# 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. Nov 2 '18 at 16:59

The diffraction pattern at the focal plane created by a circular aperture is called an Airy Disk or Airy Pattern. Both the outer opening and the inner hole plus secondary contribute to the exact function. This is usually not easily observed with ground based telescopes because the seeing fluctuations due to air turbulence smears it out. These images use filters that are a few 100 Angstroms wide, so although not monochromatic, they are narrow enough to clearly see the pattern. The spectral shape of the star's light within the filter makes the effective width of frequencies more narrow.

• Aha, very nice! That works mathematically. In this answer I tracked down some specific Hubble images and plotted the transmission spectra of the specific filters used in those images. Is it possible here to at least name the group of filters that are a few hundred Ångstroms wide? Thanks!
– uhoh
Nov 3 '18 at 2:42
• 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
• @uhoh - If you click on the photo and scroll down one sees the filter list. It used the Wide J and H on WFC3. The Wide J is center at 1248.6 nm and is 284.5 nm wide and the Wide H is centered at 1536.9 nm and is 268.3 nm (from the WFC3 Handbook). Wider than I thought but still narrow enough, evidently, to provide a clear Airy pattern. Nov 5 '18 at 16:33
• No, those are too wide to get "15 or more sharp, distinct rings." I can count 20 on the right-hand star in the monochrome cropped image. But they may isolate narrow emission lines. This is what really caught my eye, so many rings yet the last one as narrow as the first!
– uhoh
Nov 5 '18 at 16:39
• I checked the archive at STScI which lists these exposures. The heic0901f Carina Nebula was taken with F502n, F656N, and F673N narrow band filters in the red in camera WFC3. The Serpens Nebula was taken with F125W, F160W wide band and F164N narrow band filters in the near IR on WFC3. So, you are correct there is a narrow band image included that was not mentioned in the annotations. Nov 20 '18 at 18:19