{"product_id":"askar-91f","title":"Askar 91F Flat-Field Astrograph","description":"\u003cul class=\"DAV-feature-list\"\u003e\n\u003cli\u003e91mm Aperture Quadruplet APO Optics\u003c\/li\u003e\n\u003cli\u003e609mm Focal Length at f\/6.7\u003c\/li\u003e\n\u003cli\u003e44mm Full-Frame Corrected Image Circle\u003c\/li\u003e\n\u003cli\u003eIntegrated Field Flattener Design\u003c\/li\u003e\n\u003cli\u003e3.92kg Net Weight\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003c!-- more --\u003e\n\u003cul class=\"tabs\"\u003e\n\u003cli class=\"active\"\u003eDescription\u003c\/li\u003e\n\u003cli\u003eFAQ\u003c\/li\u003e\n\u003cli\u003eSpecifications\u003c\/li\u003e\n\u003cli\u003eIn the Box\u003c\/li\u003e\n\u003cli\u003eDownloads\u003c\/li\u003e\n\u003cli\u003eWarranty\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003cul class=\"tabs-content\"\u003e\n\u003cli class=\"active\"\u003e\n\u003cdiv\u003e\n\u003cdiv class=\"DAV-bg-light\"\u003e\n\u003cdiv class=\"DAV-section-center\"\u003e\n\u003ch2\u003eAskar 91F Flat-Field Astrograph\u003c\/h2\u003e\n\u003cp\u003eThe Askar 91F astrograph integrates a 91mm aperture with a 609mm focal length for a native f\/6.7 optical system. Its quadruplet APO objective, which includes one ED glass element, delivers a flat, color-corrected 44mm image circle that illuminates full-frame sensors without an external flattener. The entire optical tube weighs only 3.92kg, creating a portable yet powerful imaging instrument.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-bg-light\"\u003e\n\u003cdiv class=\"DAV-section\"\u003e\n\u003ch3\u003eA 91mm f\/6.7 Quadruplet APO with a 44mm Image Circle\u003c\/h3\u003e\n\u003cp\u003eThe core of the Askar 91F is its self-flattened optical design. Unlike traditional refractors that require a separate, carefully spaced field flattener, this quadruplet system has the correction built in. This eliminates the need to calculate back focus or purchase additional accessories, allowing you to connect your camera and start imaging immediately after achieving focus.\u003c\/p\u003e\n\u003cp\u003eThe 44mm image circle is specifically designed to cover full-frame CMOS and DSLR sensors, ensuring sharp, pinpoint stars from the center to the corners of your frame. The inclusion of one ED (Extra-low Dispersion) glass element in the air-spaced objective effectively suppresses chromatic aberration, resulting in high-contrast images with true-to-life color rendition, free of distracting purple fringing around bright stars.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-bg-light\"\u003e\n\u003cdiv class=\"DAV-section\"\u003e\n\u003ch3\u003e3-inch Rack-and-Pinion Focuser with 360° Rotation\u003c\/h3\u003e\n\u003cp\u003eTo support heavy full-frame cameras and filter wheels without flexure, the Askar 91F is equipped with a rigid 3-inch dual-speed rack-and-pinion focuser. The 1:10 fine adjustment knob allows for critical focus, while the 30mm of focuser travel accommodates a wide range of accessories. A 360° rotator at the rear of the focuser lets you frame your target perfectly without rotating the entire optical tube.\u003c\/p\u003e\n\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003eIncluded Adapters:\u003c\/strong\u003e The package comes with a set of photographic adapters, including M54x0.75 and M48x0.75 connections, ensuring compatibility with most popular astronomy cameras and T-rings. The M48 adapter also includes a thread for 2-inch filters.\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eVisual Versatility:\u003c\/strong\u003e For visual use, the scope includes standard 2-inch and 1.25-inch adapters, making it a capable instrument for observing star clusters and nebulae, not just for imaging them.