Good morning, everyone.
Just about everyone here has had some experience with X-rays; you get your teeth X-rayed at the dentist, if you break a bone, you get X-rayed at the hospital. Critical welds in gas pipelines are quality checked using X-rays (they are supposed to be, anyway). The inner workings of machines can be examined using X-rays, and even fossils of dinosaurs can be examined while still encased in rock. It seems that X-rays can be used to see through just about anything.
However, consider the photograph at the top of this article. It is an image of Sagittarius A*, the supermassive black hole at the center of our galaxy, and it was taken with the X-ray telescope on the Chandra X-Ray Observatory.
An X-ray telescope? How do you make a telescope to work with X-rays? How do you make a mirror or a lens to focus photons that are energetic enough to penetrate pretty much any material that we know of?
It’s all done with mirrors.
See? A nice, simple explanation, and now you can all go back to work. But, I suppose that at least some of you would like a less-concise explanation. I suppose I could be bothered to continue, and I still have a half-cup of coffee left. So let us first take a quick look at what X-rays are and where they come from.
X-rays are a form of high energy electromagnetic radiation. They have a shorter wavelength and higher energy than ultraviolet radiation, and a lower energy and longer wavelength than gamma rays. Rather than throw a bunch of frequencies, energies and wavelength numbers at you, I refer you to the chart above which breaks them down nicely. It also contains lots of additional information about the various wavelengths that is quite useful. Like most UV and all gamma rays, X-rays are an ionizing radiation, meaning that they knock electrons off of whatever atoms they strike, which is why you don’t want them striking you a lot, or to use a better term, irradiating you.
Earthly X-rays are generated by three primary methods: X-ray fluorescence, bremsstrahlung (braking radiation), and synchrotron radiation. Astrophysical X-rays are generated primarily by heat, from about a million kelvin up through hundreds of millions of kelvins and beyond. Stars fall into this category, as does matter falling onto a neutron star or into a black hole from its accretion disc. There are astrophysical objects that emit, fluoresce, and reflect X-rays, as well as bremsstrahlung, black body radiation, synchrotron radiation, and a thing called inverse Compton scattering. They are all over the place out there. You can’t swing a cat, etc, etc.
X-rays are absorbed by the atmosphere, so if you make an X-ray telescope you have to hoist it into orbit if you want to see anything. This simple chart shows what wavelengths are absorbed.
X-rays act like little bullets when they strike matter, just like if you were shooting a gun at a wall. However, if you take the gun and shoot it from a very shallow angle to the wall, very nearly parallel, you can get a ricochet, and X-ray telescopes use this same principle. They are called Wolter telescopes and they use grazing incidence optics. The mirrors in a Wolter telescope are a series of nested tubes with very specific curvatures that direct the X-rays into a focal point. In the case of the Chandra telescope, that would be onto one of two detectors . One is a CCD spectrometer, and the other is a high resolution camera using two micro channel plate components. Here are a couple of diagrams showing the light path in the Chandra X-ray telescope:
The four pairs of mirrors in the Chandra had to be manufactured to incredible tolerances. They are so smooth that if they were surface of the Earth, the highest point would be 78 inches tall. Ooooo. They are coated with iridium, which is both dense and highly reflective, important characteristics for a mirror dealing with X-rays. They were then installed and aligned with such precision in the support structure that the alignment of the mirrors from one end of the nine foot assembly to the other is accurate to 1.3 micrometers, or 50 millionths of an inch.
For the size of the Chandra telescope, its focal length being 10 meters or 33 feet, it has a collecting area or ‘aperture’ of only .43 square feet at 1 keV. That would be absolutely puny for an optical telescope, especially one that was hauled up to high Earth orbit. People would definitely talk. The wavelengths that the Chandra telescope can observe are from 0.1 to 10 keV, and I’m not sure how the higher energies alter the effective ‘aperture’. That was never made clear to me from the literature I had access to. Everything about X-ray telescopes is very different from their optical counterparts and I’m behind the curve on them, but it’s fun trying to get familiar with how they work.
So the next time you see an X-ray image of a celestial object, you’ll have some idea of what took the picture, even if you’re not quite certain how it works.
I was concerned that I didn’t include enough complicated charts, so I’m throwing in an extra that has the added attraction of being hard to read in places. No need to thank me.
This actually my favorite EM spectrum chart. It has all sorts of interesting information on it, but unfortunately it’s not a very good image and I’d like to find a better, clearer one. The problem is, I don’t know who made it or where it came from, as I found it by accident in a hires archive on 4chan. All of the the images that I found using Google are of the same picture, although some are of different sizes. If anyone knows where I can find a better copy, please let me know in comments if you would be so kind. Thanks.
All those bloody hyperlinks in the body of the text.