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 CGEM II, with a 40 lb load capacity, is the latest update to the venerable CGEM mount and provides a sturdy, rigid platform for a fully-loaded RASA 8
- The drive system, consisting of low-cog DC Servo motors with integrated optical encoders, steel worm gears supported by precision sealed bearings, and brass worm wheels, ensures smooth, accurate slewing and tracking
- 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 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.
CGEM II is a sturdy equatorial mount capable of rigidly supporting the 8” RASA. Its 40-lb load capacity can handle the 17-lb optical tube with ease, leaving lots of capacity to spare for your camera and any accessories. The heavy-duty adjustable height tripod has 2” diameter steel tripod legs with graduated markings that make it easy to extend the legs to the same height. The 8” RASA connects directly to the CGEM II with its CGE dovetail mounting bar.
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. Designed with imaging in mind, the CGEM II motor drive system accurately tracks during an exposure. This system incorporates low-cog DC servo motors with integrated optical encoders. The design helps the motors run smoothly, while the integrated optical encoders provide accuracy. The motors turn steel worm gears, which are supported by precision bearings, against brass worm wheels.
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 CGEM II 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 CGEM II also has a dedicated autoguider port to connect an autoguiding camera directly to the mount.
Conditions for imaging are best when celestial objects are at their highest point in the sky with the darkest background, 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. CGEM II 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.
When imaging, cable management can be a problem. The camera has a cable, the mount’s power source has a cable, and additional cables can come from a computer connection and augtoguider. To minimize cable management issues and prevent unwanted snags, all cabling for the mount is housed internally. Transporting the entire system to a dark site is easy. The optical tube, mount head, tripod, counterweight bar, and counterweight all disassemble, without tools, into separate manageable pieces.
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|
|Instrument load capacity||40 lbs (18 kg)|
|Height adjustment range (includes mount and tripod)||1346mm - 1651mm (53" – 65")|
|Tripod Leg Diameter||50.8mm (2") Steel tripod with graduated markings on lower section|
|Latitude adjustment range||15° - 70°|
|Mount Head Weight||40 lbs (18.14 kg)|
|Tripod Weight||20 lbs (9 kg)|
|Weight of Counterweights||1 x 17 lbs|
|Slew Speeds||9 slew speeds - max speed 4.5°/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 (1x Hand control port and 1x AUX Port for optional accessories)|
|USB Port||Yes, input on Hand Control|
|Power Requirements||12V DC, 3.2 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||Celestron's Starry Night Special Edition Software, SkyPortal App|
|Total Kit Weight||95 lbs (43 kg)|
|Included Items||CGEM II Equatorial Head |Tripod | Accessory Tray | 1 x 17 lbs counterweight | NexStar+ Hand Control | DC Power Cable (item#18769) | Hand Control Holster|