\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003c\/div\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-bg-light\"\u003e\n\u003cdiv class=\"DAV-section\"\u003e\n\u003ch3\u003eA 3.92kg Lightweight Design with Dual Finder Bases\u003c\/h3\u003e\n\u003cp\u003eEngineered for portability, the Askar 91F's main tube weighs just 3.92kg (4.52kg with rings and plate), making it an excellent match for mid-sized equatorial mounts. The retractable dew shield reduces the telescope's length to as little as 513mm for transport and storage. Thoughtful mechanical details enhance usability in the field.\u003c\/p\u003e\n\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003eDual Finder Mounts:\u003c\/strong\u003e Two finder bases are mounted on either side of the focuser, providing flexible mounting options for a guide scope and a finder scope simultaneously.\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eMounting Hardware:\u003c\/strong\u003e The included 230mm Vixen-style dovetail plate provides a secure connection to your mount and features 1\/4\" and 3\/8\" screw holes for direct attachment to a photo tripod for terrestrial viewing.\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003c\/div\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-bg-light\"\u003e\n\u003cdiv class=\"DAV-section\"\u003e\n\u003ch3\u003eAskar 91F vs. Triplet APOs: The Advantage of an Integrated Flattener\u003c\/h3\u003e\n\u003cp\u003eWhen comparing the Askar 91F to a traditional triplet apochromat like the William Optics FLT 91, the primary distinction is the optical design philosophy. While both deliver excellent color correction, the triplet design requires an additional, separately purchased field flattener to achieve a flat field for astrophotography.\u003c\/p\u003e\n\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003eSimplicity and Cost:\u003c\/strong\u003e The Askar 91F's integrated flattener means there's no complex back-focus spacing to calculate and no extra accessory to buy. This simplifies the imaging train and reduces overall cost.\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eGuaranteed Correction:\u003c\/strong\u003e Because the flattener is part of the objective design, the optical correction is perfectly matched to the telescope, eliminating potential issues with third-party flatteners.\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eTrade-offs:\u003c\/strong\u003e A triplet with a separate flattener\/reducer can offer multiple focal ratios. The Askar 91F is optimized for its native 609mm focal length, prioritizing out-of-the-box imaging readiness over focal length flexibility.\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003c\/div\u003e\n\u003c\/div\u003e\n\u003c\/div\u003e\n\u003c\/li\u003e\n\u003cli\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eWhat is the main benefit of the Askar 91F's quadruplet design?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eThe primary benefit is its \u003cstrong\u003eself-flattened optical system\u003c\/strong\u003e. Unlike standard refractors that require a separate accessory, the Askar 91F has the field flattener built into its quadruplet objective. This ensures a perfectly flat 44mm image circle for full-frame cameras without the need to calculate back focus or buy extra components.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eDo I need to calculate back focus for the Askar 91F?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eNo. The integrated flattener design means you do not need to calculate back focus. Simply attach your camera using the provided adapters, bring the telescope into focus using the focuser, and you are ready to image.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eHow does the Askar 91F perform on full-frame cameras like a Sony A7 series?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eThe Askar 91F is designed specifically for full-frame cameras. Its \u003cstrong\u003e44mm corrected image circle\u003c\/strong\u003e fully illuminates the sensor of a Sony A7, Canon R, or Nikon Z series camera, providing pinpoint stars across the entire frame without vignetting or distorted stars in the corners.