Astronomical Telescope Bahtinov Focusing Mask High Quality Aluminium Alloy Focusing-mask with Fixed Diameter 60-80mm/80-120mm Product Parameters: Item: Telescope Bahtinov Focusing Mask Material: Aluminium Alloy CNC made Size: 60mm/80mm 60mm: Suitable caliber: 60~80 caliber telescope (fixed outer diameter 63~85mm; fixed inner diameter 77~99mm) 80mm: Suitable caliber: 85-100 caliber telescope (fixed outer diameter 83-115mm; fixed inner diameter 97-129mm) Weight: 70g Color: Black Function: D eep space photography accuratefocus Attention: There is 1-3mm error because of manual measurement. Place the Bahtinov Focus-Mask over the end of your telescope, then locate a bright star near your target, take a 3-5 second picture and view the diffraction spikes. Change your focus, take another picture and again, view your diffraction spikes. Is the center spike moving further from center? Then change the direction your focusing and try another picture. Watch the direction the spikes are moving and when the center spike passes the middle of the other two spikes, change direction and fine tune your tuning knobs until the center spike is in the middle. Your now in perfect focus. Product Features: 1. Suitable for different caliber astronomicaltelescopes and camera lens; 2. Full 6061 Aluminum Alloy CNC made withScrubed andBlack Anodizedfor long life use; 3. 3pcs metal screws for fastening the focusing mask; 4. Metal screws with silicone mat for protecting the telescope lens . Product Package: 1pc focusing mask 3pcs screws Product Pictures: