James Webb: 'Fully focused' telescope beats expectations

What is the 'dark sky' Hubble image?

They represent light that was diffracted by the edges of the hexagonal primary mirror segments and secondary mirror support struts.

The reason they are so prominent is because of how extremely overexposed the star is. Every star will have them but in a normally exposed star they are too faint to see.

In the Hubble Space Telescope you have four spikes that are just from the support struts of the secondary mirror. The circular mirror generates something called an Airy Disk https://en.wikipedia.org/wiki/Airy_disk.

Diffraction effects are unavoidable in any telescope and can tell you a lot about how well the telescope is working. In this case they indicate that Webb is aligned extremely well.

> The "spike" structures were a function of the design of Webb's primary mirror, explained Prof Mark McCaughrean from the European Space Agency (Esa).

> "The shape of those 18 hexagons imprints a faint diffraction pattern that makes bright stars look like spiky snowflakes - this isn't a problem for the science, but will give Webb images a very distinctive look," he told BBC News.

> "Indeed, the fact that we can see those spikes so crisply also confirms that the mirrors have been perfectly lined up - this is brilliant news."