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eWhat kind of mount is needed for the 4.52kg Askar 91F OTA?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eWith a fully loaded weight of 4.52kg (including rings and dovetail plate), the Askar 91F is well-suited for mid-sized equatorial mounts with a payload capacity of 25-30 lbs (11-14 kg) or more. Mounts like the Sky-Watcher HEQ5 or iOptron CEM26\/GEM28 can carry this telescope and associated imaging gear with excellent stability.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eCan I use the Askar 91F for visual observation of planets like Jupiter?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eYes. While optimized for astrophotography, its high-quality APO optics make it a very capable visual instrument. At 609mm, it provides medium power for observing Jupiter's moons and cloud bands or Saturn's rings. You would simply use the included 1.25\" or 2\" adapter with your choice of eyepieces.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eWhat do the different adapter threads on the Askar 91F connect to?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eThe Askar 91F includes a set of adapters to connect various devices. The primary connection types are:\u003c\/p\u003e\n\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003eM64×1:\u003c\/strong\u003e The thread on the focuser drawtube itself.\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eM54×0.75:\u003c\/strong\u003e A common thread size for many larger-format astronomy cameras and filter wheels.\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eM48×0.75:\u003c\/strong\u003e The standard thread for 2\" filters and the most common connection for full-frame camera T-rings.\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003c\/div\u003e\n\u003c\/li\u003e\n\u003cli\u003e\n\u003ctable class=\"DAV-specifications\"\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd\u003eAperture\u003c\/td\u003e\n\u003ctd\u003e91mm\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eFocal Length\u003c\/td\u003e\n\u003ctd\u003e609mm\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eFocal Ratio\u003c\/td\u003e\n\u003ctd\u003ef\/6.7\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eObjective Lens\u003c\/td\u003e\n\u003ctd\u003eQuadruplet air-spaced APO (including one ED glass)\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eImage Circle\u003c\/td\u003e\n\u003ctd\u003e44mm\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eFocuser\u003c\/td\u003e\n\u003ctd\u003e3-inch dual-speed Rack \u0026amp; Pinion\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eFocuser Travel\u003c\/td\u003e\n\u003ctd\u003e30mm\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eNet Weight (OTA only)\u003c\/td\u003e\n\u003ctd\u003e3.92 kg\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eGross Weight (w\/ rings \u0026amp; plate)\u003c\/td\u003e\n\u003ctd\u003e4.52 kg\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eTotal Length (Visual Config)\u003c\/td\u003e\n\u003ctd\u003e513mm (retracted) \/ 599mm (extended)\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eTotal Length (Photo Config)\u003c\/td\u003e\n\u003ctd\u003e550mm (retracted) \/ 636mm (extended)\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eRear-end Adapters\u003c\/td\u003e\n\u003ctd\u003eM64x1, M54x0.75, M48x0.75 (with 2\" filter thread)\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003c\/tbody\u003e\n\u003c\/table\u003e\n\u003c\/li\u003e\n\u003cli\u003e\n\u003cul class=\"DAV-grid\"\u003e\n\u003cli class=\"DAV-grid-item\"\u003e\u003ca href=\"https:\/\/cdn.shopify.com\/s\/files\/1\/0753\/5280\/1565\/files\/Askar_91F_Flat-Field_Manual.pdf\" target=\"_blank\"\u003e\u003csvg class=\"DAV-img-box\" viewbox=\"0 0 512 512\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\"\u003e\u003cpath d=\"M0 64C0 28.7 28.7 0 64 0L224 