By Joshua N. Winn, Princeton University
There are many reasons why astronomers love their telescopes. One would be for the huge improvement in the spectral resolution they offer. This enhances their ability to measure the energies, or wavelengths, of the incoming photons. So, how does that work?
Our eyes can sense millions of different colors, and make very fine distinctions between photons with wavelengths ranging from about 0.4 to 0.7 microns. We perceive the rainbow as wild ride through all kinds of different sensations, from red to orange, yellow, green, and blue. But as miraculous as color vision is, our eyes can’t analyze the different color hues in any quantitative way, and certainly not in low light, when our vision is basically black-and-white.
Astronomers love telescopes as with them they can distinguish colors in several different ways. We can put colored filters in the light path, which admit only the light within a narrow range of wavelengths. Then we can take exposures through different filters, and combine them, sort of like how our brains combine the signals from red-, blue-, and green-sensitive cells. Or, if we want to make finer distinctions, we can put a prism in the light path, which deflects the light by an amount that depends on wavelength. That way the light from a star, instead of appearing as a point in the image, shows up as a little stripe of light, a miniature rainbow.
Now, if there are other stars nearby, we wouldn’t want all those rainbows to overlap. That would be confusing. So, before the beam of light reaches the prism we interrupt it with an opaque sheet with a slit in it, and position the slit so that the light from our favorite star, or galaxy or whatever, goes right through the slit and all the other sources of light are blocked. That’s spectroscopy. The camera takes a picture of the spectrum of light, so each pixel in the image records how many of the arriving photons have a wavelength within a particular range.
This sorting the photons by wavelength is a powerful technique for understanding the nature of a light source. For example, we can check if the light has a Planck spectrum, and if it does, we can measure the temperature of the source.
This article comes directly from content in the video series Introduction to Astrophysics. Watch it now, on Wondrium.
The Spectrum of the Sun
Modern spectrographs differ in detail from the simple prism-and-camera design. Instead of a prism, they’re more likely to use a diffraction grating; that’s a series of regularly spaced grooves in an otherwise flat surface, and the grooves have a different reflectivity from the surrounding material, so that, when a light beam bounces off, the reflections from the stripes interfere with each other in such a way that the direction of the reflected beam depends on wavelength.
In a spectrum of the Sun, recorded by a spectrograph with a diffraction grating one can see the full rainbow of colors, but if we look closely, we’ll see that it’s punctuated with thin dark lines. One can get an even better look at those lines with an echelle spectrograph, which has even higher resolution. It uses 2 diffraction gratings to spread out the spectrum in 2 dimensions, so that one can read it like a book. Photon energy increases as we go across from the left to the right, and then when we reach the end of a line, we go down to the next line.
Apparently, there are certain colors that are missing from sunlight, certain wavelengths at which the Sun looks darker than usual. Nonetheless, these spectral absorption lines do teach us about the composition of the Sun, and the conditions in its outer atmosphere.
Telescopes and Temporal Resolution
Another reason astronomers love telescopes is because they improve temporal resolution, the ability to measure the time of arrival of a burst of photons, or any more gradual variations in their rate of arrival. Our eyes have a sort of built-in exposure time of about a 10th of a second. That’s why movies work: we can watch a series of still images that’s changing every 20 milliseconds and be fooled into believing that we’re watching continuous motion.
For astronomy, a fixed exposure time would be a severe limitation. Cameras allow us to choose whichever exposure time is most appropriate for the purpose. With short exposures, we can witness events that happen quickly, like ‘slo-mo’ movies. Today, we have cameras capable of capturing scenes on the scale of milliseconds, or even microseconds. This has led the discovery of all sorts of fascinating, rapidly-varying sources.
The Crab Nebula
An example of this would be the images of the Crab Nebula. A splotch of color marks the site of a core-collapse supernova, the explosion of a massive star. The light from the explosion first reached Earth in the year 1054, and was bright enough to see in the day time, as attested by ancient Chinese records.
What the Chinese could not know, though, is that at the center of this nebula is a neutron star that blinks, periodically and the time interval between blinks is only 33 millisecond.
Common Questions about Spectral Resolution
Human eyes can’t analyze the different color hues in any quantitative way, and certainly not in low light, when our vision is basically black-and-white.
There are certain colors that are missing from sunlight, certain wavelengths at which the Sun looks darker than usual. Nonetheless, these spectral absorption lines teach us about the composition of the Sun, and the conditions in its outer atmosphere.
Our eyes have a sort of built-in exposure time of about a 10th of a second. That’s why movies work: we can watch a series of still images that’s changing every 20 milliseconds and be fooled into believing that we’re watching continuous motion.