The spikes round brilliant stars in deep pictures are brought on by the way in which mild interacts with the form of a telescope’s mirrors and the struts holding them.
Diffraction spikes are brought on by interference because of the wave nature of sunshine. The patterns produced rely on the form of the telescope’s mirror and the variety of spider struts. Credit score: Astronomy: Roen Kelly, after Cmglee & Pete Lawrence/ Wikimedia Commons
I realized in October 2022’s Breakthrough that the spikes of stars in high-power pictures are referred to as diffraction spikes. Why do they seem as two crosses at 45 levels to one another?
Graham Smith
Werribee, Australia
The diffraction spikes so acquainted to us in space pictures are an intrinsic property of the telescope. Reflecting telescopes have two mirrors: a big main and a smaller secondary. Gentle hits the first first and displays onto the secondary earlier than being mirrored again by a gap within the main mirror to the telescope’s focus (the place the digicam sits). That secondary is held in place by skinny steel struts (collectively referred to as the spider), and these struts are largely the reason for diffraction spikes.
As incoming mild waves diffract (bend barely) across the edges of the struts, the waves start to overlap one another, inflicting an interference sample like ripples overlapping in a pond. This interference sample seems within the form of the orientation of the struts; these are the diffraction spikes we see. For instance, in Hubble Area Telescope pictures, we sometimes see 4 spikes at proper angles to one another as a result of Hubble has 4 steel arms holding its secondary in place.
However you’ll discover that pictures from the James Webb Area Telescope (JWST) have six massive spikes and two small ones. That is the place issues get a bit extra advanced. JWST’s spider has solely three struts, however on this case, not solely these struts are diffracting incoming mild; so are the 18 hexagonal segments of JWST’s main mirror. (Hubble’s main mirror doesn’t trigger the identical impact as a result of it’s a single spherical piece of glass.) Incoming mild diffracts when it encounters the sides of every of JWST’s non-round segments. The telescope’s design is such that the spikes from the mirrors and the spikes from two of the spider struts mix, creating the six bigger spikes. The 2 small horizontal spikes are from the remaining strut.
You will get many sorts of diffraction spikes, relying on the form of your mirror (or aperture) and the quantity and orientation of secondary mirror struts. All level sources (i.e., stars) are affected by diffraction spikes, however brighter sources have bigger spikes, which is why diffraction spikes seem apparent across the brightest stars however are unnoticeable in fainter ones.
Alison Klesman
Senior Editor
