Focusing Masks

How Focusing Masks Work

A mask with precisely angled slots placed over the aperture produces diffraction spikes whose relative position depends on focus with far more sensitivity than the star image itself.

• Bahtinov masks: Three sets of slots producing an X plus a central spike. The central spike shifts off-centre with very small focus errors, making the judgement unambiguous.
• Aperture sizing: Slot spacing is calculated from aperture and focal length. A mask sized for a different telescope produces a muddled pattern that defeats the purpose.
• Tri-Bahtinov and variants: Designs that also reveal collimation error alongside focus, useful on fast reflectors.
• Hartmann masks: Simple two- or three-hole masks producing multiple star images that merge at focus — cruder but effective and easy to improvise.
• Star brightness: A moderately bright star gives the cleanest pattern. Very bright stars bloat and very faint ones do not produce visible spikes.
• Imaging versus visual use: Most valuable in imaging, where you judge focus on a screen and small errors matter. Useful visually for confirming focus on a bright star.

Why Choose David Astro for Focusing Masks

A mask is cheap, and one sized for the wrong telescope is worthless.

• Sized to your telescope: We match the mask to your aperture and focal length so the diffraction pattern is clean and readable.
• Fit over dew shields checked: The mask must sit at the aperture, and we confirm the diameter accounts for a dew shield if you use one.
• Alternatives mentioned: If you already have an electronic focuser with autofocus, we will tell you that a mask is largely redundant rather than sell one anyway.
• Authorized dealer: Genuine products with manufacturer warranty, in Canadian stock.
• Ships from Canada: No duties or brokerage fees for Canadian orders.

Focusing Masks in Canada — Frequently Asked Questions

How accurate is a Bahtinov mask?

Very. In most systems it resolves focus to within the critical focus zone, which is better than most people achieve by eye and comparable to a software autofocus routine.

Does the mask size matter?

Yes. Slot spacing is derived from aperture and focal length, so a mask made for a different telescope produces an unclear pattern. Size it to your specific instrument.

Do I need one if I have an autofocuser?

Generally not for routine focusing, though a mask is a useful independent check when setting up a new imaging train or verifying that autofocus is finding true focus.

Which star should I use?

A moderately bright one, around second or third magnitude. Very bright stars bloat and obscure the pattern; faint ones do not produce visible spikes.

Nailing focus every time?

Tell us your aperture and focal length and we will size a mask. Ships from Canada.