This is the crown element for the Jaegers 6 inch by 60 inch telescope objective. Index is 1.517. Abbe V#: 64.5. R1: +36.00, R2: +22.01. Focal length is 690mm. There may be some coating breakup. These were marked stria so it probably won’t be good for use as part of an objective but works well for other uses. Very limited supply