I am trying to understand the origin of the plus/cross-sign aberrations commonly seen for point sources in astronomical images, even ones created with professional-grade, high-quality optics. The one in the image below clearly has some chromatic dependence but its pattern tells me it's not purely chromatic aberration. I can't understand what physical cause gives rise to this particular pattern so commonly.
Thanks.
These are known as diffraction spikes. They are typically caused by diffraction on the struts that hold the secondary mirror in front of the primary:
In your picture, the chromatic effect appears because scale of the diffraction pattern changes with the wavelength, so with a white-light source you get the fringes of different colors in different places, producing the colorful mix that you observe.
For more details, see the Wikipedia link above.
