{"product_id":"orion-optics-uk-85mm-fixed-adaptor-ring-20mm-bfd","title":"Orion Optics UK 85mm Fixed Adaptor Ring with 20mm BFD","description":"\u003cul class=\"DAV-feature-list\"\u003e\n\u003cli\u003eConnects to 85mm threaded correctors and focusers\u003c\/li\u003e\n\u003cli\u003eProvides a fixed 20mm back focus distance (BFD)\u003c\/li\u003e\n\u003cli\u003eEnsures rigid, non-flexing camera connection\u003c\/li\u003e\n\u003cli\u003eDesigned for Orion Optics UK imaging systems\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\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\u003eOrion Optics UK 85mm Fixed Adaptor Ring with 20mm BFD\u003c\/h2\u003e\n\u003cp\u003eThe Orion Optics UK 85mm Fixed Adaptor Ring is a precision-machined component designed to establish a critical 20mm of back focus distance in an astrophotography imaging train. This single-piece solution provides a rigid, tilt-free connection between large-format correctors and your camera setup.\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\u003eAchieving 20mm Back Focus with 85mm Threads\u003c\/h3\u003e\n\u003cp\u003eThis fixed adaptor ring is an essential component for connecting imaging cameras to Orion Optics UK correctors that utilize a large 85mm thread. It establishes the exact 20mm back focus distance required for the optical corrector to deliver a flat field, ensuring sharp stars from the center to the edge of your sensor. Machined for a secure, non-flexing connection, this ring eliminates a common source of tilt and spacing errors that can compromise image quality in demanding astrophotography setups.\u003c\/p\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 primary purpose of the Orion Optics UK 85mm fixed adaptor ring?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eIts sole function is to set a precise, rigid \u003cstrong\u003e20mm spacing\u003c\/strong\u003e between an optical corrector with an \u003cstrong\u003e85mm thread\u003c\/strong\u003e and the next component in the imaging train, typically leading to the camera sensor. This spacing is essential for achieving optimal optical correction and a flat field.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eIf my coma corrector requires 55mm of back focus, can I use this 20mm BFD adaptor ring?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eNo. This adaptor provides a fixed 20mm distance. You would need a different combination of spacers that add up to the required 55mm. Using the wrong spacing will result in poor star shapes and field curvature.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eI'm connecting a camera with 17.5mm of back focus to my Orion Optics UK corrector. Will this 20mm adaptor be enough?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eThis 20mm ring is just one component in your total back focus calculation. You must account for the distance required by the corrector (e.g., 55mm) and subtract the camera's own back focus (17.5mm) and the spacing this ring provides (20mm) to determine what additional spacers you need. Always calculate the full imaging train distance from the corrector's flange to the sensor.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eWhy use a fixed 20mm BFD ring instead of a variable spacer?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eA fixed, single-piece adaptor like this 85mm ring provides a more rigid and stable connection than stacking multiple smaller spacers or using a variable extender. This significantly minimizes the risk of tilt, which is critical for achieving a perfectly flat imaging field with large sensors.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eIs the 85mm thread on this adaptor a standard size?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eThe 85mm thread is specific to certain larger format correctors and focusers, particularly those from Orion Optics UK. It is not a common standard like M42 or M48 threads, so you must confirm your equipment uses an 85mm thread before purchasing.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eMy stars are elongated in the corners. Could this 85mm adaptor ring be the cause?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eIf the adaptor is providing the correct 20mm of spacing as part of a properly calculated back focus distance, it is unlikely to be the cause. Elongated corner stars are the classic sign of incorrect back focus. Double-check the total spacing requirement for your specific corrector and ensure all components in your imaging train add up to that exact distance.\u003c\/p\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\u003eAdaptor Thread\u003c\/td\u003e\n\u003ctd\u003e85mm\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eBack Focus Distance (BFD)\u003c\/td\u003e\n\u003ctd\u003e20mm\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eBrand\u003c\/td\u003e\n\u003ctd\u003eOrion Optics UK\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\n\u003cdiv\u003e\n\u003cp class=\"DAV-item-title\"\u003e85mm fixed adaptor ring 20mm BFD\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\u003ca href=\"https:\/\/davidastro.com\/pages\/orion-optics-uk-warranty-policy\" rel=\"noopener\" target=\"_blank\"\u003eOrion Optics UK 1-Year Warranty\u003c\/a\u003e\n\u003c\/li\u003e\n\u003c\/ul\u003e","brand":"Orion Optics UK","offers":[{"title":"Default Title","offer_id":52942434533661,"sku":"OOUK-85MM-FIXED-ADAPTOR-RING-20MM","price":141.11,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0753\/5280\/1565\/files\/Orion-Optics-UK-Logo_bd506535-9b1d-4ab1-8d6f-258eec09f072.jpg?v=1789607585","url":"https:\/\/davidastro.com\/en-us\/products\/orion-optics-uk-85mm-fixed-adaptor-ring-20mm-bfd","provider":"David Astro","version":"1.0","type":"link"}