Bahtinov focusing mask: design & fit notes

An uncommon dew shield or lens hood may fall between stock sizes. This original cap-style mask uses its actual outside diameter and an independent optical pattern diameter. A solid seating rim rests on the hood’s front lip; a straight sleeve surrounds its outside. The removal tab has a 5 mm hole. One black PETG mask is included per quantity.

Measure a straight, round mounting section with calipers, including any raised lip. The sleeve’s total diameter clearance is added once. Check the full engagement depth for protrusions, and ensure the hood rim stops the mask before the face can touch any glass. The 192-sided sleeve bore circumscribes the requested circle, preserving its minimum clearance. Petal hoods, tapered ends, threads, inside mounts and irregular outlines are outside this design’s scope. Presets are examples without brand-fit claims.

Set the grating pattern to cover the optical opening, leaving at least 3 mm between its edge and the measured hood outside edge for seating. The solid center T web connects the three regions: horizontal slits on the upper half, and slits at +20° and −20° on the lower quarters. Every bar joins the outer rim. Pattern diameter describes the faceted grating boundary, not an unobstructed aperture; the mask intentionally blocks part of the light.

Use your working focal length, including reducers or Barlows. Period equals diffraction order × focal length ÷ spacing factor. Each slit and each bar occupies half a period. Factors 150, 175 and 200 are available. Third-order spacing triples the period, allowing wider printable bars for shorter telephoto examples. This changes the diffraction pattern; focusing sensitivity and brightness have not been physically tested. This is a classic focusing mask, without a tri-Bahtinov collimation pattern.

Both slit and bar widths must be at least 1.2 mm. The pattern must span at least eight periods, and the center support web cannot exceed one eighth of its diameter. These are design and printability guards, not an optical performance certification. Physical print, fit, opacity and star-focusing tests are pending; no focusing accuracy is promised. Inspect and clear every slit of stringing before use.

Print the grating face on the bed with the sleeve upward, black PETG, 100% infill, with a 0.4 mm nozzle and no supports. All bars start on the bed. The complete part, including the 12 mm removal tab, must fit the store’s 245 mm usable envelope inside the Bambu A1’s 256 mm bed. Lift by the tab rather than pressing the grating.

Use briefly on a bright star at night, then remove before imaging or observing. This is never a solar filter. Black PETG is the only offered color/material combination. The default measured example costs $14 before shipping and tax. Price follows the generated solid volume and surface area, including slots, sleeve, support webs and tab; checkout validates the dimensions and recalculates the price.

Read the original CAD builder and shared pattern and pricing geometry. Live preview and matching STL generation run locally in your browser. The CAD is original; no competitor mesh or generator source code was copied.