A couple of you have mentioned the Bahtinov mask for use in photographing stars. Here's an stl code for a 150mm mask which can be easily resized in Cura when generating the gcode file for printing. A png is included showing the mask. The mask will also work if 2D printed on clear media (not translucent) making a film version which can be placed in front of a lens. The stl will result in a flat (2mm thick) mask which may need a retainer rim to hold it for a given lens. If sized properly, it will fit in a filter ring (use a spare UV or Haze filter).
Thanks to arach101 for putting me on to Fusion360 which was used to generate the mask. Fusion360 is free if you go through all the registration process and declare yourself an educational or hobby user, but it's not the smartest program on the block in some ways and is a bear to use offline because it takes so long to boot (not fast booting on line either). It is however, a very useful and feature filled 3D modeling program which converts generated models to stl format easily. Those can then be used directly in Cura to create the gcode files for many 3D printers.
Last edited by Bob 256; 07-20-2019 at 12:15 PM.