Celestron 11″ Rowe-Ackermann Schmidt Astrograph (RASA) f/2.2, V2 Optical Tube Assembly (OTA Only)
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- Fast, wide-field 11” f/2.2 optical design with rare-earth glass for images free of false color, coma, and field curvature
- Large 43.3mm optimized image circle maintains pinpoint stars to the far corners of even the largest astroimaging sensors, while the usable field extends even further to 52mm for larger format sensors
- New Ultra-Stable Focus System (USFS) minimizes focus shift and mirror flop
- Integrated air-cooling system features a quiet, high-output 12V MagLev fan and vents with mesh filters to prevent dust ingress
- Common camera adapters (T-thread and M48) included for easy attachment to popular CCD and DSLR cameras
Due to availability, RASA 11″ will take around 6 weeks
Capturing impressive deep-sky astroimages is easier than ever with Celestron’s Rowe-Ackermann Schmidt Astrograph (RASA) f/2.2, the perfect companion to today’s top DSLR or astronomical CCD cameras. This fast, wide-field f/2.2 system allows for shorter exposure times compared to traditional f/10 astroimaging, without sacrificing resolution. Because shorter sub-exposure times are possible, your equatorial mount won’t need to accurately track over extended periods. The short focal length also lessens equatorial tracking demands. In many cases, autoguiding will not be required.
RASA 11 f/2.2 builds on the legacy of Celestron’s Schmidt Cameras, which allowed astrophotographers to produce images with much shorter exposure times on film in the 1970s.
Today, with CCD sensor sizes as large as film or larger, the RASA 11 f/2.2 offers a 43.3mm optimized image circle to capture pinpoint stars on the largest imaging chips. Optical performance for huge sensors with diagonals of up to 52mm wide is still excellent.
Combine this large image circle with a focal length of just 620mm and you have an instrument suitable for wide-field imaging, creating huge mosaics of the night sky, surveying, and even comet hunting.
The RASA 11 f/2.2 features optics with 4-element rare-earth glass for images free of false color and aberrations like coma and field curvature. 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. The design also provides minimal vignetting.
RASA 11 f/2.2 utilizes the new Ultra-Stable Focus System (USFS). At the heart of this system is a precision linear ball bearing. The bearing serves to minimize focus shift (unwanted lateral motion of the primary mirror during focusing which causes shifting of the image) and mirror flop (movement of the primary mirror when the telescope is pointing to different positions in the sky). The USFS is also compatible with the optional Celestron Focus Motor (#94155-A). The integrated 12V DC MagLev fan reduces cooldown time and provides optimal airflow through the dust filtered optical tube.
Engineered as a complete astroimaging system, every component of the RASA 11 f/2.2 is optimized for peak performance with DSLR and astronomical CCD cameras. Down to the thickness of the glass used in the included fully-multicoated optical window or optional imaging filter, every component of the system has been taken into careful consideration to work together seamlessly. The Dovetail CGE bars on the top of the optical tube provides a connection for use of optional accessories like a guidescope.
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- Celestron 11-inch RASA Astrograph OTA.
- M48 Camera Adapter.
- 42 mm T-Thread Camera Adapter.
- 2 Year Warranty.
|Aperture||279 mm (11″)|
|Camera/Eyepiece Connection||Male T—Thread (M42x0.75)|
|Corrected Image Circle||43.3mm|
|Dawes Limit||0.42 arcseconds|
|Focal Length||620 mm|
|Light Gathering Power||1589x|
|Optical Design||Rowe Ackermann Schmidt Astrograph|
|RMS Spot Size||4.4 microns|
|Tube Weight||43 lbs|
The sensor must be 72.8mm (+/- 1.0mm) from the tilt collar.
We recommend Astrodon’s 5nm narrowbands for imaging with the RASA or any telescope faster than f/2.8
(More info below)
Using Astrodon Filters at Focal Ratios below f/3.5
Astrodon officially puts the recommend focal ratio limit at f/3.5. That said, it is a conservative limit and other filter manufacturers specify their limit much more optimistically. But we (Astrodon) maintain that filter performance will be somewhat degraded at focal ratios below f/3.5. Let’s explore the consequences of “pushing the limits” and using Astrodon filters (or any brand of filters) at focal lengths below f/3.5. Many imagers will be willing to make these tradeoffs if they understand the pros and cons.
Anything between f/2.8 and f/3.6 will result in a 0.8nm blue shift, which will easily remain within the bandpass width of a 3nm bandwidth filter. Also, the eye won’t be able to pick up a shift this small. At some point, the blue shift will be large enough to start to move out of the bandpass of the filter. So, for imaging at focal ratios at or below f/2.8, we recommend Astrodon’s 5nm bandpass filters.
Halo Size and Focal Ratio
The short answer is that filter thickness and coating quality are significant factors in determining halo size, but there are limits as to what the filter can do. Different setups are going to perform differently because the (1) distance between sensor and filter and (2) focal ratio are both important variables.
The Halo Formula is:
Distance Traveled = Halo Size X Focal Ratio
Solving for Halo Size:
Halo Size = Distance Traveled divided by focal ratio
Where Distance Traveled:
Is the filter thickness divided by the refractive index (assume 1.5 as a ballpark refractive index but this is where higher quality filters come in) plus the distance from the filter to the sensor.
Astrodon filters are thin (3mm) and the coatings are of high quality. This is a critical factor in determining the final halo size. So Astrodon’s are famous for their small halo size; they perform much better than most other filters, but there are limits to what any filter can do. The Distance Traveled in your system is an important factor as well. Obviously, closer is better. But focal ratio is another critical factor. That “divided by the focal ratio” is inescapable.
Let’s compare halo size in an f/8 system and an f/2 system. If all the parameters in the f/8 and the f/2 scope are otherwise equal…
Divide the Distance Traveled by 8 (with the f/8 scope) and you get a pretty small halo. Divide the Distance Traveled by 2 (with the f/2 scope) and the halo is going to be 4 times bigger.
There is no escaping the math. You are going to have some halo even with Astrodon filters if the distance between filter and sensor is large and/or the focal ratio is fast.