Orion Optics UK ODK12 Optical Tube Assembly

ODK Astrograph: 52mm Flat Field with 1.8µm Stars

The Optimised Dall-Kirkham (ODK) design is built from the ground up for astrophotography. The ODK12 uses a 300mm primary mirror and a matched 120mm secondary, with a built-in 3-lens corrector group to eliminate off-axis coma and field curvature. This results in a corrected image circle larger than 52mm, capable of illuminating the largest commercially available astronomy camera sensors without vignetting or distortion.

The optical correction delivers incredibly tight stars across this massive field. On-axis spot sizes are a mere 1.8µm, and even at the extreme edge of the field, stars remain compact at 5.6µm. This level of correction ensures that you are capturing all the resolution the 300mm aperture can provide, making the ODK12 a premier instrument for high-resolution mosaics and large-field imaging of nebulae and galaxy clusters.

Carbon Fiber Tube & Active Cooling at 19kg

To maintain focus stability during temperature drops, the ODK12 features a carbon fibre sandwich tube. This material has virtually zero thermal expansion, preventing the 2040mm focal plane from shifting during long exposures or throughout an imaging session. The tube's open design and short 625mm length promote rapid thermal acclimation.

This process is accelerated by three controllable cooling fans mounted behind the honeycomb CNC-machined primary mirror cell. This active cooling system quickly brings the 300mm mirror to ambient temperature, minimizing tube currents and ensuring the sharpest possible images from the moment you begin observing. At approximately 19kg, the OTA is manageable for observatory-class mounts rated for higher payloads.

218mm Back Focus with a 3" Crayford Focuser

A critical feature for serious imagers is ample back focus, and the ODK12 provides an exceptionally long 218mm of working distance from the rear of the mirror cell. This generous space allows you to assemble a complete imaging train with any combination of accessories—including off-axis guiders, large filter wheels, rotators, and adaptive optics—without concern for reaching focus.

Focusing is controlled by a robust 3" Crayford focuser with a 10:1 dual-speed reduction mechanism for precise adjustments. This heavy-duty focuser is designed to carry the weight of modern large-format cameras and accessory stacks without flexure or slippage, ensuring your focal plane remains locked and perfectly orthogonal to the optical axis.

ODK12 vs. 12" Ritchey-Chrétien: Collimation and Correction

The ODK12 is often compared to Ritchey-Chrétien (RC) astrographs of similar aperture. While RCs are renowned for their corrected fields, they have a key disadvantage.

• RC Advantage: Ritchey-Chrétien telescopes typically feature longer focal ratios (f/8 or slower), which can provide more image scale for smaller targets without the need for Barlow lenses.
• ODK12 Advantage: The ODK is significantly easier to collimate. Unlike an RC, which requires precise alignment of both the primary and secondary mirrors, the ODK's primary mirror is fixed. Collimation is achieved by adjusting the secondary mirror only, a much simpler and more stable process. Furthermore, the ODK's 3-lens corrector is an integrated part of the design, whereas many RCs require an additional, separate corrector to achieve a fully flat field.