Orion Optics UK ODK14Tr Truss Optical Tube Assembly

ODK Optics: A 350mm f/6.8 System with a >52mm Corrected Field

The heart of the ODK14Tr is its Optimized Dall-Kirkham optical system, which pairs an ellipsoidal primary mirror with a spherical secondary. This configuration is inherently easier to figure to a high degree of smoothness than more complex aspheric designs. The system's native coma is fully addressed by a dedicated, multi-coated 3-lens corrector, resulting in a diffraction-limited, flat field across a massive >52mm image circle.

This optical design delivers incredibly sharp stars, with a measured spot size of just 2.1μm on-axis and 5.6μm at the edge of the field. This ensures that the telescope's resolution is not the limiting factor, even when paired with modern CMOS cameras that have very small pixels. The 350mm aperture gathers significant light, while the f/6.8 focal ratio provides a balance of image scale and exposure speed suitable for imaging everything from distant galaxy clusters to faint nebulae.

Truss Structure: Carbon Fibre Sandwich Construction at 26kg

The ODK14Tr utilizes an open truss framework made from a carbon fibre sandwich material, a design choice that offers superior thermal performance and rigidity. Unlike a solid tube, the open design prevents the formation of tube currents and allows the optics to acclimate to ambient temperatures quickly. The carbon fibre construction minimizes focus shift caused by temperature changes, a critical factor for maintaining sharpness over long imaging sessions.

Despite its large 400mm tube diameter, the OTA weighs approximately 26kg and has a compact length of 700mm, making it manageable for high-capacity observatory-class mounts. The primary mirror is housed in a honeycomb CNC machined aluminum cell, providing rigid support without inducing mechanical stress on the optics.

Thermal Control: Active Cooling with 3 Controllable Fans

Achieving thermal equilibrium is essential for high-resolution imaging, and the ODK14Tr addresses this directly with an integrated active cooling system. Three controllable fans are positioned behind the primary mirror to accelerate the cooling process, drawing air across the back of the 350mm mirror to bring it to ambient temperature rapidly.

By minimizing the temperature differential between the glass and the surrounding air, this system drastically reduces mirror seeing—the localized turbulence that can soften fine details. This allows you to start imaging sooner and maintain optimal optical performance throughout the night. For observers in dew-prone locations, optional heaters can be integrated for complete environmental control.

Imaging Train Support: 210mm Back Focus and a 3" Crayford Focuser

The ODK14Tr is designed to accommodate complex and heavy imaging trains. It provides a generous 210mm of back focus, measured from the rear of the mirror cell. This extensive travel leaves ample room for accessories like off-axis guiders, large filter wheels, rotators, and large-format cameras without concern for reaching focus.

Focusing is handled by a robust 3" Crayford focuser with a 10:1 reduction ratio. The large diameter prevents vignetting on full-frame and larger sensors, while the fine-focus knob allows for the precise adjustments necessary to achieve critical focus at the 2380mm focal length. The entire assembly attaches to your mount via a sturdy, CNC-machined aluminum dovetail plate with integrated supports for maximum stability.

ODK14Tr vs. CDK Designs: Understanding Astrograph Trade-Offs

When choosing a high-end astrograph, the Orion Optics UK Optimized Dall-Kirkham (ODK) is often compared to Corrected Dall-Kirkham (CDK) designs. Both aim for a large, flat, coma-free field, but they achieve it through different optical philosophies.

• ODK Approach: The ODK uses an ellipsoidal primary and a spherical secondary. A spherical surface is the simplest to manufacture to an extremely high degree of smoothness, minimizing scatter. The significant off-axis coma inherent in this design is then eliminated with a dedicated 3-lens corrector placed before the focal plane.
• CDK Approach: A CDK typically uses an ellipsoidal primary and an aspheric (hyperbolic) secondary, along with a corrector. The aspheric secondary corrects for coma natively, reducing the workload of the corrector lenses. The trade-off is the increased complexity and cost of figuring the aspheric secondary surface.

The ODK14Tr's design represents a pragmatic choice, prioritizing the manufacturability of its core components to achieve exceptional surface quality while using a powerful, modern corrector to deliver a final image that is sharp and flat across its >52mm field.