Starbeam's unique flip mirror feature provides comfort and flexibility for viewing straight through and 90°. Viewed straight through, you see the natural sky with a 10 arc-minute (1/3 Moon dia.) "red-star" showing your scope's position. When your scope is pointed high, flip the mirror into position for more comfortable viewing. The size, color and adjustable brightness of the "red-star" makes it easy to see against any twilight to dark sky background.With its 39mm aperture, the projected image is easily seen any distance behind Starbeam, whether 2 inches or 2 feet! The finely made glass lens acts as a beamsplitter to combine the natural sky background with the superimposed red pointing "star." The first lens surface acts as a collimating mirror so the "red-star" stays stationary against the sky background regardless of your eye position.