This Celestron 11” f/2.2 Rowe-Ackermann Schmidt Astrograph (RASA) optical tube 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
- Mirror locks
- 42mm T-thread and 48mm camera adapters
- Two CGE (Losmandy-style “D-plate”) dovetails
- 2-year warranty
“I don’t get it. What do you look through?”
This might be the most common question people ask when you unpack your new Celestron Rowe-Ackermann Schmidt Astrograph (RASA). It’s not only missing something that comes with virtually every telescope ever produced . . . an eyepiece . . . it doesn’t even have a place to put an eyepiece. That’s because the 620mm focal length f/2.22 Celestron 11” RASA is strictly a deep space imaging scope and cannot be used visually.
But what an imaging scope! It has a huge 70mm image circle that can handle full frame DSLRs and the largest sensor size CCD cameras with minimal vignetting. It provides 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). Those advantages? 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 in a fraction of the time it used to take, even without using an autoguider.
The Celestron RASA concept was to modernize the Celestron Schmidt camera, an instrument that had a loyal following, as its very fast focal ratio allowed amateur astrophotographers to produce wide field deep space images in the 1970s. Schmidt cameras could produce great images, but they wre and imaging scope only a really hard-core astrophotographer could love.
Those Schmidt cameras used a single frame of 35mm film, cut from a roll of film. You flexed the chip of film in total darkness (being careful not to touch the emulsion side) to fit snugly into a curved holder that matched the camera’s curved focal plane, then loaded it into the camera by feel inside a black cloth bag to avoid image-spoiling stray light. You manually guided your scope during the entire exposure, keeping your eye glued to the crosshairs of an illuminated reticle eyepiece so your stars wouldn’t turn into elongated squiggles. Finally, after a sometimes multiple hour single exposure, you tediously processed the small chip of film in your own darkroom before you could even begin to see if you had captured a usable image.
No more. Telescope mount drive accuracy has improved tremendously, electronic eyeballs have taken over guiding, and fast digital photography 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 Celestron Rowe-Ackermann Schmidt Astrograph has provided that advance in the performance of deep space imaging scopes. With it, today’s amateur astrophotographers can produce results rivaling that of the best professional observatory photos of only a few short years ago.
As with most advances in optics, the Celestron Rowe-Ackermann Schmidt Astrograph was designed not by committee, but by two dedicated optical experts who believed things could be done better – Dave Rowe and Mark Ackermann.
Dave Rowe – amateur astronomer, telescope maker, and optical designer – studied astronomy and astrophysics at Caltech, has published more than 50 papers, and holds 15 patents. Rowe has designed and fabricated many telescopes for Celestron and PlaneWave Instruments, including PlaneWave's corrected Dall-Kirkham and CDK700 telescope. He also worked closely with Celestron engineers in the development of the unique StarSense technology.
Mark Ackermann – amateur astronomer and experienced optical designer – earned a BS in mathematics and physics from the United States Air Force Academy, an MS in solid state physics, and a PhD in nonlinear optics from the University of New Mexico. He has published dozens of papers on optical telescope design and holds six US patents related to optical systems.
Engineered as a complete astroimaging system, every component of the Celestron Rowe-Ackermann Schmidt Astrograph is optimized for peak performance with DSLR and astronomical CCD cameras. Every component of the system has been designed to work together seamlessly, right down to the thickness of the glass used in the scope’s fully-multicoated optical window. 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 include a custom-engineered linear brass focuser bearing to reduce image shift and 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 equipment for the highest possible light throughput. 42mm T-thread DSLR and 48mm CCD camera adapters are supplied as standard equipment with the 35 pound optical tube. Two Losmandy-style “D-plate” dovetails are standard, one on the bottom of the tube for installing it on your equatorial mount, and one on top for installing accessories (such as a 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.