This bahtinov astrophotography focusing mask is a must-have for astronomy enthusiasts who own a Rokinon or Samyang 135mm F/2 lens. This mask is perfect for use with refractor telescopes with outer diameter to the optical tube less than 93mm. It is a high-quality accessory that will greatly enhance your astrophotography experience.The mask is easy to use and will help you achieve sharp focus in your astrophotography images. This mask is ideal for use by both amateur and professional astronomers.