Askar 91F 0.75× Focal Reducer

Askar 91F 0.75× Focal Reducer — For the Askar 91F Flat-Field Astrograph

The Askar 91F 0.75× Reducer is a dedicated focal reducer designed exclusively for the Askar 91F Flat-Field astrograph. It reduces the 91F’s native f/6.7 focal ratio down to f/5, shortening the effective focal length from 609mm to 455mm. This delivers a noticeably wider field of view and a faster optical system that meaningfully reduces exposure times — particularly useful for large nebulae, wide star fields, and imaging from light-polluted locations.

Alongside focal reduction, the reducer incorporates built-in field-flattening correction that eliminates field curvature across the full 44mm image circle. Stars are sharp and round from the centre to the corners of a full-frame sensor, with no additional flattener required at the reduced ratio.

The optical design uses a triplet lens group for smooth, well-corrected reduction. The standard back focus distance is 55mm from the M48×0.75 male thread. Both ends of the reducer body feature M59×1 threads. The 91F’s photographic extension tube connects directly via the included M54×0.75 and M48×0.75 adapter rings. The rear adapter ring has a built-in M48×0.75 standard 2" filter thread for direct filter installation. The body is precision CNC-machined and ships with protective metal thread caps on both ends to guard against dust when not in use.

Who Is This Product Best For?

• Askar 91F owners who want a wider field of view and faster exposures for large nebulae and star fields.
• Imagers in light-polluted skies who benefit from the faster f/5 ratio to shorten sub-exposure times.
• Travel astrophotographers already using the compact 91F who want to maximise field width on a lightweight setup.
• Narrowband and broadband filter users who want more imaging speed from the 91F system.
• Users wanting flexibility between the native f/6.7 (tighter framing, higher resolution) and f/5 (wider field, faster) without a second telescope.

Why Choose the Askar 91F 0.75× Reducer?

• Dedicated design — optically optimized specifically for the 91F for consistent, well-corrected performance across the full 44mm image circle at f/5.
• Built-in field flattening — one accessory handles both reduction and corner correction simultaneously.
• Significantly faster system — f/5 vs f/6.7 means roughly 44% less exposure time required for the same signal.
• Wider field — 455mm vs 609mm focal length gives approximately 25% more field width per axis.
• Standard 55mm back focus — no spacing guesswork; direct adapter connections are included.
• Filter-ready rear thread — built-in M48×0.75 accepts standard 2" astronomy filters directly.
• Lightest F Series reducer — at just 0.3kg, adds virtually no weight penalty to the 91F travel setup.
• CNC body with protective caps — professional build quality with dust-proof metal end caps for storage and transport.

Recommended Uses

• Wide-field deep-sky nebula and star cluster imaging with the Askar 91F
• Large emission and reflection nebulae (Orion, Rosette, Heart and Soul, etc.)
• Milky Way core and wide star field imaging
• Narrowband and broadband filter imaging from light-polluted sites
• Portable and travel astrophotography setups

Compatibility and Accessory Notes

Compatible telescope: Askar 91F Flat-Field only. Not compatible with the 111F, 131F, or any other telescope.

Connection: Connects via the 91F’s photographic extension tube using the included M54×0.75 or M48×0.75 adapter rings. Both ends of the reducer body are M59×1 threads.

Back focus: 55mm from the base of the M48×0.75 male thread. No additional spacing calculation required when using the supplied adapters.

Filter compatibility: Built-in rear M48×0.75 thread accepts standard 2" astronomy filters directly.

Important Limitations

• 91F only — not compatible with the 111F (which uses a 0.74× reducer with M68 threads) or the 131F (which uses a 0.74× reducer with M72 threads).
• Not for visual observing — focal reducers are imaging-only accessories and are not used with eyepieces.
• 55mm back focus must be respected — using additional spacers that alter the back focus from 55mm will degrade corner star quality.

Frequently Asked Questions

What does this reducer do to the 91F?
It reduces the focal length from 609mm to 455mm and the focal ratio from f/6.7 to f/5, giving a wider field of view and faster exposures.

Does it work on the 111F or 131F?
No. Each F Series telescope has its own dedicated reducer with different thread sizes. The 111F uses a 0.74× Reducer (M68 thread) and the 131F uses a 0.74× Reducer (M72 thread).

Do I need a flattener at f/5?
No. Field flattening is built into the reducer and covers the full 44mm image circle.

What is the back focus?
55mm from the base of the M48×0.75 male thread on the rear of the reducer.

Is this suitable for a travel setup?
Yes. At 0.3kg it adds minimal weight and the compact size fits easily into any imaging kit bag.

Our Recommendation

At Ontario Telescope and Accessories, we recommend the Askar 91F 0.75× Reducer as an ideal companion for the 91F — particularly for imagers who primarily target large nebulae or who shoot from light-polluted sites where a faster focal ratio makes a real practical difference. At f/5, the 91F system becomes one of the most capable and portable wide-field full-frame imaging setups available at this price point. For 91F owners who haven’t yet added a reducer, this is the single most impactful accessory purchase you can make for the telescope.