0l0 128c0 17.7 14.3 32 32 32l128 0 0 144-208 0c-35.3 0-64 28.7-64 64l0 144-48 0c-35.3 0-64-28.7-64-64L0 64zm384 64l-128 0L256 0 384 128zM176 352l32 0c30.9 0 56 25.1 56 56s-25.1 56-56 56l-16 0 0 32c0 8.8-7.2 16-16 16s-16-7.2-16-16l0-48 0-80c0-8.8 7.2-16 16-16zm32 80c13.3 0 24-10.7 24-24s-10.7-24-24-24l-16 0 0 48 16 0zm96-80l32 0c26.5 0 48 21.5 48 48l0 64c0 26.5-21.5 48-48 48l-32 0c-8.8 0-16-7.2-16-16l0-128c0-8.8 7.2-16 16-16zm32 128c8.8 0 16-7.2 16-16l0-64c0-8.8-7.2-16-16-16l-16 0 0 96 16 0zm80-112c0-8.8 7.2-16 16-16l48 0c8.8 0 16 7.2 16 16s-7.2 16-16 16l-32 0 0 32 32 0c8.8 0 16 7.2 16 16s-7.2 16-16 16l-32 0 0 48c0 8.8-7.2 16-16 16s-16-7.2-16-16l0-64 0-64z\" fill=\"#0047a7\"\u003e\u003c\/path\u003e\u003c\/svg\u003e\n\u003cdiv\u003e\n\u003cp class=\"DAV-item-title\"\u003eDownload Manual\u003c\/p\u003e\n\u003c\/div\u003e\n\u003c\/a\u003e\u003c\/li\u003e\n\u003cli class=\"DAV-grid-item\"\u003e\n\u003cdiv\u003e\n\u003cp class=\"DAV-item-title\"\u003eAskar 91F OTA\u003c\/p\u003e\n\u003cp class=\"DAV-item-subtitle\"\u003e× 1\u003c\/p\u003e\n\u003c\/div\u003e\n\u003c\/li\u003e\n\u003cli class=\"DAV-grid-item\"\u003e\n\u003cdiv\u003e\n\u003cp class=\"DAV-item-title\"\u003eTube Rings\u003c\/p\u003e\n\u003cp class=\"DAV-item-subtitle\"\u003e× 1 Pair\u003c\/p\u003e\n\u003c\/div\u003e\n\u003c\/li\u003e\n\u003cli class=\"DAV-grid-item\"\u003e\n\u003cdiv\u003e\n\u003cp class=\"DAV-item-title\"\u003eCarry Handle\u003c\/p\u003e\n\u003cp class=\"DAV-item-subtitle\"\u003e× 1\u003c\/p\u003e\n\u003c\/div\u003e\n\u003c\/li\u003e\n\u003cli class=\"DAV-grid-item\"\u003e\n\u003cdiv\u003e\n\u003cp class=\"DAV-item-title\"\u003e230mm Vixen Dovetail Plate\u003c\/p\u003e\n\u003cp class=\"DAV-item-subtitle\"\u003e× 1\u003c\/p\u003e\n\u003c\/div\u003e\n\u003c\/li\u003e\n\u003cli class=\"DAV-grid-item\"\u003e\n\u003cdiv\u003e\n\u003cp class=\"DAV-item-title\"\u003ePhotographic Adapter Set\u003c\/p\u003e\n\u003cp class=\"DAV-item-subtitle\"\u003e× 1\u003c\/p\u003e\n\u003c\/div\u003e\n\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003c\/li\u003e\n\u003cli\u003e\n\u003cul class=\"DAV-grid\"\u003e\n\u003cli class=\"DAV-grid-item\"\u003e\u003ca href=\"https:\/\/cdn.shopify.com\/s\/files\/1\/0753\/5280\/1565\/files\/Askar_91F_Flat-Field_Manual.pdf\" target=\"_blank\"\u003e\u003csvg class=\"DAV-img-box\" viewbox=\"0 0 512 512\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\"\u003e\u003cpath d=\"M0 64C0 28.7 28.7 0 64 0L224 0l0 128c0 17.7 14.3 32 32 32l128 0 0 144-208 0c-35.3 0-64 28.7-64 64l0 144-48 0c-35.3 0-64-28.7-64-64L0 64zm384 64l-128 0L256 0 384 128zM176 352l32 0c30.9 0 56 25.1 56 56s-25.1 56-56 56l-16 0 0 32c0 8.8-7.2 16-16 16s-16-7.2-16-16l0-48 0-80c0-8.8 7.2-16 16-16zm32 80c13.3 0 24-10.7 24-24s-10.7-24-24-24l-16 0 0 48 16 0zm96-80l32 0c26.5 0 48 21.5 48 48l0 64c0 26.5-21.5 48-48 48l-32 0c-8.8 0-16-7.2-16-16l0-128c0-8.8 7.2-16 16-16zm32 128c8.8 0 16-7.2 16-16l0-64c0-8.8-7.2-16-16-16l-16 0 0 96 16 0zm80-112c0-8.8 7.2-16 16-16l48 0c8.8 0 16 7.2 16 16s-7.2 16-16 16l-32 0 0 32 32 0c8.8 0 16 7.2 16 16s-7.2 16-16 16l-32 0 0 48c0 8.8-7.2 16-16 16s-16-7.2-16-16l0-64 0-64z\" fill=\"#0047a7\"\u003e\u003c\/path\u003e\u003c\/svg\u003e\n\u003cdiv\u003e\n\u003cp class=\"DAV-item-title\"\u003eDownload Manual\u003c\/p\u003e\n\u003c\/div\u003e\n\u003c\/a\u003e\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003c\/li\u003e\n\u003cli\u003e\n\u003ca href=\"https:\/\/davidastro.com\/pages\/askar-warranty-policy\" rel=\"noopener\" target=\"_blank\"\u003eAskar 1-Year Warranty\u003c\/a\u003e\n\u003c\/li\u003e\n\u003c\/ul\u003e","brand":"Askar","offers":[{"title":"Default Title","offer_id":52851329925405,"sku":"ASK-ASAPO91F","price":999.0,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0753\/5280\/1565\/files\/Askar-91F-Flat-Field-1_23348316-55b6-4015-90af-dd56471b084f.jpg?v=1788401678","url":"https:\/\/davidastro.com\/fr-eu\/products\/askar-91f","provider":"David Astro","version":"1.0","type":"link"}