Celestron 8” RASA OTA

The 8" RASA: An f/2.0 Astrograph for Portable Wide-Field Imaging

The RASA 8 is a dedicated imaging telescope built for capturing stunning deep-sky objects without the challenges of longer focal length instruments. Its f/2.0 focal ratio lets imagers use shorter exposure times to capture detail in faint objects. When combined with a sensitive CMOS camera and live-stacking software, the RASA 8 can provide an almost real-time observing experience, showing images on a computer that are brighter and more detailed than can be seen in much bigger telescopes with the naked eye.

This speed also reduces the demands on your equatorial mount. Because shorter exposures are possible and the 400mm focal length lessens tracking demands, autoguiding will not be required in many cases. Weighing 17 lb, the optical tube is easy to transport to remote dark sky locations and pairs with a wide variety of mounts.

The 4-Element Rowe-Ackermann Schmidt Optical System

The Rowe-Ackermann Schmidt Astrograph (RASA) design is an evolution of the classic Schmidt camera, engineered for modern digital sensors. Light enters through a Schmidt corrector, reflects off a spherical primary mirror, and then passes through a four-element corrector lens group, which forms a flat field without optical aberrations.

Unlike a traditional telescope where the camera attaches at the rear, the RASA's focal plane is located at the front of the instrument. Your camera is mounted in front of the lens group at the prime focus, which is why the RASA cannot accommodate a traditional eyepiece.

A 400mm Focal Length and 22mm Image Circle at f/2.0

The RASA 8's optical specifications are tailored for wide-field astrophotography. The 400mm focal length captures a vast 3.15° field of view across its optimized image circle.

• Optimized Image Circle (22mm): Delivers the highest optical performance with spot sizes less than 2.3 μm RMS, ensuring sharp stars from edge to edge on sensors with up to a 22mm diagonal.
• Useable Field (32mm): The field remains well-corrected out to a 32mm diagonal, with only minimal performance loss at the edge; the APS-C sensors used in many mirrorless cameras are a good choice, while field illumination in the corners is reduced.
• Wavelength Range (400-800 nm): The optics are specified from 400 to 800 nm, with StarBright XLT coatings.

Camera Integration: Sensor Sizing, 29mm Back Focus, and Adapters

The RASA 8 is designed for modern astronomical CMOS cameras, smaller CCDs, and mirrorless cameras. Due to its front-mounted design and specific back focus requirements, it is not compatible with standard DSLR cameras. Since the camera mounts in front of the optics, choose cameras that block as little of the aperture as possible: cylindrical bodies under 100mm in outer diameter work best, and avoid any camera body more than 4 inches in diameter. Full-frame (42mm) sensors work, but performance is poor at the edges and field illumination is reduced.

Achieving the correct sensor distance is critical for edge-of-field performance. The RASA 8 requires the camera sensor to be placed precisely at one of two distances, depending on the adapter used.

• Back Focus from Collar (29mm): This is the native back focus distance measured from the top surface of the main threaded collar on the lens group cell. Custom adapters should be designed to meet this spacing.
• Back Focus with M42 Adapter (25mm): When using the included M42 T-thread adapter, the required back focus is 25mm. You will need to add the correct M42 extension rings (not included) to make up the difference between this requirement and your camera's native back focus.
• Included Adapters: The RASA 8 ships with an M42x0.75 T-thread adapter and a C-thread adapter, accommodating a wide range of popular astronomical cameras.

Filter Integration: Using the Integrated Filter Mount

The RASA 8 has an internal filter mount: the removable 46mm optical window at the front of the lens group can be replaced by Celestron's Light Pollution Imaging Filter for the RASA 8 (sold separately). If you use a filter anywhere in the light path, or if your camera has its own optical window, remove the RASA's window to keep the best optical performance.

To install a filter, you first remove your camera and the camera adapter. Then, you can carefully unthread the cell holding the 46mm optical window and replace it with a compatible filter, such as Celestron's Light Pollution Imaging Filter designed for the RASA 8 (sold separately).

The Ultra-Stable Focus System

The RASA 8 is equipped with Celestron's Ultra-Stable Focus System (USFS), a design that mitigates the lateral "mirror shift" that can occur when focusing or slewing. The key to this system is a precision linear ball bearing, which Celestron tests during assembly.

The focuser moves the primary mirror forward and back within the optical tube. While a single turn of the knob produces very fine movement, it takes approximately 30 full turns to move through the entire focus range. For automated and remote focusing, the RASA 8 is fully compatible with the Celestron Focus Motor (sold separately), allowing you to achieve critical focus from the same computer controlling your camera.

The 12V DC Active Cooling System

For an optical system to deliver its best performance, it must be in thermal equilibrium with the ambient air. The RASA 8 features an integrated air-cooling system to speed up this process. A 12V DC MagLev fan in the rear cell pulls air through the mesh vents on the perimeter of the rear cell; the mesh keeps dust out.

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 can supply at least 100mA through a 5.5mm/2.1mm tip-positive plug. Store the astrograph in a cool area and keep it outside for an hour or so before imaging, then unplug the battery pack to turn the fan off when you are ready to image.

Collimation and Camera Rotation for Pinpoint Stars

While the RASA 8 optics are aligned at the factory, provisions exist for fine-tuning collimation if necessary. Adjustments are made to the tilt of the front lens group, not the primary mirror or corrector. To check collimation, you point the telescope at a bright star, defocus the image, and inspect the resulting "donut" pattern. A perfectly collimated system will show a concentric pattern.

Adjustments are made using three pairs of push-pull screws on the lens group cell, which are reached through the three slotted openings in the camera adapter, using 3mm and 2mm hex keys with a camera installed. Camera orientation can also be easily adjusted. Simply loosen the main retaining ring by no more than 1/8th of a turn, rotate your camera to the desired framing, and gently retighten the ring.

The 17 lb OTA and CGE Dovetail Bar

The RASA 8 optical tube assembly weighs only 17 lbs (7.7 kg), making it an easy payload for a wide range of equatorial mounts. Its physical dimensions are also manageable, with an optical tube length of 628mm (24.7") and a diameter of 235mm (9.3").

It comes equipped with a wide CGE-style dovetail bar. This format is also compatible with mounts that accept Losmandy-D style plates. The astrograph weighs 17 lb, so a mount with a load capacity greater than that is required for good performance once a camera and accessories are added. The included CGE dovetail accessory bar can carry an optional guidescope.

Included & Optional Accessories: What's in the Box and What to Add

The RASA 8 optical tube assembly includes the essential adapters to get you started with common astronomical cameras. The box contains the main optical tube, a 42mm T-thread camera adapter, a C-thread camera adapter, a retaining ring for securing them, and a battery pack for the cooling fan.

To complete your imaging setup, several optional accessories are available from Celestron:

• Camera Adapters: Specific adapters for Canon and Sony mirrorless cameras are available to ensure correct back focus.
• Celestron Focus Motor: Allows for precise, remote-controlled focusing from your computer.
• Light Pollution Filter: Mounts directly into the RASA 8's optical path in place of the optical window.
• 8" Dew Shield: The easiest way to prevent dew on the Schmidt corrector; a dew heater can be added in more severe conditions.

Solar Warning: Never look directly at the Sun with the naked eye or with an optic (unless you have the proper solar filter). Permanent and irreversible eye damage may result. Never use your optic to project an image of the Sun onto any surface. Internal heat build-up can damage the optic and any accessories attached to it. Never leave your optic unsupervised.