{"product_id":"askar-91f-flat-field-quadruplet-astrograph","title":"Askar 91F Flat-Field Quadruplet Astrograph","description":"\u003ch2\u003eAskar 91F Flat-Field Quadruplet Astrograph — 91mm f\/6.7\u003c\/h2\u003e\n\n\u003cp\u003eThe \u003cstrong\u003eAskar 91F Flat-Field\u003c\/strong\u003e 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.\u003c\/p\u003e\n\n\u003cp\u003eThe 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.\u003c\/p\u003e\n\n\u003cp\u003eThe 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.\u003c\/p\u003e\n\n\u003cp\u003eThe mechanical package includes a 3\" R\u0026amp;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.\u003c\/p\u003e\n\n\u003ch2\u003eWho Is This Product Best For?\u003c\/h2\u003e\n\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003eBeginner to intermediate astrophotographers\u003c\/strong\u003e who want a full-frame-capable, self-flattening instrument with no setup complexity.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eTravel and portable rig users\u003c\/strong\u003e — 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.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eDual-purpose visual and imaging users\u003c\/strong\u003e — the included eyepiece and erecting prism kit makes it ready for daytime spotting and nighttime visual observing right out of the box.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eBirders and nature observers\u003c\/strong\u003e looking for a versatile, optically excellent spotting scope that doubles as an astrograph.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eDeep-sky imagers on smaller mounts\u003c\/strong\u003e who need a flat-field full-frame instrument within a tight weight budget.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003eWhy Choose the Askar 91F?\u003c\/h2\u003e\n\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003eSelf-flattening quadruplet\u003c\/strong\u003e — no separate flattener, no backfocus calculation. Attach and shoot.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003e44mm full-frame image circle\u003c\/strong\u003e — covers all modern full-frame DSLR, mirrorless, and dedicated astronomy sensors.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eLightest F Series telescope\u003c\/strong\u003e — at 3.92kg OTA (4.52kg gross), suitable for the widest range of mounts including travel-grade systems.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eMost complete standard package\u003c\/strong\u003e — the 91F includes 8mm and 20mm eyepieces plus a 1.25\" erecting prism, unlike the larger F Series models.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eED glass chromatic control\u003c\/strong\u003e — one ED element minimizes false colour across the full image field.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eBuilt-in 360° rotator\u003c\/strong\u003e — frame targets at any angle without repositioning the mount.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eMatting felt interior\u003c\/strong\u003e — stray light absorption lining inside the tube for better contrast.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eRetractable dew shield\u003c\/strong\u003e — compact for transport, extended for use.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eTripod-mountable dovetail\u003c\/strong\u003e — 230mm Vixen plate with 1\/4\" and 3\/8\" screw holes for direct tripod use.\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 astrophotography (nebulae, star clusters, large emission regions)\u003c\/li\u003e\n  \u003cli\u003eFull-frame DSLR, mirrorless, and dedicated astronomy camera imaging\u003c\/li\u003e\n  \u003cli\u003eNarrowband and broadband filter imaging (built-in 2\" filter thread)\u003c\/li\u003e\n  \u003cli\u003eVisual deep-sky observing with included 8mm and 20mm eyepieces\u003c\/li\u003e\n  \u003cli\u003eBirdwatching, wildlife, and terrestrial spotting\u003c\/li\u003e\n  \u003cli\u003eTravel and portable mount setups\u003c\/li\u003e\n  \u003cli\u003eBeginner astrophotography and astronomy outreach\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003eCompatibility and Accessory Notes\u003c\/h2\u003e\n\n\u003cp\u003e\u003cstrong\u003eCamera connections:\u003c\/strong\u003e 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.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eMount compatibility:\u003c\/strong\u003e 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.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eOptional accessories:\u003c\/strong\u003e 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.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eIncluded visual accessories:\u003c\/strong\u003e 8mm eyepiece (76× magnification), 20mm eyepiece (30× magnification), and 1.25\" 45° erecting prism for correct-image terrestrial and astronomical viewing.