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365Astronomy Bahtinov Focusing Mask 250-290mm

365Astronomy Bahtinov Focusing Mask 250-290mm
365Astronomy Bahtinov Focusing Mask 250-290mm
365Astronomy Bahtinov Focusing Mask 250-290mm
365Astronomy Bahtinov Focusing Mask 250-290mm
365Astronomy Bahtinov Focusing Mask 250-290mm
365Astronomy Bahtinov Focusing Mask 250-290mm
365Astronomy Bahtinov Focusing Mask 250-290mm
365Astronomy Bahtinov Focusing Mask 250-290mm
365Astronomy Bahtinov Focusing Mask 250-290mm
Ex Tax: £22.50
  • Stock: 4
  • Model: 365Bahtinov250-290
  • Shipping Weight: 0.35kg
  • SKU: 365Bahtinov250-290
  • GTIN: 0702403401063
  • MPN: 365Bahtinov250-290
  • Location: D8-13

365Astronomy Bahtinov Telescope Focusing Mask 250-290
for telescope tubes between appr. 245mm outer diameter and appr. 300mm inner  tube diameter

The 365Astronomy Bahtinov Focusing Mask features variable fixing rods made of brass and covered by a soft transparent plastic tube for safer grab. The mask itself is made from a black, lightweight, flexible plastic material. 

Just like a solar filter, place the Bahtinov mask in front of the telescope tube and secure it with the plastic covered rods from inside or outside of the tube. When you use it with a camera lens, most likely you'll be able to secure it only at the outer perimeter of the lens as camera lenses don't usually have a long enough edge to place the rods inside it, unless you'll use the lens with an optional lens shade if such available for your camera lens. The problem might be that many camera lens shades are of not simple tubular that case better to install the Bahtinov on the camera lens' body rather than onto an asymmetric lens shade.

How to use it
With the help of the resulting diffraction pattern of a star precise focus can be determined.  From the star’s diffraction spike there are three rays formed; when the distance between them is the same then the optimum focus is reached.  
This can be used of course not only visually but via a dSLR, CSC, mirrorless compact camera, CCD or CMOS camera as well.  Especially in combination with live-view cameras, the otherwise painful focusing process becomes a breeze.
 If you need to use a telescope dew shield, that’s fine, please use the Bahtinov mask inside the dew shield or from outside at the end.

As you can see on the pictures there are three appr. 8mm long obstructions on the mask with the slots for the fixing rods that are about 7.8mm thick and 14mm long.  This makes the overall diameter appr. 16mm wider than the nominal (290mm) diameter of the mask. So the overall diameter of this mask is app 306mm.

Nominal size: 250-290mm
Max inner diameter of telescope tube: appr. 300mm (value "A", see image above)
Min outer diameter of telescope tube: appr. 245mm (value "B", see image above)
Thickness of plastic covered copper rod fixing: appr. 7.8mm

The difference between the nominal size and real size is due to the thickness of the plastic covered rods that hold the Bahtinov mask into the telescope tube. These can be used to hold the mask either from inside the tube or from outside, but please consider the above provided size info to decide about the correct size for your telescope.

Our Bahtinov masks can be found in the following sizes:



Nominal sizetelescope tube min outer
diameter (appr.)
telescope tube max inner
diameter (appr.)
dew shield size
sample compatible telescopes
65-100mm60mm110mm small camera lenses or small refractors like the TS 50mm APO, INED70 or other 60-80mm refractors etc.
85-120mm80mm130mm116mm, etc.80, 100 or 102mm ED Skywatcher and other refractors
105-150mm 100mm160mm 100, 120mm Skywatcher refractors, Celestron 4 SE
125-180mm120mm190mm145, 157 etc127mm Celestron or Skywatcher Maksutovs,
Meade 127 ED APO, Celestron 5 SE
150-200mm 145mm210mm181mm, etc6" Skywatcher or Celestron Newton, C6 SCT
175-220mm 170mm230mm 7" Maksutovs
195-240mm 190mm250mm236mm, etc8" SkyWatcher or Celestron Newton
215-260mm210mm270mm236mm, etc8" SkyWatcher or Celestron Newton, C8 SCT
250-290mm245mm300mm289mm, etc.10" SkyWatcher or Celestron Newton, C925 SCT
290-340mm 285mm350mm 12" Skywatcher Newtonians, C11 SCT


The sample dew shield size and sample telescope in the above table are just examples, there might be many more telescopes compatible with that size within the given range of diameters...please measure the size of your camera lens or telescope and compare with the above table or contact us for advise if unsure. Please note, above table is based on our best knowledge, but to be considered as experimental and we would appreciate customer feedback about tested compatibility.

(Bahtinov mask, mispelled Bathinov telescope focusing mask)




General Information
Weight in Kg0.35

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