Farpoint Adjustable Bahtinov Focus Mask — 5.5" to 8.5" OTAsNot sure this is the right mask for your telescope? Use our Focus Mask Finder match your scope to the right Standard, Tri Bahtinov, or Hyperstar RASA mask. Adjustable Bahtinov focus mask for OTAs from 5. 5" to 8. 5" front aperture. Sets onto the front of your telescope, gives you perfect focus in seconds, and lifts off cleanly when you're done. The adjustable peg system fits any scope in this aperture range including OTAs we don't make a custom fit