Askar 131F 0.74× Focal Reducer

Askar 131F 0.74× Focal Reducer — For the Askar 131F Flat-Field Astrograph

The Askar 131F 0.74× Reducer is a dedicated focal reducer designed exclusively for the Askar 131F Flat-Field astrograph. It reduces the 131F’s native f/7.5 focal ratio down to f/5.6, shortening the effective focal length from 983mm to 733.6mm. This delivers a wider field of view and significantly faster exposures — roughly 80% of the original exposure time — making it particularly valuable for capturing large nebulae, expansive star fields, faint emission structures, and fast-moving or time-sensitive celestial events.

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

The optical design uses a triplet lens group for smooth, well-corrected reduction across the full field. The standard back focus distance is 55mm measured from the M48×0.75 male thread. Both ends of the reducer body feature M72×1 threads. The package includes M54×0.75 and M48×0.75 adapter rings for direct connection to the 131F’s photographic extension tube, and 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.

Who Is This Product Best For?

• Askar 131F owners who image large nebulae, emission regions, or wide star fields and want a broader field of view without changing telescopes.
• Imagers in light-polluted skies who benefit from the faster f/5.6 ratio to shorten sub-exposure times.
• Narrowband and broadband filter users who want the faster ratio for more efficient filter imaging.
• Astrophotographers who want to switch between the native f/7.5 (for tighter framing) and f/5.6 (for wider field) without buying a second telescope.

Why Choose the Askar 131F 0.74× Reducer?

• Dedicated design — optically optimized specifically for the 131F, not a generic reducer. Delivers consistent correction across the full 44mm image circle at the reduced ratio.
• Built-in field flattening — no additional flattener needed at f/5.6; the reducer handles both reduction and correction simultaneously.
• Faster imaging — f/5.6 vs f/7.5 means roughly 44% less exposure time needed for the same signal level.
• Wider field — 733.6mm focal length vs 983mm gives approximately 26% more field width in each direction.
• Standard back focus — 55mm from M48 thread eliminates guesswork; direct adapter connections included in the box.
• Filter-ready rear — built-in M48×0.75 thread accepts standard 2" astronomy filters without an additional adapter.
• CNC precision body with dust caps — professional-grade build quality with protective metal end caps.

Recommended Uses

• Wide-field deep-sky nebula imaging with the Askar 131F
• Large emission and reflection nebulae (Orion, Rosette, North America, etc.)
• Galaxy groups and clusters requiring wider framing
• Narrowband and broadband filter imaging in light-polluted sites
• Time-lapse or short-exposure widefield astrophotography

Compatibility and Accessory Notes

Compatible telescope: Askar 131F Flat-Field only. Not designed for use with any other telescope.

Connection: The 131F’s photographic extension tube connects directly to the reducer via the included M54×0.75 or M48×0.75 adapter rings. Both end threads on the reducer body are M72×1.

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: The built-in rear M48×0.75 thread accepts standard 2" (48mm) astronomy filters directly.

Camera compatibility: Works with any camera system that connects via the included M54 or M48 adapter rings — DSLRs with T-rings, and dedicated astronomy cameras with M54 or M48 nose threads.

Important Limitations

• 131F only — this reducer is not compatible with the 91F, 111F, or any other telescope. Each F Series model has its own dedicated reducer.
• Not for visual observing — focal reducers are imaging-only accessories and are not used with eyepieces.
• 55mm back focus must be respected — using spacers or adapters that change the back focus distance from the specified 55mm will degrade corner star quality.

Frequently Asked Questions

What does the 0.74× reducer do to the 131F?
It reduces the focal length from 983mm to 733.6mm and the focal ratio from f/7.5 to f/5.6, giving a wider field of view and faster exposures.

Does it work on the 91F or 111F?
No. Each F Series telescope has its own dedicated reducer. The 91F uses the 0.75× Reducer and the 111F uses the 0.74× Reducer — both are different products.

Do I still need a field flattener at f/5.6?
No. The reducer has field-flattening correction built in and covers the full 44mm image circle with sharp stars to the corners.

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

Our Recommendation

At Ontario Telescope and Accessories, we consider the Askar 131F 0.74× Reducer a worthwhile companion purchase for any 131F owner who does wide-field deep-sky imaging. The improvement in field width and exposure speed is significant, and the fact that it handles both reduction and field flattening in one unit keeps the imaging train simple. If you own a 131F and shoot large nebulae or work under moderate light pollution, this reducer will meaningfully expand what you can do with the telescope.