Celestron Ultima apochromatic Bar lens with a factor of 2, compatible with eyepieces with a rim diameter of 1.25 inch. Excellent correction of chromatic aberration.
The main advantage of using Barlow lenses is that they allow you to limit the number of eyepieces while providing a wide range of available magnifications. For example, having two eyepieces with 30 and 15 mm focal lengths, and using a Barlow lens, we get two additional magnifications corresponding to eyepieces with 10 and 5 mm focal lengths. In addition, Barlow lenses do not change the offset of the exit pupil of the glasses ( eye relief ), allow the focus to be extended further from the tube (when we have a problem with sharpening the image) and allow to double the focal length when taking photos of the sky in the focus of the lens.
HOW TO ATTACH A BARL LENS TO A TELESCOPE