Celestron RASA 8″ f/2 Optical Tube Assembly (OTA Only)
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- Incredible clarity from edge to edge of your image circle
- Portable and powerful for professionals and amateurs alike
- Aperture: 203 mm (8-inch)
- Telescope weight: 17 lbs.
- Back focus: 29 mm (25 mm without included adapters). Working distance from the T2 thread is 25mm.
- Optimized for sensors with a 22 mm diagonal.
- PLEASE NOTE: The 8-inch RASA will not work with standard DSLR + T-Ring systems.
The Rowe-Ackermann Schmidt Astrograph, known mainly as the Celestron RASA, became one of the best astrophotography telescopes the moment it was released. Since then, it’s popularity has only grown due to the incredibly fast optics for efficient imaging, absurdly clear optics from edge to edge of the aperture, and the fact that it’s one of the most reasonably priced, professional-grade telescopes available. Any astrophotographer will say the same great things about the Celestron RASA astrograph telescope. Experience as close to perfection as you can get in a portable package with the RASA series telescopes.
FAST OPTICS FOR SHORT EXPOSURES & “REAL-TIME” OBSERVING
Use shorter exposure times to capture detail in faint objects with its f/2.0 optical system. When combined with sensitive cameras and the proper “live stacking” software, the 8-inch RASA can provide an almost real-time observing experience. Instantly view images on a computer that are brighter and more detailed than what you would see with the naked eye in bigger telescopes.
Because shorter exposure times are possible, your equatorial mount won’t need to accurately track over extended periods. The 8-inch RASA’s relatively short 400 mm focal length also lessens equatorial tracking demands. In many cases, autoguiding will not be required.
AN ASTROGRAPH FOR EVERYONE
The 8-inch RASA is an imaging telescope that delivers a flat field without optical aberrations for razor sharp stars across a wide field of view. It can capture stunning deep-sky astronomical images without the challenges typically presented by longer focal length instruments at a fraction of the cost.
It has many of the same thoughtfully designed features as its “big brother” RASA 11, including, the integrated air-cooling system, internal filter mount, and sturdy CGE dovetail mounting bar.
FOCUSING MADE EASY
Focusing is easier, more accurate and more stable with its new focuser design that mitigates lateral movement of the primary mirror when tracking with the astrograph, focusing or slewing. Two sets of precision bearings are precisely aligned and tested during assembly to ensure optimal results.
IMAGING IN THE MODERN AGE
Designed to pair with color astronomical CMOS cameras, smaller CCD cameras, and mirrorless cameras, it is optimized for sensors with up to a 22 mm diagonal. It can accept larger sensors (up to 32 mm diagonal) as well. APS-C sized sensors used in many mirrorless cameras.
Camera adapters to connect C-mount cameras and cameras which mount with M42 threads are included.
Please note: The 8-inch RASA will not work with standard DSLR + T-Ring systems. Celestron offers optional adapters to connect your 8-inch RASA to Sony or Canon mirrorless cameras.
HIGHLIGHTED FEATURES OF THE CELESTRON RASA
- Designed with a Schmidt corrector, primary mirror, lens group and optical window.
- The 8-inch RASA’s optics are designed to perform over a wider spectral range, from 390-800 nm. This allows more of the light emitted from the astronomical object to be sharply focused in the image.
- A removable optical window, so you can maintain peak optical performance if a filter is added or if a camera has its own optical window.
- Celestron offers a Light Pollution Imaging Filter designed specifically for the 8-inch RASA, which mounts in place of the optical window.
- All refractive optical surfaces are coated with StarBright XLT coatings, while the primary mirror uses enhanced aluminum coatings.
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- 8-inch RASA Telescope.
- M42 Camera Adapter.
- C-thread Camera Adapter.
- Fan Battery Pack.
|Aperture||203 mm (8″)|
|Corrected Image Circle||22mm|
|Dawes Limit||0.57 arcseconds|
|Focal Length||400 mm|
|Light Gathering Power||841x|
|Optical Design||Rowe Ackermann Schmidt Astrograph|
|RMS Spot Size||4.6 microns|
|Tube Weight||17 lbs|
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.