This Celestron 11” f/2.2 Rowe-Ackermann Schmidt Astrograph (RASA) CGEM DX mount combination has:
- Very fast 11" f/2.2 Rowe-Ackermann Schmidt optics
- StarBright XLT optical multicoatings for the highest possible light transmission
- MagLev DC cooling fan and cooling vents
- Built-in four-element rare earth corrector lens
- 42mm T-thread and 48mm camera adapters
- CGEM DX go-to German equatorial mount
- 2-year warranty
Details on the Celestron 11” RASA optical tube . . .
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 an instrument that only a really hard-core astrophotographer could love.
Schmidt cameras use a single frame of 35mm film, cut from a roll of film. You flex the chip of film in your darkroom (being careful not to touch the emulsion side) to fit snugly into a curved holder that matches the Schmidt camera’s curved focal plane, then load it into the camera by feel inside a black cloth bag to avoid image-spoiling stray light. You manually guide your scope during the entire exposure, keeping your eye glued to the crosshairs of an illuminated reticle eyepiece the whole time so your stars don’t turn into elongated squiggles. Finally, after a sometimes multiple hour single exposure, you tediously process the small chip of film in your darkroom before you can even begin to see if you have captured a usable image.
No more! Telescope mount drive accuracy has improved tremendously, electronic eyeballs have taken over guiding, and fast DSLR and CCD electronic imaging has taken the place of slow 35mm film.
Today’s CCD camera can have sensors as large, if not larger, than film. To compensate for the 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.
Some of the advanced features of the Celestron 11” RASA optical tube include a custom-engineered linear brass focuser bearing to reduce image shift 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 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.
You can see some examples of the RASA’s imaging capabilities in the feature images above, which are just portions of the 2136 x 1752 pixel original. Taken at the 2014 Texas Star Party, the original image of Markarian’s Chain of galaxies is the result of 58 x 90 second shots with the Celestron RASA 11” on the Celestron CGEM DX mount, using a QHY11 color camera. The imagers were working on 90 second subs and getting 17th magnitude detail, as the camera would oversaturate at two minutes, due to the RASA’s very fast f/2.2 focal ratio. Comparable subs with a refractor would probably take 12 to 15 minutes. Star party participants had trouble believing that the subs were only 90 seconds long . . . until the photographers took a 90 second exposure and showed them the results immediately.
John Davis took the tri-color RASA image of the Propeller Nebula (DWB-111), an emission nebula in Cygnus, with a QSI 583 monochrome camera from his Bucksnort Observatory in Texas. This first RASA image of the Propeller is scheduled to appear in the issue of August Sky & Telescope magazine. Total exposure time was 1.3 hours, consisting of 40 minutes H-alpha; 21 minutes luminance; and six minutes each of red, blue, and green. The image is a portion of the 1600 x 1153 pixel original.
Naturally, an astrograph as good as the RASA demands an equally good mount. The Celestron CGEM DX is that mount.
Details on the Celestron CGEM DX go-to German equatorial mount . . .
The Celestron CGEM DX mount is light enough to transport to a dark sky imaging site, yet stable and accurate enough for serious imaging. The CGEM DX mount head contains dual-axis slewing/tracking motors on each axis for go-to computer control. Steel worm gears and 90mm diameter brass worm wheels assure long-term reliability and high accuracy performance. The use of low cog DC Servo motors with integrated optical positioning encoders offers 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.
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. An AC adapter is also available for backyard or observatory use.
No polar alignment finderscope is needed for the precise polar alignment required for long exposure astrophotography. A unique Celestron “All-Star Polar Alignment” program built into the mount’s 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.
The two supplied 22-pound counterweights are locked in place on the steel counterweight shaft with a single hand-tighten knob, making it easy to rebalance a scope if you add heavy photographic accessories. The two standard counterweights will balance any load up to the full 50-pound payload capacity of the CGEM DX mount.
For observing or imaging objects near the meridian (the imaginary line passing directly overhead from North to South), the CGEM DX is designed to track well 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.
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 easily add 100 of your own favorite objects to the list.
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. An RS-232 communication port on the hand control allows you to operate the telescope via a personal computer using the supplied RS-232 cable and NexRemote software CD-ROM.
The software built into the NexStar hand control includes mount calibration, database filter limits, hibernate, five alignment procedures, user-defined slew limits, the new "All-Star" polar alignment routine, and more. Adjustable backlash compensation is standard. 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 images. The CGEM DX mount retains the PEC recordings when it is powered down. The hand control and motor control software can be flash upgraded over the Internet to keep your CGEM DX 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 takes only a few short minutes. You can be observing in less than 15 minutes after you first take your CGEM DX-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.
The adjustable height tripod (the same as used on the Celestron CGE Pro mount) has 3” diameter stainless steel legs with a center leg brace for rigidity to provide excellent damping characteristics. The center leg brace forms a convenient accessory tray that holds 1.25” and 2” eyepieces to keep them up out of the dew-soaked grass. Setup and takedown times are exceptionally fast, as a single large hand-tighten knob holds the 11" optical tube in place. A second lock knob prevents the tube from sliding off the mount should the hand-tighten knob loosen.
The complete RASA/CGEM DX mount combination weighs 143 pounds (64.1 kg). The tripod is the heaviest component, at 45 pounds (20.5 kg). The equatorial head weighs 41 pounds (18.6kg). The two supplied counterweights weigh 22 pounds (10 kg) each. The 11” RASA optical tube weighs 35 pounds (15.9 kg).