# What is the cause of all of these sharp, concentric rings around bright stars in this HST image?

ESA's HST page heic1819 — Photo Release; Hubble reveals cosmic Bat Shadow in the Serpent’s Tail is of course beautiful and stunning, but my eyes are drawn to the diffraction artifacts of the bright stars.

I'm assuming the crosses are due to four vanes supporting the secondary mirror, but are the tight concentric rings due to Hubble's large aperture, or it's smaller secondary mirror blocking that aperture, or something else, perhaps image processing?

Without a scale for reference, it's hard to get an angular frequency and compare to an Airy-like diffraction pattern to get a diameter, and that's where it gets more puzzling, because you need a narrow wavelength range to get a coherent oscillation for so many cycles (I think I can see perhaps 15 or more sharp, distinct rings), and to zeroth order stars are mostly black-body.

Is this a bit of a puzzle, or am I missing something obvious (e.g. filters)? Or both?

Here's a cropped, monochromed, ROI:

Further stretched in contrast and size:

• Most photos like this have had heavy false-color applied both for visual contrast and for "beauty" . I wouldn't be surprised if the original is from a narrowband filter/receiver, as you surmized. – Carl Witthoft Nov 2 '18 at 16:59

• To convince myself (actually, just because it's fun) I'd like to do the calculation and get it to match. I can't track down the original HST images used to make this color image from heic1819 — Photo Release, so any information about the filter's transmission shape in general would be great. – uhoh Nov 3 '18 at 3:00