What is the primary purpose of the Orion Optics UK 85mm fixed adaptor ring?
Its sole function is to set a precise, rigid 20mm spacing between an optical corrector with an 85mm thread 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.
If my coma corrector requires 55mm of back focus, can I use this 20mm BFD adaptor ring?
No. 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.
I'm connecting a camera with 17.5mm of back focus to my Orion Optics UK corrector. Will this 20mm adaptor be enough?
This 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.
Why use a fixed 20mm BFD ring instead of a variable spacer?
A 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.
Is the 85mm thread on this adaptor a standard size?
The 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.
My stars are elongated in the corners. Could this 85mm adaptor ring be the cause?
If 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.
