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2 years ago

Why Do X-Ray Mirrors Look So Unusual?

Completed quadrant of an X-ray Mirror Assembly, under development for the JAXA/NASA XRISM mission. It is shaped like a fan with thin metal struts holding it together.

Does the object in this image look like a mirror? Maybe not, but that’s exactly what it is! To be more precise, it’s a set of mirrors that will be used on an X-ray telescope. But why does it look nothing like the mirrors you’re familiar with? To answer that, let’s first take a step back. Let’s talk telescopes.

How does a telescope work?

The basic function of a telescope is to gather and focus light to amplify the light’s source. Astronomers have used telescopes for centuries, and there are a few different designs. Today, most telescopes use curved mirrors that magnify and focus light from distant objects onto your eye, a camera, or some other instrument. The mirrors can be made from a variety of materials, including glass or metal.

Diagram showing a reflecting telescope with a pair of mirrors to focus the light on the detector — in this case, an observer’s eye. The diagram shows the “flow” of light, which starts at a distant galaxy, enters the telescope and bounces off the primary mirror at the bottom of the telescope. Then the light moves to the secondary mirror which redirects the light out of the side of the telescope tube into the observer’s eye.

Space telescopes like the James Webb and Hubble Space Telescopes use large mirrors to focus light from some of the most distant objects in the sky. However, the mirrors must be tailored for the type and range of light the telescope is going to capture—and X-rays are especially hard to catch.

X-rays versus mirrors

X-rays tend to zip through most things. This is because X-rays have much smaller wavelengths than most other types of light. In fact, X-rays can be smaller than a single atom of almost every element. When an X-ray encounters some surfaces, it can pass right between the atoms!

X-ray image of a human elbow. Denser materials, like bone, stop more X-rays than skin and muscle.

Doctors use this property of X-rays to take pictures of what’s inside you. They use a beam of X-rays that mostly passes through skin and muscle but is largely blocked by denser materials, like bone. The shadow of what was blocked shows up on the film.

This tendency to pass through things includes most mirrors. If you shoot a beam of X-rays into a standard telescope, most of the light would go right through or be absorbed. The X-rays wouldn’t be focused by the mirror, and we wouldn’t be able to study them.

Animation first showing a plane of balls face-on and an arrow passing through the space between the balls. Then the angle changes to show the balls edge-on and an arrow bouncing off the top.

X-rays can bounce off a specially designed mirror, one turned on its side so that the incoming X-rays arrive almost parallel to the surface and glance off it. At this shallow angle, the space between atoms in the mirror's surface shrinks so much that X-rays can't sneak through. The light bounces off the mirror like a stone skipping on water. This type of mirror is called a grazing incidence mirror.

A metallic onion

Telescope mirrors curve so that all of the incoming light comes to the same place. Mirrors for most telescopes are based on the same 3D shape — a paraboloid. You might remember the parabola from your math classes as the cup-shaped curve. A paraboloid is a 3D version of that, spinning it around the axis, a little like the nose cone of a rocket. This turns out to be a great shape for focusing light at a point.

A line drawing of a parabola - a cup-shaped curve, shown here on its side - spins around to create a 3D shape. The word “paraboloid” shows on the screen. Then part of the curve fades away, leaving behind two things: a small concave circle, which was one end of the paraboloid, labeled “Radio dishes; optical, infrared and ultraviolet telescope mirrors,” and a cylinder with sloping walls, which was the part of the edges of the paraboloid, labeled “X-ray mirrors.”

Mirrors for visible and infrared light and dishes for radio light use the “cup” portion of that paraboloid. For X-ray astronomy, we cut it a little differently to use the wall. Same shape, different piece. The mirrors for visible, infrared, ultraviolet, and radio telescopes look like a gently-curving cup. The X-ray mirror looks like a cylinder with very slightly angled walls.

The image below shows how different the mirrors look. On the left is one of the Chandra X-ray Observatory’s cylindrical mirrors. On the right you can see the gently curved round primary mirror for the Stratospheric Observatory for Infrared Astronomy telescope.

On the left, a technician stands next to a cylinder-shaped mirror designed for X-ray astronomy. The mirror is held in a frame a little off the ground, and is about as tall as the technician. On the right, two technicians inspect a round mirror for optical astronomy.

If we use just one grazing incidence mirror in an X-ray telescope, there would be a big hole, as shown above (left). We’d miss a lot of X-rays! Instead, our mirror makers fill in that cylinder with layers and layers of mirrors, like an onion. Then we can collect more of the X-rays that enter the telescope, giving us more light to study.

Completed X-ray Mirror Assembly for the X-ray Imaging and Spectroscopy Mission (XRISM, pronounced “crism”), which is a collaboration between the Japan Aerospace Exploration Agency (JAXA) and NASA, along with ESA participation. The assembly has thin metal struts fanning outward from a silver ring in the center of the image. Shiny ridge surfaces (actually many thin mirrors!) fill in the spaces between the struts.

Nested mirrors like this have been used in many X-ray telescopes. Above is a close-up of the mirrors for an upcoming observatory called the X-ray Imaging and Spectroscopy Mission (XRISM, pronounced “crism”), which is a Japan Aerospace Exploration Agency (JAXA)-led international collaboration between JAXA, NASA, and the European Space Agency (ESA).

The XRISM mirror assembly uses thin, gold-coated mirrors to make them super reflective to X-rays. Each of the two assemblies has 1,624 of these layers packed in them. And each layer is so smooth that the roughest spots rise no more than one millionth of a millimeter.

Chandra observations of the Perseus galaxy cluster showing turbulence in the hot X-ray-emitting gas.

Why go to all this trouble to collect this elusive light? X-rays are a great way to study the hottest and most energetic areas of the universe! For example, at the centers of certain galaxies, there are black holes that heat up gas, producing all kinds of light. The X-rays can show us light emitted by material just before it falls in.

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