Askar 91F Flat-Field Astrograph

David Astro Verified independent store

→

91mm Aperture Quadruplet APO Optics 609mm Focal Length at f/6.7 44mm Full-Frame Corrected Image Circle Integrated Field Flattener Design 3.92kg Net Weight Description FAQ Specifica tions In the Box Downloads Warranty Askar 91F Flat-Field Astrograph The Askar 91F astrograph integrates a 91mm aperture with a 609mm focal length for a native f/6.7 optical system. Its quadruplet APO objective, which includes one ED glass element, delivers a flat, color-corrected 44mm image circle that illuminates full-frame sensors without an external flattener. The entire optical tube weighs only 3.92kg, creating a portable yet powerful imaging instrument. A 91mm f/6.7 Quadruplet APO with a 44mm Image Circle The core of the Askar 91F is its self-flattened optical design. Unlike traditional refractors that require a separate, carefully spaced field flattener, this quadruplet system has the correction built in. This eliminates the need to calculate back focus or purchase additional accessories, allowing you to connect your camera and start imaging immediately after achieving focus. The 44mm image circle is specifically designed to cover full-frame CMOS and DSLR sensors, ensuring sharp, pinpoint stars from the center to the corners of your frame. The inclusion of one ED (Extra-low Dispersion) glass element in the air-spaced objective effectively suppresses chromatic aberration, resulting in high-contrast images with true-to-life color rendition, free of distracting purple fringing around bright stars. 3-inch Rack-and-Pinion Focuser with 360° Rotation To support heavy full-frame cameras and filter wheels without flexure, the Askar 91F is equipped with a rigid 3-inch dual-speed rack-and-pinion focuser. The 1:10 fine adjustment knob allows for critical focus, while the 30mm of focuser travel accommodates a wide range of accessories. A 360° rotator at the rear of the focuser lets you frame your target perfectly without rotating the entire optical tube. Included Adapters: The package comes with a set of photographic adapters, including M54x0.75 and M48x0.75 connections, ensuring compatibility with most popular astronomy cameras and T-rings. The M48 adapter also includes a thread for 2-inch filters. Visual Versatility: For visual use, the scope includes standard 2-inch and 1.25-inch adapters, making it a capable instrument for observing star clusters and nebulae, not just for imaging them. A 3.92kg Lightweight Design with Dual Finder Bases Engineered for portability, the Askar 91F's main tube weighs just 3.92kg (4.52kg with rings and plate), making it an excellent match for mid-sized equatorial mounts. The retractable dew shield reduces the telescope's length to as little as 513mm for transport and storage. Thoughtful mechanical details enhance usability in the field. Dual Finder Mounts: Two finder bases are mounted on either side of the focuser, providing flexible mounting options for a guide scope and a finder scope simultaneously. Mounting Hardware: The included 230mm Vixen-style dovetail plate provides a secure connection to your mount and features 1/4" and 3/8" screw holes for direct attachment to a photo tripod for terrestrial viewing. Askar 91F vs. Triplet APOs: The Advantage of an Integrated Flattener When comparing the Askar 91F to a traditional triplet apochromat like the William Optics FLT 91, the primary distinction is the optical design philosophy. While both deliver excellent color correction, the triplet design requires an additional, separately purchased field flattener to achieve a flat field for astrophotography. Simplicity and Cost: The Askar 91F's integrated flattener means there's no complex back-focus spacing to calculate and no extra accessory to buy. This simplifies the imaging train and reduces overall cost. Guaranteed Correction: Because the flattener is part of the objective design, the optical correction is perfectly matched to the telescope, eliminating potential issues with third-party flatteners. Trade-offs: A triplet with a separate flattener/reducer can offer multiple focal ratios. The Askar 91F is optimized for its native 609mm focal length, prioritizing out-of-the-box imaging readiness over focal length flexibility. What is the main benefit of the Askar 91F's quadruplet design? The primary benefit is its self-flattened optical system. Unlike standard refractors that require a separate accessory, the Askar 91F has the field flattener built into its quadruplet objective. This ensures a perfectly flat 44mm image circle for full-frame cameras without the need to calculate back focus or buy extra components. Do I need to calculate back focus for the Askar 91F? No. The integrated flattener design means you do not need to calculate back focus. Simply attach your camera using the provided adapters, bring the telescope into focus using the focuser, and you are ready to image. How does the Askar 91F perform on full-frame cameras like a Sony A7 series? The Askar 91F is designed specifically for full-frame cameras. Its 44mm corrected image circle fully illuminates the sensor of a Sony A7, Canon R, or Nikon Z series camera, providing pinpoint stars across the entire frame without vignetting or distorted stars in the corners. What kind of mount is needed for the 4.52kg Askar 91F OTA? With a fully loaded weight of 4.52kg (including rings and dovetail plate), the Askar 91F is well-suited for mid-sized equatorial mounts with a payload capacity of 25-30 lbs (11-14 kg) or more. Mounts like the Sky-Watcher HEQ5 or iOptron CEM26/GEM28 can carry this telescope and associated imaging gear with excellent stability. Can I use the Askar 91F for visual observation of planets like Jupiter? Yes. While optimized for astrophotography, its high-quality APO optics make it a very capable visual instrument. At 609mm, it provides medium power for observing Jupiter's moons and cloud bands or Saturn's rings. You would simply use the included 1.25" or 2" adapter with your choice of eyepieces. What do the different adapter threads on the Askar 91F connect to? The Askar 91F includes a set of adapters to connect various devices. The primary connection types are: M64×1: The thread on the focuser drawtube itself. M54×0.75: A common thread size for many larger-format astronomy cameras and filter wheels. M48×0.75: The standard thread for 2" filters and the most common connection for full-frame camera T-rings. Aperture 91mm Focal Length 609mm Focal Ratio f/6.7 Objective Lens Quadruplet air-spaced APO (including one ED glass) Image Circle 44mm Focuser 3-inch dual-speed Rack & Pinion Focuser Travel 30mm Net Weight (OTA only) 3.92 kg Gross Weight (w/ rings & plate) 4.52 kg Total Length (Visual Config) 513mm (retracted) / 599mm (extended) Total Length (Photo Config) 550mm (retracted) / 636mm (extended) Rear-end Adapters M64x1, M54x0.75, M48x0.75 (with 2" filter thread) Download Manual Askar 91F OTA × 1 Tube Rings × 1 Pair Carry Handle × 1 230mm Vixen Dovetail Plate × 1 Photographic Adapter Set × 1 Download Manual Askar 1-Year Warranty Show more