Orion Optics UK ODK14 Optical Tube Assembly

A 350mm f/6.8 Astrograph with a >52mm Flat Field

The core of the ODK14 is its Optimized Dall-Kirkham optical design, engineered specifically for astrophotography. The 350mm primary mirror gathers significant light, while the 2380mm focal length provides the image scale needed for high-resolution views of galaxies and nebulae. Unlike standard reflectors, the ODK14 incorporates a permanent three-lens corrector, which creates a diffraction-limited, color-corrected field of over 52mm—large enough to serve medium-format CCDs and full-frame CMOS sensors without vignetting or distortion.

This optical correction results in remarkably tight stars across the entire frame. On-axis, stars are focused to a spot size of just 2.1µm, expanding to only 5.6µm at the extreme edge of the field. This ensures that whether you are imaging the core of the Hercules Cluster (M13) or the faint outer arms of the Triangulum Galaxy (M33), every star is a pinpoint.

400mm Carbon Sandwich Tube and CNC Cell

To maintain optical precision, the ODK14 is built around a carbon fiber sandwich tube with a 400mm diameter. This material provides extreme rigidity at a manageable weight of approximately 26kg and, more importantly, has a near-zero coefficient of thermal expansion. This prevents the 2380mm focal plane from shifting as temperatures drop overnight, eliminating the need for constant refocusing during an imaging session.

The 350mm primary mirror is supported by a honeycomb, CNC-machined aluminum cell that prevents flexure at any orientation. To achieve rapid thermal equilibrium, three controllable fans are positioned behind the mirror cell, quickly cooling the primary mirror to ambient temperature for sharp, stable views. This open-tube design cools down significantly faster than a comparable closed-tube Schmidt-Cassegrain.

210mm of Back Focus and a 3" Crayford Focuser

The ODK14 is designed to integrate seamlessly into a modern imaging setup. It provides a massive 210mm of back focal distance, measured from the rear of the mirror cell. This generous spacing allows you to easily install any combination of accessories in the optical train, including off-axis guiders, large filter wheels, rotators, and adaptive optics units, without concern for reaching focus.

Focusing is controlled by a heavy-duty 3" Crayford focuser with a 10:1 reduction for precise fine adjustments. This focuser is capable of supporting heavy, large-format camera packages without slipping or tilting, ensuring the sensor remains perfectly orthogonal to the light path. The entire assembly is mounted using a CNC-machined aluminum dovetail system for a secure connection to your equatorial mount.

ODK14 vs. Ritchey-Chrétien: Corrected Field and Collimation

The primary competitor for an astrograph like the ODK14 is a traditional Ritchey-Chrétien (RC) telescope. While RCs are known for their coma-free fields, they typically require a separate, costly field flattener to correct for significant off-axis astigmatism and produce a truly flat field for imaging.

• Integrated Correction: The ODK14's built-in three-lens corrector delivers a flat, >52mm corrected field out of the box. An RC of similar aperture often requires an additional corrector, adding complexity and cost to the imaging train.
• Spot Size: Thanks to its corrective elements, the ODK14 maintains tight, round stars to the edge of the field, with a specified spot size of only 5.6µm. This is often superior to what an RC with a generic flattener can achieve.
• Collimation: Ritchey-Chrétien telescopes are notoriously sensitive to collimation, requiring precise alignment of both the primary and secondary hyperbolic mirrors. The ODK design is generally more forgiving to align, allowing you to spend more time imaging and less time adjusting optics.