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Astromania Bahtinov Mask Focusing Mask for Telescopes
The Bahtinov mask has become the preferred method of focusing a telescope, either visually or when imaging with a CCD camera, DSLR or webcam. They are an inexpensive tool which will allow you to achieve pin-sharp levels of focus, saving you time and improving your work. Simply place the mask over the front of your ‘scope, align the scope towards a star. The mask displays a unique diffraction profile, one part of the diffraction pattern moves in response to changes in focus and displays error as a displacement from a clearly defined point.
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Specifition
• Nominal size: 65-100mm
• Material: made of sturdy plastic.
• Thickness: 2mm
• Telescope min outer diameter：60mm
• Telescope max inner diameter：110mm
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Two Installation Modes
• Inside The Dew Tube
Place the brass screw on the inside of the the dew tube. Slide each of the three grip pegs inward until they cling to inside of the telescope’s front cell or dew shield, then tighten the thumbscrews. Suitable for use with telescopes with inter diameters ranging from 72mm to 110mm, the max inter diameter is 110mm.
• Outside The Dew Tube
Place the brass screw on the outside of the the dew tube. Slide each of the three grip pegs inward until they clamp outside of the telescope’s front cell or dew shield, then tighten the thumbscrews. Suitable for use with telescopes with outer diameters ranging from 60mm to 98mm, the min outer diameter is 60mm.
FOCUS ON A STAR FIRST, THEN MOVE THE TELESCOPE TO THE PLANET FOR IMAGING
• A “live view” image is best if your camera or software permits it, because then you’ll be able to see the results of your focus adjustments in real time, and the focusing process will be faster. If needed, though, you can take successive still exposures and review each exposure on the screen, tweaking the telescope’s focus slightly after each exposure, then taking another one. In either case, you’ll want to “zoom in” as needed so you can get a clear view of the spike pattern, in particular, the center spike’s position relative to the other two spikes that form the “X”. DSLRs often have two or more magnification settings for live view focusing, and capture programs used with a laptop often allow subframe viewing, which magnifies the image.
• As you turn your telescope’s focus knobs to move in and out of focus, you should see that center spike move to one side and the other of the X. Make fine adjustments of the focus until the center spike lands exactly in the center of the X, that is, it bisects the X. Then you’re in focus!
USING THE FOCUSING MASK TO FOCUS
• With the mask in place over the front of your telescope and your camera attached to the scope and turned on, center a bright star in the field of view. You should see the diffraction spike pattern on the camera’s LCD screen (for DSLRs) or on your laptop computer running your preferred image capture software (see above figure) . If the scope is grossly out of focus you may see the grid pattern of the mask, in which case you should adjust the scope’s focus to shrink down the grid pattern until it turns into a dot with spikes.
• Once focus is achieved, remove the focusing mask. Then you’re all set to photograph any object at infinity focus.
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Make sure the mask is centered on the front of the scope
Adjust the pegs so that each is at roughly the same position in its slot. To ensure a tight grip, loosen one thumbscrew slightly, then push the grip peg inward while pulling the mask gently toward the loosened grip peg. This will make sure that the other two grip pegs are contacting the telescope when you retighten the third thumbscrew. Now the mask should be well secured in position over the front of the telescope. You’re ready to focus!
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