Celestron RASA 6" OTA

The RASA 6 Concept: An f/2.2 Portable Astrograph

The RASA 6 is engineered for one purpose: capturing deep-sky images with exceptional speed and portability. Its f/2.2 design drastically shortens exposure times, enabling you to produce detailed images of faint nebulae and galaxies without the heavy burden of extended tracking. This makes it an ideal instrument for imagers at all levels, from those seeking a powerful grab-and-go setup to those adding a dedicated wide-field astrograph to their observatory.

The optical speed fundamentally changes the imaging experience. When paired with a sensitive CMOS camera and live-stacking software, you can watch detailed images build on your screen in near-real time. The fast focal ratio also lessens the tracking demands on your equatorial mount, simplifying your entire workflow from setup to final image.

RASA Heritage: Professional Technology in a 6" Astrograph

Since RASA technology debuted in 2014, it has become a trusted choice for space surveillance and satellite tracking, relied on by defense contractors, professional researchers and governments. The RASA 6 brings that same design to a compact, grab-and-go package, including the flat field that keeps stars sharp from edge to edge.

The RASA 6 collects more light and provides higher resolution than the refractors in its price range, so you can capture faint detail in galaxies, nebulae and star clusters in less time. It also makes a capable second astrograph for imagers who already own a larger instrument.

Rowe-Ackermann Schmidt Optics: A 3-Element Corrected Design

The patented Rowe-Ackermann Schmidt Astrograph optical design is what sets the RASA 6 apart. Light first passes through a Schmidt corrector before reflecting off a high-precision primary mirror. Before reaching focus, the light passes through a specialized 3-element lens group made with rare-earth glass, which ensures that more of the light from celestial objects is sharply focused onto your sensor. The flat field gives sharp stars from edge to edge at the front of the telescope, where the camera is mounted.

This imaging-only system is optimized for a wavelength range of 400 - 700 nm. All optical surfaces feature Celestron's StarBright XLT coatings for maximum light transmission and field illumination. The RASA design gives better overall performance and field illumination than a Schmidt-Cassegrain in the Fastar-style configuration; off-axis illumination is 91% at 8mm off-axis.

152mm Aperture at f/2.2: Speed and a 3.76° Useable Field

The 152mm aperture gives a light gathering power of 472x that of the human eye, and the f/2.2 focal ratio means even faint objects can be imaged in short exposures. The 335mm focal length creates an expansive field of view, ideal for large celestial targets.

The optical system is optimized for a 16mm image circle (2.74°) but provides a useable field up to 22mm (3.76°) with minimal performance loss at the edges. The central obstruction is 77mm, or 50.7% of the aperture diameter.

Camera Integration: Achieving 17.5mm Back Focus with M42 Threads

The RASA 6 is designed for use with modern, compact astronomy cameras. The camera mounts at the front of the optical tube, so cylindrical cameras with an outer diameter under 75mm are recommended to minimize obstruction. The included M42 T-thread camera adapter establishes the critical back focus distance of 17.5mm from the adapter's top surface to your camera's sensor.

The optical design is optimized for sensors up to 16mm diagonal and still performs well with sensors up to 22mm diagonal, with reduced illumination in the corners. The RASA 6 works with a wide range of pixel sizes and, thanks to its f/2.2 focal ratio and 6" aperture, can work well even with small pixels under 3 μm. Many astronomy cameras sold as "HyperStar-compatible" are well suited to it. With larger sensors, set the back focus within 1mm of the required 17.5mm for the best results.

• Correct Spacing: If your camera's back focus is less than 17.5mm, M42 extension rings (sold separately) are required to place the sensor at the correct focal plane. Cameras with a back focus greater than 17.5mm are not compatible.
• DSLR Incompatibility: Due to their large central obstruction and long back focus requirements, DSLR cameras are not compatible with the RASA 6.
• Installation: The camera adapter connects through a retaining ring to your camera's M42 threads. This assembly then threads onto the filter drawer at the front of the RASA 6.

