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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 don't know which size should to choose you can ask me before buy it , Thanks

Product Features:

1. Suitable for different caliber astronomical telescopes and camera lens;

2. Full Acrylic made for 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 brass screws

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	80、100 or 120mm ED Skywatcher and other refractors
105-150mm	100mm	160mm		100、120mm 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	8'Skywatcher or Celestron Newton
215-260mm	210mm	270mm	236mm.etc	8'Skywatcher 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