This customizable design generates custom Bahtinov masks, which are used to aid focusing telescopes during astrophotography. They are named after Paul Bahtinov, who developed the first designs.
A Bahtinov mask creates diffraction spikes visible to the viewer, with each direction of slots in the mask corresponding to a spike. To focus your telescope or camera, first center a bright, solitary star in the view, then place the mask over the end of the telescope or camera lens. As focus is neared, the diffraction spikes will cross at the same point; before or after focus will force the spikes apart. Once the spikes are crossed, remove the mask and observe in focus.
Because of telescope sizes, this thing is difficult to print for larger telescopes. The default values generate a mask that fits on the front of a Celestron 8 (NexStar GPS 8). The size of this default mask means that a large 3d printer must print this. A MendelMax printer, with a bed width of approximately 220mm, is capable of printing it. Generally speaking, an 8" scope is the practical upper bound for masks.
To generate a mask for your telescope, carefully measure the inside diameter of the optical tube -- this is the Outer Diameter of the mask.
Set Aperture to the advertised size of your scope.
If your scope has a secondary mirror holder (all SCTs do), measure the diameter of the external mount.
And finally, adjust the size of the gaps.
Once printed, attach some self-adhesive felt pads on the bottom of the mask (the side that will face the corrector) to protect the corrector lens.