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IMPORTANT: Our store is located in Canada. All orders that include products manufactured outside of Canada or USA will be subject to tariffs and duties, regardless of the order value. US customers are responsible for all applicable duties and tariffs, and those will be billed by the carrier, except for the Lacerta upgrade kit, for which we collect them at checkout.
IMPORTANT: Our store is located in Canada. All orders that include products manufactured outside of Canada or USA will be subject to tariffs and duties, regardless of the order value. US customers are responsible for all applicable duties and tariffs, and those will be billed by the carrier, except for the Lacerta upgrade kit, for which we collect them at checkout.
William Optics 105 -148mm Bahtinov Focusing Mask
William Optics 105 -148mm Bahtinov Focusing Mask
William Optics 105 -148mm Bahtinov Focusing Mask

William Optics 105 -148mm Bahtinov Focusing Mask

by William Optics
SKU WO-BM-DS-GR55
$68.00 - $68.00
$68.00
$68.00 - $68.00
$68.00
Price Match Policy!
  • Fits Telescope Dew Shields from 105mm to 148mm
  • Generates Clear Diffraction Spikes for Focusing
  • Removes Subjectivity from Manual Focusing
  • Lightweight and Durable Construction
  • Ideal for Astrophotography
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  • Description
  • FAQ
  • Specifications
  • In the Box
  • Warranty

  • William Optics 105-148mm Bahtinov Focusing Mask

    The William Optics Bahtinov Focusing Mask provides a fast, repeatable method for achieving critical focus for astrophotography. It is designed to fit any telescope with an outer dew shield diameter between 105mm and 148mm, making it a versatile tool for a wide range of optical tubes.

    Universal Fit for Dew Shields from 105mm to 148mm

    This mask features adjustable mounting posts to securely grip the outer edge of your telescope's dew shield. This simple yet effective design allows it to fit a wide variety of instruments, from small refractors to mid-sized reflectors, as long as the dew shield diameter falls within the 105mm to 148mm range.

    • Adjustable Fit: Securely mounts on any dew shield from 105mm to 148mm in diameter, ensuring broad compatibility.
    • Lightweight Construction: Adds minimal weight to the front of your telescope, preventing balance issues when imaging.
    • Simple Installation: Attaches and removes in seconds without any tools, letting you get to focusing quickly.

    The Bahtinov Diffraction Pattern: Visualizing Perfect Focus

    Instead of guessing when a star is at its smallest point, the Bahtinov mask creates a bright, easy-to-see diffraction pattern around it. As you adjust your focuser, you'll see a central diffraction spike move between two outer spikes. Perfect focus is achieved only when the central spike is perfectly centered and symmetrical.

    This visual confirmation is immediate and unambiguous, even on a small camera screen. It works on any bright star and is significantly more accurate than relying on visual estimation, which can be affected by eye fatigue and atmospheric conditions.

    Bahtinov Mask vs. FWHM: Why Visual Confirmation Matters

    While autofocus routines using Full Width at Half Maximum (FWHM) are powerful, they can be misled by atmospheric turbulence ("seeing"), which causes star sizes to fluctuate moment by moment. A Bahtinov mask is largely immune to seeing; the diffraction pattern remains stable and readable even when the air is unsteady.

    • Seeing Immunity: Provides a clear result even in turbulent atmospheric conditions that can fool software metrics.
    • No Software Required: An essential analog tool that works directly at the eyepiece or on any live-view camera feed without needing a computer.
    • Faster Than Software: For manual focusing, finding the symmetrical spike pattern is often faster than waiting for software to analyze multiple exposures and calculate FWHM values.

  • How do I use the William Optics 105-148mm Bahtinov Mask?

    Simply place the mask over the front of your telescope's dew shield and point at a moderately bright star. Use your camera's live view, zoomed in on the star, to see the diffraction pattern. Adjust your focuser until the central spike is perfectly centered between the two outer spikes. Once centered, your focus is perfect, and you can remove the mask to begin imaging.

    Will this William Optics Bahtinov mask fit my telescope?

    This mask is designed to fit any telescope with an outer dew shield diameter that measures between 105mm and 148mm. Measure the outside diameter of your dew shield to confirm compatibility. The adjustable posts will securely hold it in place within this range.

    What kind of star should I use to focus the William Optics Bahtinov Mask?

    Choose a bright star that is not overly brilliant, like Vega or Arcturus. An ideal star is bright enough to produce a clear diffraction pattern on your camera's live view but not so bright that it completely saturates the sensor, which can make the spikes difficult to judge. It's often best to choose a star near your imaging target to minimize focus shift from mirror flop or temperature changes.

    Can I use the William Optics 105-148mm Bahtinov Mask for visual observing?

    While designed for astrophotography, a Bahtinov mask can be used for visual focusing. However, the diffraction pattern is much easier to see and evaluate on a camera screen than through an eyepiece. For imaging, it is an essential tool; for visual, it can be helpful but is generally less necessary.

    I have a William Optics ZenithStar 81; will this 105-148mm mask fit?

    Yes, this mask is a good fit for many popular refractors. To be certain, you should measure the outer diameter of your ZenithStar 81's dew shield. If it falls within the 105mm to 148mm range, the mask will fit perfectly.

    How does the William Optics Bahtinov Mask help when imaging a target like the Orion Nebula (M42) with a DSLR?

    When imaging a faint object like the Orion Nebula (M42), achieving perfect focus is critical to capturing fine detail. Before you slew to the nebula, point your DSLR and telescope to a nearby bright star like Rigel. Use the Bahtinov mask with your camera's live view to set focus precisely. Then, remove the mask and move to M42, confident that your stars will be as sharp as possible in your final stacked image.

  • Mask Type Bahtinov
    Fitment Method Mounts on Telescope Dew Shield
    Fitment Diameter Range 105mm - 148mm

    • William Optics 105-148mm Bahtinov Focusing Mask

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  • William Optics 2-Year Warranty