Review: WalkingWay star point filter diffraction grating $18-26
Amazon Affiliate Link (49mm version): https://amzn.to/3Wp3dfL
Price as reviewed: $18-26 depending upon size
Walking Way 49mm Variable Star Filter, 4-8 Point Starburst Lens Filter with HD Optical Glass, Cross-Screen Sparkle Effect Filter for Holiday, Nightscape
This filter has hair thin lines running perpendicular in a grid, which causes a 4 point spike around point light sources (it acts as a diffraction grating). The wrinkle with this filter is that there are two layers of glass, that can be rotated, so that you can select either a 4-point or 8-point star pattern depending upon how you line them up (parallel or at a 45 degree angle).
The effect shows up best with small point light sources (Christmas tree lights, candles, stars etc...) but the lines have very little detrimental effect on the rest of the image as they are not in focus. The cost of the filter is relatively low and an easy way to add this effect optically (as opposed to trying to add it in post production). If you like this effect, I can recommend this filter.
If you have a telescope with an aperture smaller than 82mm, you could buy one and affix it to the end of your telescope to get "star points" in your astro photography images.