Celestron 11″ Rowe-Ackermann Schmidt Astrograph (RASA) Optical Tube Assembly

The Celestron 11″ Rowe-Ackermann Schmidt Astrograph (RASA) Optical Tube Assembly represents the most capable and large-format model in the RASA lineup, engineered for demanding astrophotographers who require both extreme imaging speed and a wider usable imaging field. Unlike smaller RASA systems, the 11″ version is specifically optimised to deliver improved performance with larger sensor formats, making it a more versatile platform for APS-C and full-frame astrophotography.

At its core is a 279mm (11-inch) aperture optical system paired with a 620mm focal length, producing an ultra-fast f/2.2 focal ratio. This combination allows exceptionally rapid light collection, significantly reducing exposure times while maintaining strong signal depth across extended deep-sky structures. The system is particularly effective for large nebula complexes, expansive galaxy regions, and wide-field survey imaging where speed and field coverage are critical.

A key differentiator of the 11″ RASA is its larger corrected imaging circle and improved field performance across larger sensors compared to the 6″ and 8″ models. While all RASA telescopes are designed for fast wide-field imaging, the 11″ version provides a more consistent and better-corrected image across APS-C sensors and is notably more suitable for full-frame CMOS cameras. This results in improved star shape consistency toward the edges of larger sensors and more usable imaging area for high-resolution astrophotography workflows.

The optical system is based on the Rowe-Ackermann Schmidt design, which integrates a precision Schmidt corrector plate, a spherical primary mirror, and a multi-element corrector lens group positioned near the focal plane. This configuration corrects spherical aberration, coma, and field curvature, producing a flat imaging field designed specifically for modern large-format CMOS sensors.

Celestron’s StarBright XLT coatings are applied to all optical surfaces, enhancing transmission efficiency and contrast, which is particularly important in ultra-fast f/2 systems where maximising photon throughput directly impacts imaging efficiency.

The optical tube includes an integrated filter drawer system compatible with both 1.25″ and 2″ astronomical filters. A removable clear optical window is supplied as standard, allowing filters to be introduced into the optical path without altering spacing or optical alignment. This supports efficient switching between broadband and narrowband imaging configurations.

Thermal stability is supported by a built-in 12V MagLev cooling fan located at the rear of the optical tube. This active cooling system accelerates thermal equilibration of the primary mirror, reducing internal air currents and improving early-session image stability.

Mechanically, the 11″ RASA is built around a rigid aluminium optical tube assembly designed for heavy-duty equatorial mounts. A CGE-style dovetail ensures compatibility with professional-grade imaging systems capable of supporting its size and camera payload requirements.

As with all RASA systems, the 11″ model is a dedicated astrophotography instrument and is not suitable for visual observing due to its front-mounted camera configuration and highly specialised optical layout.

Overall, the Celestron 11″ RASA OTA stands out as the most versatile and large-format option in the series, combining extreme imaging speed with improved support for larger sensor cameras, making it particularly well suited to advanced deep-sky imaging and wide-field survey work.