Astromar Bahtinov Mask Generator

About this generator

The Astromar Bahtinov Mask Generator sits at the end of a focusing guide on Astromar, a UK astrophotography site run by Martyn Allen. A Bahtinov mask is a cap with three groups of slits that fits over the front of a telescope or lens. On a bright star it shows three diffraction spikes that only line up at exact focus. The guide explains how to read the pattern and where to find each number the tool asks for.

You enter the focal length, clear aperture, tube outer diameter and rim wall width, and the page shows the focal ratio and a flat preview of the slit pattern that updates as you type. An Advanced panel sets the plate thickness, slits per grating, strut width, grating offset angle, fit clearance, mounting skirt depth and the width of the center spine and divider bars, each explained in plain words. The STL is one piece: a slit plate on top of a collar that slides over the tube like a lens cap, printed collar end down without supports.

It is free, needs no account and runs in the browser, so nothing is uploaded. The mesh engine is on GitHub under the MIT license, which covers the code; the site says nothing about the prints. It suits astrophotographers who focus by hand and want a mask sized to their own scope instead of buying one.

What you can make

• Bahtinov focusing masks sized to a telescope
• Masks with a slip-on collar for the tube
• Custom slit count and grating angle

How to use Astromar Bahtinov Mask Generator

1. Look up the focal length and clear aperture and measure the outside of the tube
2. Enter the four numbers and check the pattern preview
3. Change slits, clearance or skirt depth under Advanced if you need to
4. Download the STL and print it collar end down