Earlier this week, I showcased the newly-released Hubble Ultra Deep Field in the infrared, and compared it with the older image of the same region taken in visible light.
As many astute readers noticed, the newer image looks blurrier than the old one! This is true, and there’s a good reason for this. Here’s one of the old images from the Ultra Deep Field (in visible light):
and here’s that same region in infrared light.
Yes, the latter image is blurrier. Why is it blurrier? One of the biggest differences between infrared light and visible light is their wavelengths; visible light goes from about 400 to 700 nanometers, but infrared light has a longer wavelength than visible. In fact, as you get redder and redder (or infra-redder, as the case may be), your wavelength continues to increase substantially.
Hubble’s new camera, in the infrared, can see from about 800 to 1700 nanometers, or wavelengths more than twice as long as visible light! Well, your telescope’s resolution, or how sharply you can see, is directly related to the wavelength of light you look at.
Why is that? Take a look at the Hubble Space Telescope’s mirror:
It’s got a certain, fixed diameter (2.4 meters). If you send violet light (400 nm) at it, it takes 6 million wavelengths to get across it, meaning it can resolve angles down to 0.034 arc-seconds! (Remember, there are 60 arc-minutes in one degree, and 60 arc-seconds in one arc-minute.) But if you look at the longest wavelength infrared light that Hubble can see (1700 nm), it only takes 1.4 million wavelengths before the light gets across the mirror, meaning that it has a resolution of 0.146 arc-seconds, more than four times worse than the violet light case!
So that’s why the newer image looks blurrier, because you’re looking at a longer wavelength of light! Incidentally, that’s why lone radio telescopes are as big as they are — even built into mountains, sometimes — because the wavelength of radio light is measured in meters, not nanometers! Here’s a shining example: the 305 meter Arecibo telescope!
So why? Why would we look in a wavelength with inferior resolution? Because resolution isn’t the only important factor for astronomy. When galaxies are far away, the light from them gets redshifted, meaning you need to look at longer wavelengths to find them. You look in the infrared instead of the visible, and suddenly you’re able to see at higher redshifts, and hence greater distances! Can you spot any blobs in the newer, fuzzier images that simply don’t exist in the older one? If you can, you’ve likely found a new, high-redshift galaxy! Take a look at this side-by-side view of a tiny region of both images:
That extra bright red spot on the left? That’s an ultra-high-redshift galaxy that cannot be seen in visible light! Know what else is probably an ultra-high-redshift galaxy? That green smudge below and to the left of the bright red spot; it isn’t in the old image!
So you can be upset all you want at the lower resolution; there’s plenty of fantastic science that we need the infrared for! And if you just can’t see how the infrared will ever compete with the visible, remember that the James Webb Telescope is coming up. It’s an infrared space telescope, and it’s just a little bigger than Hubble.