Bahtinov Mask Focusing Mask for Telescopes - FM270mm for a tube outer diameter (OD) from 290mm-340mm - can be mounted with the brass screws inside or outside the drawtube We have 10 sizes to choose from! Note : A : When you buy it , you need to check your out-diameter of the tube .Not just the Objective lensdiameter .Becasie you need to put the item one the tube. if you dont know which size should to choose you can ask me before buy it , Thanks For example : your telescope is 80mm , but you need to know that the telescpe tube diameter must be bigger then the 80mm , so at least you need to choose the 85-120mm . 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. Nominal size telescope tube min outer diameter(appr.) telescope tube max inner diameter(appr.) sample dew shield size sample compatible telescopes 65-100mm 60mm 110mm small camera lenses or small refractors like the TS 50mm APO. INED70 or other 60-80mm refractors etc. 85-120mm 80mm 130mm 116mm.etc 80100 or 120mm ED Skywatcher and other refractors 105-150mm 100mm 160mm 100120mm Skywatcher refractors Celestron 4 SE 125-180mm 120mm 190mm 145.157 etc 127mm Celestron or Skywatcher Maksutovs. meade 127 ED APO. Celestron 5SE 150-200mm 145mm 210mm 181mm.etc 6 Skywatcher or Celestron Newton C6 SCT 175-220mm 170mm 230mm 7 Maksutovs 195-240mm 190mm 250mm 236mm.etc 8Skywatcher or Celestron Newton 215-260mm 210mm 270mm 236mm.etc 8Skywatcher or Celestron Newton. C8 SCT 250-290 245mm 300mm 289mm.etc 10 Skywatcher or Celestron Newton. C925 SCT 290-340 285mm 350mm 12 Skywatcher Newtonians. C11 SCT