Bresser Measurement eyepiece 23 mm. For calibration a micrometer slide would be necessary Specifications for Bresser Measurement eyepiece: Manufacturer: Bresser Magnification: 10 x Diameter: 23 mm Color: Black Magnification Type: Fixed Features of Bresser Measurement eyepiece Magnification: 10x Diameter: 23 mm Package Contents: Bresser Measurement eyepiece Bresser 59-41980: Measurement eyepiece 23 mm