Celestron 36 cm RASA V2 OTA - CGE Dovetail

The Celestron RASA 36 V2: A 355.6mm Astrograph for SSA and Research

Celestron built the 36 cm RASA for applications that need large aperture, high speed and a wide, flat field together. Its white paper explains that the 14-inch version was intended primarily for Space Situational Awareness and other space surveillance work, where a large area of sky has to be scanned in a short time to detect very small and faint objects, and that it was meant to be housed in an observatory rather than carried.

For advanced wide-field astroimaging it is the largest RASA Celestron manufactures, and it is the biggest and fastest (f/2.2) optical instrument of its kind available off-the-shelf.

The Rowe-Ackermann Schmidt Design: A 4-Element Prime-Focus System

The RASA optical system is a modern evolution of the classic Schmidt camera. Light first passes through a 355.6mm Schmidt corrector plate, which pre-corrects for spherical aberration. It then reflects off a large, spherical primary mirror and converges toward the front of the tube. Before reaching focus, the light passes through a four-element corrector lens group, and the star images form on a flat plane ahead of it, where the camera sensor sits.

This is a convenient external prime-focus image capture location with a flat focal plane, providing small spot sizes to the edge of a wide field.

f/2.2 Speed Across a 70mm Usable Field

The defining feature of the RASA 36 is its f/2.2 focal ratio combined with a 355.6mm aperture and a 790mm focal length.

This speed is delivered across a massive field of view. The optical system produces an optimized image circle of 60.1mm and a usable field of 70mm, with only minimal performance loss at the edge of the usable field. Off-axis illumination is 83% at 30mm off-axis, and spot sizes stay under 6.3 μm RMS across the field. With a custom camera adapter, the field of view reaches up to 4.4 degrees.

77.5mm Back Focus for Large-Format Sensors

The RASA 36 is designed for seamless integration with a wide variety of imaging systems. It provides a generous 77.5mm of back focus, measured from the top of the lens group's threaded collar. This ample back focus accommodates a wide variety of imaging sensors.

The included M48 camera adapter gives 55mm of back focus. It attaches a DSLR with a 48mm "wide T-ring" for your camera make, and astronomical cameras with sensors smaller than full frame (43mm diagonal) may also use it with optional adapters such as an M48-to-M42 adapter and M48 extension rings. Cameras with larger sensors need a custom camera adapter; Celestron's "Camera Adapter Requirements" drawing gives the design.

Using Filters with the RASA 36 and its Removable Optical Window

The RASA 36's front lens cell includes a 104mm optical window. If you plan to use any other glass in the light path, such as a filter or a camera's own optical window, remove the RASA's built-in optical window to maintain the highest level of optical performance.

Removing the window is a straightforward process. With the camera and camera adapter removed, grasp the knurled cell of the optical window and unthread it. The window housing uses M110 x 0.75 threads on its exterior.

The Ultra-Stable Focus System (USFS) at f/2.2

The RASA 36 V2 features Celestron's Ultra-Stable Focus System (USFS). At its heart is a precision linear ball bearing, which minimizes two critical issues: focus shift (lateral image movement during focusing) and mirror flop (mirror movement when slewing across the sky).

The manual adds that the USFS minimizes unwanted shifting of the primary mirror when focusing and during long exposures. A single focus knob moves the primary mirror within the rear cell, and the USFS is compatible with the optional Celestron Focus Motor (#94155-A).

Active Cooling: The 12V DC MagLev Fan for Thermal Stability

Achieving peak optical performance requires the telescope to be in thermal equilibrium with the ambient air. The RASA 36 is equipped with an integrated 12V DC MagLev fan to accelerate this cooldown process. The fan in the rear cell pulls air through vents with micromesh, which keeps dust out of the tube.

The included battery pack holds eight AA batteries (not included) and powers the fan. Store the astrograph in a cool area and keep it outside for an hour or so before imaging; when the optics have cooled down and you are ready to image, unplug the battery pack to turn the fan off.

Lens Group Collimation and Camera Rotation for a Perfect Field

While the RASA 36 is factory-aligned and should not require frequent adjustment, provisions are made for precise collimation. The tilt of the 4-element lens group can be adjusted using three pairs of push-pull screws, to collimate the entire optical system. It must be done with a camera installed: defocus a bright star (Polaris if possible) by about one turn of the focus knob and adjust one pair of screws at a time until the defocused star pattern is concentric.

Framing your target is also straightforward. The camera is secured by a knurled retaining ring. By slightly loosening this ring (no more than 1/8th of a turn), you can rotate the camera to the desired orientation, then retighten the ring without overtightening it.

Mounting the 75 lb OTA with Dual CGE/Losmandy-D Dovetails

The RASA 36 is a substantial instrument, with an optical tube weight of 75 lbs (34.02 kg) and a length of 1079.5mm (42.5"). Choose a mount with a load capacity of more than 75 lb, so the mount performs well when a camera and accessories are added. Moving and mounting the OTA is a two-person job, facilitated by integrated handles on the rear cell.

To provide flexible and secure mounting, the OTA is equipped with two CGE/Losmandy D-style dovetail bars. The main rail attaches the telescope to the mount, and the accessory rail can carry an optional guidescope. The rear cell has pairs of threaded holes for a Celestron finderscope and two sets of two M8x1.25 threaded inserts for custom mounting.

Included Components and Optional Accessories for the RASA 36

The RASA 36 V2 system arrives ready for camera integration. The primary included components are:

• RASA 36 V2 Optical Tube Assembly: The complete 355.6mm f/2.2 astrograph.
• M48 Camera Adapter: Provides a standard connection with 55mm of back focus.
• Fan Battery Pack: Powers the internal cooling fan (requires 8x AA batteries, not included).

Several optional accessories complete your imaging setup. A focus motor is an essential accessory you will likely want to add, and the Celestron Focus Motor is fully compatible. For dew, the easiest step is an optional 14" dew shield; in more severe conditions a dew heater can keep the corrector dry, and Celestron's dew prevention products are compatible with the RASA 36 cm.

A Step-by-Step Guide to Your First Night with the RASA 36

Getting started with the RASA 36 involves a few key steps. First, securely mount the 75 lb OTA onto a suitable high-capacity equatorial mount; this should always be done with two people. Next, attach your camera using either the included M48 adapter or a custom adapter to achieve the correct back focus. Once the camera is installed, you can adjust its rotational orientation for framing.

Before imaging, allow the telescope to cool to ambient temperature. Run the fan from the included battery pack and keep the telescope outside for an hour or so. Once cooled, point the telescope to a bright star to perform your initial focus. With focus achieved, you are ready to slew to your target and begin capturing the deep, wide-field images the RASA 36 is designed for.