A Bahtinov Mask for helping with focusing a Hadley telescope.
=== Update 2023/03/03 ===
Added a thin edges version that reduces overall thickness to 1.5mm and has angled edges to each slit, in the hopes of making a sharper diffraction pattern.
For this version, I'd recommend printing with a thin layer height (I used 0.15mm) and with elephant foot compensation turned off.
When you use it, I would face the larger flat side (labeled “space side”) towards the stars.
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https://en.wikipedia.org/wiki/Bahtinov_mask
Inspired by Gyoenastaader's Bahtinov mask (although I did not use that model as a starting point), this version uses a pattern with more shallowly angled 20 degree slits, which some find easier to use than the 45 degree slits of the above model. (That said, I did find an article saying that for automatic computer based focusing, a higher angle can be helpful - maybe try both this model and the one linked above.)
The pattern used in this model was generated using Satoru Takagi's Bahtinov Mask Generator website with a focal length of 900mm to match the standard Hadley mirrors.
Tested on my ½" aluminum rod Hadley.
Diffraction spikes look much crisper to the eye than in the included picture captured with my phone camera.
Many thanks to Gyoenastaader for including the step files with their original mirror covers!
The author remixed this model.