Askar FRA600 108mm f/5.6 Quintuplet APO

Quintuplet Petzval Optics: A 66mm Flat-Field Image Circle

The core of the FRA600 is its 5-element (quintuplet) optical system. This advanced Petzval design uses a front triplet and a rear doublet, including two ED elements, to achieve a high degree of apochromatic correction. The result is a 600mm focal length system that produces a 66mm image circle, fully illuminating and correcting for field curvature on sensors far larger than the APS-C format, including full-frame and medium-format chips.

• Built-in Flattener: Unlike traditional refractors that require a separate, correctly-spaced flattener, the FRA600's correction is integral to the design. This eliminates a common source of tilt and spacing errors in the imaging train.
• Apochromatic Correction: The inclusion of two ED (Extra-low Dispersion) glass elements minimizes chromatic aberration, ensuring that stars are tight, high-contrast points without distracting purple or green fringing.
• Full Illumination: The rear lens group is intentionally oversized to reduce vignetting and maintain even field illumination out to the edge of the 66mm circle, critical for producing clean, gradent-free images.

A Portable 493mm Design with a 6.5kg Payload

Engineered for both backyard and field use, the FRA600 packs its 108mm aperture into a surprisingly compact and manageable form factor. With its dew shield retracted, the optical tube measures just 493mm in length, making it easy to transport and mount. The total weight, including the supplied tube rings and dovetail, is only 6.5kg, placing it well within the capacity of many modern portable equatorial mounts.

A robust rack-and-pinion focuser provides the rigidity needed to carry heavy imaging trains without slipping or flexure. The focuser features a 1:10 micro-adjustment knob for precise critical focus, a 360-degree rotator for camera framing, and a series of threaded adapters to ensure a solid connection to your equipment.

160mm Back Focus and a Multi-Thread Adapter System

The FRA600 provides a generous 160mm of back focus measured from the M90x1 thread on the focuser rotator. This ample distance accommodates complex imaging setups, including off-axis guiders, filter wheels, and rotators. To simplify connections, Askar includes a conical adapter that provides M68x1, M54x0.75, and M48x0.75 male threads, covering the most common standards for flatteners, cameras, and other accessories.

• Secure Threaded Connections: The system is designed around threaded adapters rather than compression rings, preventing camera tilt and ensuring a rigid optical path from the focuser to the sensor.
• Included Adapter Versatility: The conical adapter directly accommodates full-frame cameras with M48 threads and provides larger M54 and M68 threads for connecting to other specialized equipment.
• Standard 2" and 1.25" Adapters: For visual use or connecting planetary cameras, standard 2" and 1.25" adapters are also included, making the scope ready for observation out of the box.

Askar FRA600 vs. Triplet APOs: The Built-in Flattener Advantage

While a premium 108mm triplet APO can deliver excellent color correction, it almost always requires an external field flattener to produce a flat field for astrophotography. The Askar FRA600's quintuplet design integrates this function into the main optics, offering distinct advantages.

• System Simplicity: You don't need to purchase and correctly space a separate flattener. The FRA600 is flat by design, simplifying your imaging train and reducing potential points of failure.
• Optimized Correction: The optical prescription is a single, cohesive system. The lenses are designed from the ground up to work together, providing correction across a massive 66mm circle that is often difficult to achieve with bolt-on accessories.
• Cost-Effectiveness: When factoring in the cost of a high-quality, large-format field flattener for a comparable triplet, the integrated design of the FRA600 often represents a superior value for the dedicated astro-imager.