The Danubia Erecting Image Eyepiece 7-21mm Zoom is for telescopes with a 1.25 inch eyepiece aperture and reverses the image in the telescope to upright, for easy viewing, so is perfect when spending long periods star gazing. It will also help when using sky maps, as it is quicker to work out what is being seen.. Useful for both Danubia by Dorr reflector and refractor telescopes, the Eyepiece is really easy to fix..