Askar 91F Flat-Field Quadruplet Astrograph

Askar 91F Flat-Field Quadruplet Astrograph — 91mm f/6.7

The Askar 91F Flat-Field is a 91mm aperture, 609mm focal length quadruplet APO astrograph and the most accessible telescope in the Askar F Series. At a native f/6.7 focal ratio, it combines a compact and lightweight build — just 3.92kg OTA — with the same self-flattening, full-frame-capable optical performance that defines the entire F Series lineup.

The four-element optical design (three front elements, one rear correcting element) includes one piece of ED glass to suppress chromatic aberration. The system natively supports a 44mm image circle covering full-frame sensors with no external flattener required and no backfocus calculation needed. Simply attach your camera, achieve focus, and begin imaging.

The 91F is the most complete package in the F Series out of the box: in addition to the standard photographic extension tube with M48×0.75 and M54×0.75 adapters, it ships with an 8mm eyepiece (76×), a 20mm eyepiece (30×), and a 1.25" 45° erecting prism — making it immediately ready for visual observing, birdwatching, and terrestrial use the moment it arrives.

The mechanical package includes a 3" R&P focuser with 1:10 fine adjustment, a 360° camera rotator, a retractable dew shield, a matting-felt-lined tube interior, two finder bases on the focuser plus a handle slot, and a 230mm Vixen-style dovetail plate with 1/4" and 3/8" tripod holes. The exterior features the signature Askar blue-and-white CNC-machined finish.

Who Is This Product Best For?

• Beginner to intermediate astrophotographers who want a full-frame-capable, self-flattening instrument with no setup complexity.
• Travel and portable rig users — at 3.92kg OTA and a maximum length under 640mm with dew shield extended, it pairs well with compact travel mounts like the ZWO AM5, iOptron HEM27, Sky-Watcher Star Adventurer GTi, and similar.
• Dual-purpose visual and imaging users — the included eyepiece and erecting prism kit makes it ready for daytime spotting and nighttime visual observing right out of the box.
• Birders and nature observers looking for a versatile, optically excellent spotting scope that doubles as an astrograph.
• Deep-sky imagers on smaller mounts who need a flat-field full-frame instrument within a tight weight budget.

Why Choose the Askar 91F?

• Self-flattening quadruplet — no separate flattener, no backfocus calculation. Attach and shoot.
• 44mm full-frame image circle — covers all modern full-frame DSLR, mirrorless, and dedicated astronomy sensors.
• Lightest F Series telescope — at 3.92kg OTA (4.52kg gross), suitable for the widest range of mounts including travel-grade systems.
• Most complete standard package — the 91F includes 8mm and 20mm eyepieces plus a 1.25" erecting prism, unlike the larger F Series models.
• ED glass chromatic control — one ED element minimizes false colour across the full image field.
• Built-in 360° rotator — frame targets at any angle without repositioning the mount.
• Matting felt interior — stray light absorption lining inside the tube for better contrast.
• Retractable dew shield — compact for transport, extended for use.
• Tripod-mountable dovetail — 230mm Vixen plate with 1/4" and 3/8" screw holes for direct tripod use.

Recommended Uses

• Wide-field deep-sky astrophotography (nebulae, star clusters, large emission regions)
• Full-frame DSLR, mirrorless, and dedicated astronomy camera imaging
• Narrowband and broadband filter imaging (built-in 2" filter thread)
• Visual deep-sky observing with included 8mm and 20mm eyepieces
• Birdwatching, wildlife, and terrestrial spotting
• Travel and portable mount setups
• Beginner astrophotography and astronomy outreach

Compatibility and Accessory Notes

Camera connections: Ships with M48×0.75 and M54×0.75 adapters. DSLR users need a standard T-ring. Dedicated astronomy cameras with M54 or M48 connections attach directly. Built-in 2" filter thread on the extension tube accepts standard astronomy filters.

Mount compatibility: The 230mm Vixen dovetail fits any Vixen-compatible saddle. The 1/4" and 3/8" tripod holes allow direct tripod mounting without a mount at all. At 4.52kg gross, compatible with lightweight mounts including ZWO AM5, iOptron HEM27, Sky-Watcher HEQ5, Star Adventurer GTi, and similar. A mount rated for at least 6–8kg payload is recommended for imaging.

Optional accessories: The Askar 91F 0.75× Reducer (sold separately) brings the system to f/5 and 455mm focal length for wider-field, faster imaging. An electronic focuser motor is available separately.

Included visual accessories: 8mm eyepiece (76× magnification), 20mm eyepiece (30× magnification), and 1.25" 45° erecting prism for correct-image terrestrial and astronomical viewing.

Important Limitations

• Smaller aperture than 111F/131F — 91mm collects less light than larger F Series instruments; exposures will be longer on faint targets. Consider the optional 0.75× reducer or a larger telescope for very faint, small-scale objects.
• f/6.7 is moderate speed — not ultrafast; the optional 0.75× reducer (sold separately) brings it to f/5 for shorter exposures.
• Not ideal for planetary imaging — the 609mm focal length is optimized for wide-field deep sky, not high-magnification planetary detail.
• No electronic focuser included — motorized focus requires a separately purchased motor.

Frequently Asked Questions

Do I need a flattener?
No. The self-flattening quadruplet produces a flat field across the 44mm image circle natively.

Does it cover full-frame sensors?
Yes. The 44mm image circle fully covers all current full-frame DSLR, mirrorless, and dedicated astronomy cameras.

Is it good for beginners?
Yes — it is the most beginner-friendly instrument in the F Series. The included eyepieces and erecting prism mean it can be used for visual observing immediately, and the self-flattening design removes the most common astrophotography setup hurdle.

What mount do I need?
For imaging, a mount rated for at least 6–8kg payload is recommended. For visual use only, it can be used on a camera tripod via the built-in tripod screw holes.

Is a reducer available?
Yes. The Askar 91F 0.75× Reducer (sold separately) reduces the system to f/5 and 455mm focal length.

How does it compare to the 111F?
The 91F is lighter (3.92kg vs 5.74kg), shorter, includes eyepieces and an erecting prism, and is better suited for smaller mounts and travel. The 111F has more aperture and a longer focal length for more resolution on smaller targets.

Can I use it for birdwatching?
Yes. The included 20mm eyepiece and erecting prism give a correct-image view for terrestrial spotting. It functions as an excellent spotting scope in addition to its astronomy role.

Our Recommendation

At Ontario Telescope and Accessories, we recommend the Askar 91F Flat-Field as the best entry point into the F Series for most buyers. It is the lightest, most portable, and most complete package — ready for visual observing, birdwatching, and astrophotography the moment it arrives. If you are using a compact or travel mount, or you want one instrument that handles astronomy and daytime optics equally well, the 91F is an outstanding choice.

If you need more aperture for fainter targets or higher resolution and have a heavier mount, consider the 111F or 131F. But for first-time F Series buyers and travel users, the 91F delivers the full F Series experience in the most accessible package.

Optical Diagram Reports

The following optical and mechanical diagrams are provided for customers who wish to review the Askar 91F’s optical performance, mechanical dimensions, and connection specifications.