A recent press release from the Herschel Space Observatory noted the fact that Herschel has the “largest mirror ever launched into space.” This is true. Herschel’s primary mirror is 3.5 meters in diameter, while Hubble’s primary mirror is only 2.4 meters across. Given that knowledge, one might expect the images from Herschel to surpass those of Hubble. After all, bigger is better, right?
Within the press release is the above Herschel image of the galaxy cluster Abell 2218, including a comparison to a Hubble image. It may surprise you to see that Herschel’s view is incredibly fuzzy compared to Hubble’s view. What happened? Perhaps size doesn’t matter?
The important point to recognize is that the Herschel image captures infrared light, while Hubble’s view uses visible light. The resolution of a telescope depends not only on the size of the mirror, but also on the wavelength of light it is observing. In fact, for the same size mirror (and equivalent optics), resolution is inversely proportional to wavelength. Shorter wavelengths have better resolution.
Herschel’s infrared wavelengths are about 1,000 times longer that Hubble’s visible wavelengths. To achieve equivalent resolution, Herschel would need a mirror 1,000 times larger than Hubble’s. While Herschel’s mirror is about 50% larger than Hubble’s mirror, the wavelengths it observes are many times longer. Hence, its infrared resolution is not near what Hubble can provide at visible wavelengths.
In astronomy, size does matter, but only in proportion to wavelength.
However, do not discount these truly wonderful observations from Herschel. For an infrared telescope, it surpasses anything that has come before. Once one understands the wavelength region Herschel is looking at, one can recognize that details like this have not been seen before at these wavelengths. And it is the multi-wavelength character of modern astronomy that has helped us to gain much more information about planets, stars, nebulae, and galaxies, and to genuinely see the universe in another light.