A 0.9-inch Barlow lens is an optical device used in telescopes to increase the effective focal length of the telescope, thereby enhancing its magnification capabilities. A Barlow lens is typically a concave lens that is placed between the objective lens (or primary mirror) of the telescope and the eyepiece. The 0.9-inch specification refers to the diameter of the lens.
By using a Barlow lens, astronomers and stargazers can get a closer look at celestial bodies without having to switch between multiple eyepieces. It effectively multiplies the magnification power of your eyepiece—commonly by 2x or 3x—allowing for more versatile viewing options with the same set of equipment. This makes it a valuable accessory for both amateur and professional astronomers. It is important to ensure compatibility between the Barlow lens and your telescope to achieve optimum performance.