Fieldring Eyepiece Planner

Fieldring helps visual astronomers choose from the eyepieces they already own. Enter your telescope aperture and focal length, then add eyepiece focal length, apparent field and optional field stop. See magnification, exit pupil and estimated true sky field immediately. Select an eyepiece to view its angular field circle. Compare your whole kit from wide to narrow, or check reference angular spans for the Moon, Pleiades, Orion Nebula and Andromeda. A printable PDF field card lists your optical inputs and calculated results for the telescope case. Share it using the iOS Share sheet. Fieldring is an optical planning aid, not a live sky viewer or a simulation of how bright or detailed an object will appear. Target spans are approximate angular references; actual visibility, seeing, eye relief and illumination are not modeled. When available, a field-stop diameter yields a more direct true-field estimate; otherwise Fieldring uses apparent field divided by magnification. Check maker specifications before use. A new install begins with an empty kit. You may add your own telescope or try a clearly labeled example. All kit data is stored locally. One upfront purchase, no account, subscription, ads, analytics, tracking, cloud processing or developer-server upload.