Filter System: An Integrated Drawer for 1.25" and 2" Filters

An integrated filter drawer is built into the front lens assembly, and this system is designed to maintain the RASA's optical performance by replacing the included 2" clear filter with your imaging filter, ensuring no extra glass is added to the f/2.2 light path.

The filter holder is secured by magnets and can be removed by pulling its handle. It natively accepts standard 2" mounted filters. For smaller sensors, the included 1.25" filter adapter threads into the 2" holder. It is recommended to use 1.25" filters only with sensors that have a diagonal smaller than 10mm to avoid vignetting.

Focusing the RASA 6: A Precision SCT-Style Focuser

The RASA 6 uses a standard Schmidt-Cassegrain style focuser, which moves the primary mirror forward and backward inside the optical tube. The focus knob provides fine control, with approximately 30 full turns required to move through the entire focus range.

Focus first on a bright star. The RASA 6 focuser is compatible with the optional Celestron Focus Motor. This allows you to make micro-adjustments from the same computer you use to control your camera.

Thermal Management: The 12V DC MagLev Cooling System

To achieve the best possible images, the RASA 6 must reach thermal equilibrium with the ambient air. The rear cell features an integrated 12V DC MagLev cooling fan that pulls air through the mesh vents on the perimeter of the rear cell to cool the system to ambient temperature, while the mesh keeps the internal components dust-free.

The fan is powered by the included battery pack, which requires eight AA batteries (not included). Alternatively, it can be powered by any 12V DC source that supplies at least 100mA via a 5.5mm/2.1mm tip-positive plug. Keep the astrograph outside for an hour or so before imaging, and unplug the battery pack to turn the fan off when you are ready to image.

Collimation and Camera Orientation for Edge-to-Edge Sharpness

While the RASA 6 optics are aligned at the factory, the lens group's tilt is adjustable for precise collimation if needed. This is done by adjusting two sets of three push-pull screws on the front lens cell using a 2mm hex key. By observing a defocused star, you can center the "donut hole" in the diffraction pattern until the pattern is concentric.

The camera's orientation can be easily adjusted to frame your target. Simply loosen the main retaining ring by no more than 1/8th of a turn, rotate the camera to the desired angle, and re-tighten the ring.

Mounting the 8.4 lb OTA: The CG-5 Dovetail and Physical Specs

At just 8.4 lbs and 500.5mm in length, the RASA 6 is exceptionally lightweight and compact. It is compatible with a wide range of equatorial mounts that have sufficient payload capacity for the OTA, camera, and any accessories. The astrograph attaches to your mount via an integrated CG-5 (Vixen-style) dovetail bar.

The rear cell of the astrograph includes threaded holes for attaching an optional finderscope bracket. You can also add an optional CG-5 dovetail bar to the top of the tube for mounting accessories like a guidescope or a Celestron Smart DewHeater Controller.

Recommended Accessories for the RASA 6

For the highest level of performance, Celestron strongly recommends pairing the RASA 6 with these accessories, all sold separately:

• Celestron Focus Motor: precise electronic focus from the same computer that controls your camera.
• Aluminum Dew Shield with Cover Cap - 6": slows dew formation on the corrector and doubles as a lens shade.
• Dew Heater Ring - 6": keeps the optics clear through long imaging sessions in humid conditions.
• Padded Carrying Bag for RASA 6": a protective, portable case with a pocket for the filter drawer and camera adapter.

Your First Night with the RASA 6: Setup and Workflow

Getting started with the RASA 6 is straightforward. After mounting the 8.4 lb optical tube to your equatorial mount using its CG-5 dovetail, the next step is attaching your camera. Ensure the 17.5mm back focus is correct, using M42 spacers if needed, and thread the camera onto the adapter and retaining ring assembly.

Before you begin imaging, allow the telescope to cool to ambient temperature, using the 12V DC fan to speed up the process. Once cooled, turn off the fan and point to a bright star to achieve critical focus. Finally, slew to your target, insert the desired filter into the integrated drawer, and begin capturing the deep sky with incredibly short exposures.