dark site is extremely manageable, even for one person. The optical tube, mount head, tripod, counterweight bar, and counterweight all disassemble into separate pieces. The only tool ever required is a supplied hex key, with a holder built into the mount, so you probably won't lose or forget it. The CGX mount also features two ergonomic carry handles that make carrying and setting up this heavy-duty mount much more comfortable. When it’s time to transport your mount, the tripod accessory tray “nests” into the legs when collapsed, stabilizing the tripod in your vehicle and keeping the tray from getting misplaced.
Today’s imagers do not necessarily want to be near their telescope outside when they are capturing images. Rather, they would prefer to be indoors controlling the mount and camera with a computer, while the telescope is in the backyard, or even hundreds of miles away in a dark remote observing site. Not every mount is capable of remote observing operation, but the CGX was designed specifically with this in mind. The CGX mount is designed to protect itself and your telescope if something unexpected occurs. One of the most critical problems that can occur during remote operation is the optical tube inadvertently striking the mount. This will not only ruin an exposure and require realignment with the sky, but could also damage expensive equipment. This type of strike cannot happen with CGX because optical switches on each axis stop the motors before the scope can hit the mount.
Should power be somehow interrupted, the mount has “home position” optical switches that allow easy remote realignment. The mount’s cables are all wired internally through the mount, and all external inputs, including the power jack, are on a stationary section of the mount. This prevents cables from becoming snagged or tangled and unplugged as the RASA is moving. The mount also has hard stops to physically limit the motion of the astrograph on each axis, preventing the internal cabling from being overly twisted or pulled and potentially failing. In addition, the hard stops provide a “fail safe” safety measure to protect the astrograph should the limit switches somehow fail. The power jack for the mount utilizes a threaded barrel connector, ensuring the power cable won’t become unplugged and lose power.
For remote imaging, the mount needs to be controlled by an external computer. The CGX has two ways to connect to a computer via USB 2.0. First, it has a USB port on the mount itself which is used exclusively to connect to a PC running the included Celestron PWI telescope control software. This PWI software, developed in partnership with PlaneWave Instruments, has an intuitive graphical user interface and provides an excellent solution for remotely operating the mount. It incorporates multi-point mount modeling to provide the most accurate alignment of the mount to the sky possible. In addition, the included NexStar+ hand controller also has its own USB 2.0 port for computer connection; this can be used with optional third-party telescope control software by use of an ASCOM driver.
To summarize, the CGX and RASA 8 together are an ideally-matched imaging system built for performance and ease of use. This equipment is perfect for imaging in your own backyard or at any remote dark-sky site, whether you are near the telescope or on the other side of the globe. Use the Celestron 8" RASA on a CGEM II Equatorial Mount for your astroimaging work and see your astroimage quality improve and explore new opportunities for astronomical imaging, which you have been seeking, with this high-quality Celestron system.
Note that the 8” model RASA was designed with color astronomical CMOS cameras, smaller CCD cameras, and mirrorless cameras in mind. The telescope does not work with standard DSLR cameras. It is optimized for sensors which have up to a 22 mm diagonal, but performs well with sensors up to 32 mm in diagonal. The APS-C sized sensors used in many mirrorless cameras are a good choice. 42 mm full frame sensors will also work, but performance will be poor at the edges of the sensor and field illumination will be reduced. Additionally, it is not recommended to pair RASA 8 with any camera body more than 4” in diameter. See the chart below to determine if your camera is compatible with RASA 8.
|Camera||Compatible with RASA 8?||Adapter required|
|Astronomical CMOS/CCD camera with C-mount||Yes||C-mount adapter (included)|
|Astronomical CMOS/CCD camera with M42-thread mount||Yes||M42 adapter (included) with M42 extension tubes (not included)|
|Astronomical CMOS/CCD camera with other mount||Yes||Custom camera adapter|
|Canon mirrorless with APS-C sensor||Yes|
|Canon mirrorless adapter|
|Sony mirrorless with APS-C sensor||Yes|
|Sony mirrorless adapter|
|Canon mirrorless with full frame (42mm) sensor||Yes, but not optimized across the full sensor|
|Canon mirrorless adapter|
|Sony mirrorless with full frame (42mm) sensor||Yes, but not optimized across the full sensor|
|Sony mirrorless adapter|
|Optical Design||Rowe-Ackermann Schmidt Astrograph (RASA)|
|Aperture||203 mm (8")|
|Focal Length||400 mm (15.74")|
|Central obstruction diameter||93 mm (3.66") (46% of aperture diameter)|
|Light Gathering Power (Compared to human eye)||843X|
|Resolution (Rayleigh)||0.68 arcsecond|
|Resolution (Dawes)||0.57 arcsecond|
|Image circle||22 mm (.86") Ø, 3.15°|
|Useable field||32 mm (1.26") Ø, 4.6°, only minimal performance loss at edge|
|Wavelength range||390 nm – 800 nm|
|Spot size||< 4.6 μm RMS across image circle|
|Optical coatings||XLT multi-coatings used throughout|
|Off-axis Illumination||93% at 11 mm (0.43") off-axis|
|Optical window||46 mm (1.81") Ø|
|Back focus with included camera adapter||25 mm (0.98")|
|Back focus from top of threaded collar||29 mm (1.14")|
|Optical Tube Dims.||628 mm (24.7") length | 235 mm (9.3") diameter|
|Focuser||New focuser design, minimizes focus shift|
|Optical Tube Weight||17 lb (7.7 kg)|
|Other features||Ventilation fan, built in filter mount|
|Included items||M42 camera adapter, C-Thread camera adapter, fan battery pack|
|Dovetail Mount||CGE Dovetail Bar|
|Mount Type||Computerized Equatorial Mount|
|Instrument load capacity||55 lb (25 kg)|
|Height adjustment range (includes mount and tripod)||1200.15 mm - 1968.5 mm (47.25" - 77.5")|
|Tripod Leg Diameter||50.8 mm (2") Steel tripod with graduated markings on lower section|
|Latitude adjustment range||3° - 65°|
|Mount Head Weight||44 lb (20 kg)|
|Accessory Tray||Yes, included|
|Tripod Weight||19.2 lb (8.7 kg)|
|Weight of Counterweights||2 x 11 lb|
|Slew Speeds||9 slew speeds - max speed 4°/second|
|Tracking Rates||Sidereal, Solar, and Lunar|
|Tracking Modes||EQ North & EQ South|
|Dovetail Compatibility||Dual saddle plates (Vixen and CGE saddle)|
|Number of Auxiliary ports||2 Aux ports (Hand Control can use either Aux port)|
|USB Port||Yes, input for Mount and Hand Control|
|Power Requirements||12 Volts DC, @ 4 amps|
|Motor Drive||DC servo motors|
|Alignment Procedures||2-Star Align, 1-Star Align, Solar System Align, Last Alignment, Quick Align|
|Periodic Error Correction||Yes|
|Computerized Hand Control||2 line x 18 character backlit Liquid Crystal Display, 19 LED backlit buttons, USB 2.0 port for PC connection|
|NexStar+ Database||40,000+ objects, 100 user defined programmable objects. Enhanced information on over 200 objects|
|Software||PWI Telescope Control Software, Celestron's Starry Night Special Edition Software, SkyPortal App|
|Total Kit Weight||124.2 lb (56.3 kg)|
|Included Items||CGX Equatorial Head | CGX Tripod | Accessory Tray | 2 x 11 lb counterweights | NexStar+ Hand Control | 8 mm Allen Wrench | 12V DC Power Cable | Hand Control Holster|