\u003c\/p\u003e\n\n\u003ch2\u003eImportant Limitations\u003c\/h2\u003e\n\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003eSmaller aperture than 111F\/131F\u003c\/strong\u003e — 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.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003ef\/6.7 is moderate speed\u003c\/strong\u003e — not ultrafast; the optional 0.75× reducer (sold separately) brings it to f\/5 for shorter exposures.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eNot ideal for planetary imaging\u003c\/strong\u003e — the 609mm focal length is optimized for wide-field deep sky, not high-magnification planetary detail.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eNo electronic focuser included\u003c\/strong\u003e — motorized focus requires a separately purchased motor.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003eFrequently Asked Questions\u003c\/h2\u003e\n\n\u003cp\u003e\u003cstrong\u003eDo I need a flattener?\u003c\/strong\u003e\u003cbr\u003eNo. The self-flattening quadruplet produces a flat field across the 44mm image circle natively.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eDoes it cover full-frame sensors?\u003c\/strong\u003e\u003cbr\u003eYes. The 44mm image circle fully covers all current full-frame DSLR, mirrorless, and dedicated astronomy cameras.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eIs it good for beginners?\u003c\/strong\u003e\u003cbr\u003eYes — 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.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eWhat mount do I need?\u003c\/strong\u003e\u003cbr\u003eFor 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.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eIs a reducer available?\u003c\/strong\u003e\u003cbr\u003eYes. The Askar 91F 0.75× Reducer (sold separately) reduces the system to f\/5 and 455mm focal length.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eHow does it compare to the 111F?\u003c\/strong\u003e\u003cbr\u003eThe 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.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eCan I use it for birdwatching?\u003c\/strong\u003e\u003cbr\u003eYes. 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.\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 Flat-Field\u003c\/strong\u003e 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.\u003c\/p\u003e\n\n\u003cp\u003eIf 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.\u003c\/p\u003e\n\n\u003ch2\u003eOptical Diagram Reports\u003c\/h2\u003e\n\n\u003cp\u003eThe following optical and mechanical diagrams are provided for customers who wish to review the Askar 91F’s optical performance, mechanical dimensions, and connection specifications.\u003c\/p\u003e\n\n\u003cimg src=\"https:\/\/omo-oss-image.thefastimg.com\/portal-saas\/pg2024111409483993253\/cms\/image\/6d3433e9-069f-4e8f-8fe5-f64659c95b6e.jpg\" alt=\"Askar 91F Product Size Diagram - tube length and mechanical dimensions\" style=\"max-width:100%;display:block;margin:1em 0;\"\u003e\n\u003cimg src=\"https:\/\/omo-oss-image.thefastimg.com\/portal-saas\/pg2024111409483993253\/cms\/image\/d8b7fe34-d5d0-4555-bbc9-ead405dfacf2.jpg\" alt=\"Askar 91F Connection Diagram - photographic adapter and camera thread connection details\" style=\"max-width:100%;display:block;margin:1em 0;\"\u003e\n\u003cimg src=\"https:\/\/omo-oss-image.thefastimg.com\/portal-saas\/pg2024111409483993253\/cms\/image\/37f4ab88-3b9a-4c66-88f2-1674920272bb.jpg\" alt=\"Askar 91F Locking Screw Position Diagram - focuser and adapter lock locations\" style=\"max-width:100%;display:block;margin:1em 0;\"\u003e\n\u003cimg src=\"https:\/\/omo-oss-image.thefastimg.com\/portal-saas\/pg2024111409483993253\/cms\/image\/1acc5643-0b25-4789-a1b5-11c889ba55e2.jpg\" alt=\"Askar 91F Electronic Focuser Assembly Diagram - motor mounting instructions\" style=\"max-width:100%;display:block;margin:1em 0;\"\u003e\n\u003cimg src=\"https:\/\/omo-oss-image.thefastimg.com\/portal-saas\/pg2024111409483993253\/cms\/image\/fabf66d1-bbc7-47d2-95d9-13cbfcf8d59e.jpg\" alt=\"Askar 91F Optical Path Diagram - quadruplet lens group and light path layout\" style=\"max-width:100%;display:block;margin:1em 0;\"\u003e\n\u003cimg src=\"https:\/\/omo-oss-image.thefastimg.com\/portal-saas\/pg2024111409483993253\/cms\/image\/9204b293-5dea-45bd-a45d-91db45661241.jpg\" alt=\"Askar 91F Spot Diagram - star spot performance across the full-frame image circle\" style=\"max-width:100%;display:block;margin:1em 0;\"\u003e","brand":"Askar","offers":[{"title":"Default Title","offer_id":53154396504175,"sku":"91F-FLAT-FIELD","price":1398.6,"currency_code":"CAD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0686\/0049\/6239\/files\/4a0aa511-087e-46a0-897a-b099e77a876e.png_560xaf.webp?v=1779668266","url":"https:\/\/ontariotelescope.com\/products\/askar-91f-flat-field-quadruplet-astrograph","provider":"Ontario Telescope and Accessories","version":"1.0","type":"link"}