By Joshua N. Winn, Princeton University
There are three main cultures in observational astronomy—optical, radio, and X-ray, each of which uses telescopes to collect data on invisible radiation. The telescopes are aimed at going beyond the visible range to study and explore all the other orders of magnitude.
Lenses or Mirrors?
Unlike the x-ray culture, in the optical and radio domains, photons don’t pack energy, so the optical and radio telescopes can make use of mirrors to focus them. This allows us to collect much more light than a pinhole camera.
At optical wavelengths, you could use a lens instead of a mirror, but lenses have a problem. Because they’re made of glass, they act like prisms, even when you don’t want them to—the amount they deflect the light depends at least slightly on its wavelength. That introduces chromatic aberration, and so what should be a white point in the image turns into a multicolored blob.
That’s one reason why astronomers use reflective optics: glass mirrors, coated with aluminum or silver. Another reason is that a big mirror is easier to support than a lens: only one side of the mirror matters, the reflective side, so you can lay it shiny-side up on a stable supporting structure. As opposed to a lens, which you can only grip on the rim, or else you’ll block the light.
Type of Mirror
The mirror used in the telescope has to be curved, to focus light. One possibility is to use a mirror whose surface has the shape of a parabola. What’s special about the parabola is that all the photons coming in along the symmetry axis get bounced to the same point: the focal point.
So, a parabolic bucket not only collects them but also re-directs and concentrates them into a small area, and that’s where we can put a detector.
Off-axis, the rays hitting different parts of the parabola land in slightly different locations in the focal plane. That causes image distortions, or aberrations. Stars near the middle of the image appear pretty sharp, but away from the center, they look like little cones, or comets. That’s why this type of distortion is called coma.
This article comes directly from content in the video series Introduction to Astrophysics. Watch it now, on Wondrium.
Parabola in Radio Telescopes
Parabolic dishes are used in radio telescopes. You point it straight at a source of radio waves and measure the radio static coming from that direction. And to make an image, you can slew the dish through a range of directions, recording the intensity of the static as you go.
At shorter wavelengths, it’s more common to use the imaging property of the parabola, in addition to the focusing property. Photons arriving head-on, straight down the symmetry axis of the parabola, all end up at the focal point.
But what about the ones coming from a slightly different angle, off-axis? They don’t get directed to the focal point, but they do get concentrated near a different point, displaced to the side from the focal point.
For small angles the displacement is proportional to the incoming angle, which is just what we want, to make an image. It’s a mapping between incident direction, and location in a surface. To capture the image, we insert a 2D detector with lots of separate pixels, each of which can record the intensity at a point in the focal plane.
Optical Telescopes
For optical astronomers, the detector of choice is the charge-coupled device, or CCD, which uses a thin layer of pure silicon. Photons hit the silicon and knock loose some electrons, which can then be trapped and counted by electronics mounted on the silicon surface.
Interestingly, optical astronomers rarely use parabolas. You see, it’d be wonderful if all the photons from a given direction get sent to a single point in the focal plane. But that’s not the case; only the on-axis light is focused perfectly.
Should we search for a better shape than the parabola, one that will focus light to a single point no matter which way it’s coming from? Nope. There’s no such shape for a mirror, even in principle, that lacks aberrations. James Clerk Maxwell and Ernst Abbe proved that in 1856.
Instead, what astronomers do to reduce aberrations is use multiple mirrors. The light hits a primary mirror, and bounces to a secondary mirror, which is also curved, before going to the detector. Having more than one surface that one can adjust gives the optical designer the freedom to reduce whatever kind of aberration is worrying the most. We can use mirrors with elliptical, or hyperbolic cross sections instead of parabolic or use three mirrors.
Common Questions about Optical and Radio Telescopes
The mirror used in the telescope has to be curved, to focus light. One possibility is to use a mirror whose surface has the shape of a parabola. What’s special about the parabola is that all the photons coming in along the symmetry axis get bounced to the same point: the focal point. So, a parabolic bucket not only collects them but also re-directs and concentrates them into a small area, and that’s where we can put a detector.
Interestingly, optical astronomers rarely use parabolas. It is only the on-axis light that is focused perfectly. Off-axis, the rays hitting different parts of the parabola land in slightly different locations in the focal plane, causing image distortions, or aberrations.
What astronomers do to reduce aberrations is use multiple mirrors. The light hits a primary mirror, and bounces to a secondary mirror, which is also curved, before going to the detector. Having more than one surface that one can adjust gives the optical designer the freedom to reduce whatever kind of aberration is worrying the most. We can use mirrors with elliptical, or hyperbolic cross sections instead of parabolic or use three mirrors.
