{"product_id":"askar-91f-0-75-focal-reducer","title":"Askar 91F 0.75× Focal Reducer","description":"\u003ch2\u003eAskar 91F 0.75× Focal Reducer — For the Askar 91F Flat-Field Astrograph\u003c\/h2\u003e\n\n\u003cp\u003eThe \u003cstrong\u003eAskar 91F 0.75× Reducer\u003c\/strong\u003e 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.\u003c\/p\u003e\n\n\u003cp\u003eAlongside 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.\u003c\/p\u003e\n\n\u003cp\u003eThe 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.\u003c\/p\u003e\n\n\u003ch2\u003eWho Is This Product Best For?\u003c\/h2\u003e\n\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003eAskar 91F owners\u003c\/strong\u003e who want a wider field of view and faster exposures for large nebulae and star fields.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eImagers in light-polluted skies\u003c\/strong\u003e who benefit from the faster f\/5 ratio to shorten sub-exposure times.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eTravel astrophotographers\u003c\/strong\u003e already using the compact 91F who want to maximise field width on a lightweight setup.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eNarrowband and broadband filter users\u003c\/strong\u003e who want more imaging speed from the 91F system.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eUsers wanting flexibility\u003c\/strong\u003e between the native f\/6.7 (tighter framing, higher resolution) and f\/5 (wider field, faster) without a second telescope.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003eWhy Choose the Askar 91F 0.75× Reducer?\u003c\/h2\u003e\n\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003eDedicated design\u003c\/strong\u003e — optically optimized specifically for the 91F for consistent, well-corrected performance across the full 44mm image circle at f\/5.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eBuilt-in field flattening\u003c\/strong\u003e — one accessory handles both reduction and corner correction simultaneously.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eSignificantly faster system\u003c\/strong\u003e — f\/5 vs f\/6.7 means roughly 44% less exposure time required for the same signal.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eWider field\u003c\/strong\u003e — 455mm vs 609mm focal length gives approximately 25% more field width per axis.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eStandard 55mm back focus\u003c\/strong\u003e — no spacing guesswork; direct adapter connections are included.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eFilter-ready rear thread\u003c\/strong\u003e — built-in M48×0.75 accepts standard 2\" astronomy filters directly.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eLightest F Series reducer\u003c\/strong\u003e — at just 0.3kg, adds virtually no weight penalty to the 91F travel setup.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eCNC body with protective caps\u003c\/strong\u003e — professional build quality with dust-proof metal end caps for storage and transport.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003eRecommended Uses\u003c\/h2\u003e\n\n\u003cul\u003e\n  \u003cli\u003eWide-field deep-sky nebula and star cluster imaging with the Askar 91F\u003c\/li\u003e\n  \u003cli\u003eLarge emission and reflection nebulae (Orion, Rosette, Heart and Soul, etc.)\u003c\/li\u003e\n  \u003cli\u003eMilky Way core and wide star field imaging\u003c\/li\u003e\n  \u003cli\u003eNarrowband and broadband filter imaging from light-polluted sites\u003c\/li\u003e\n  \u003cli\u003ePortable and travel astrophotography setups\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003eCompatibility and Accessory Notes\u003c\/h2\u003e\n\n\u003cp\u003e\u003cstrong\u003eCompatible telescope:\u003c\/strong\u003e Askar 91F Flat-Field only. Not compatible with the 111F, 131F, or any other telescope.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eConnection:\u003c\/strong\u003e 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.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eBack focus:\u003c\/strong\u003e 55mm from the base of the M48×0.75 male thread. No additional spacing calculation required when using the supplied adapters.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eFilter compatibility:\u003c\/strong\u003e Built-in rear M48×0.75 thread accepts standard 2\" astronomy filters directly.\u003c\/p\u003e\n\n\u003ch2\u003eImportant Limitations\u003c\/h2\u003e\n\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003e91F only\u003c\/strong\u003e — 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).\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eNot for visual observing\u003c\/strong\u003e — focal reducers are imaging-only accessories and are not used with eyepieces.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003e55mm back focus must be respected\u003c\/strong\u003e — using additional spacers that alter the back focus from 55mm will degrade corner star quality.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003eFrequently Asked Questions\u003c\/h2\u003e\n\n\u003cp\u003e\u003cstrong\u003eWhat does this reducer do to the 91F?\u003c\/strong\u003e\u003cbr\u003eIt 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.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eDoes it work on the 111F or 131F?\u003c\/strong\u003e\u003cbr\u003eNo. 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).\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eDo I need a flattener at f\/5?\u003c\/strong\u003e\u003cbr\u003eNo. Field flattening is built into the reducer and covers the full 44mm image circle.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eWhat is the back focus?\u003c\/strong\u003e\u003cbr\u003e55mm from the base of the M48×0.75 male thread on the rear of the reducer.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eIs this suitable for a travel setup?\u003c\/strong\u003e\u003cbr\u003eYes. At 0.3kg it adds minimal weight and the compact size fits easily into any imaging kit bag.\u003c\/p\u003e\n\n\u003ch2\u003eOur Recommendation\u003c\/h2\u003e\n\n\u003cp\u003eAt Ontario Telescope and Accessories, we recommend the \u003cstrong\u003eAskar 91F 0.75× Reducer\u003c\/strong\u003e 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.\u003c\/p\u003e","brand":"Askar","offers":[{"title":"Default Title","offer_id":53154399125615,"sku":"91F-REDUCER-075X","price":278.6,"currency_code":"CAD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0686\/0049\/6239\/files\/697d8e02-5798-4712-9d20-77ec86f1ed18.png?v=1779667568","url":"https:\/\/ontariotelescope.com\/products\/askar-91f-0-75-focal-reducer","provider":"Ontario Telescope and Accessories","version":"1.0","type":"link"}