# If the Hubble telescope can take excellent pictures of stars and galaxies forming millions of light years away, why can't it take a good picture of Pluto?

Nov 24, 2015

The Hubble telescope has an angular resolution of $.05 \text{ arc-seconds}$. Galaxies are much larger than that, but Pluto is not.

#### Explanation:

Hubble uses a special type of camera called a CCD, or charged couple device. These cameras use a grid of "bins" which collect and count photons. These "bins" correspond to pixels in the resulting image. If we look at a Hubble image of Pluto from 1996, we can see these pixels.

Currently, Hubble's resolution is about $.05 \text{ arc-seconds}$. That means that each of these pixels is $.05 \text{ arc-seconds}$ wide. A single arc-second is $1 \text{/} 3600$th of a degree. We measure resolution in arc seconds because it tells us how much of our view of the sky an object takes up. To put this in perspective, the entire sky is a ${360}^{o}$ field of view, or $1 , 296 , 000 \text{ arc-seconds}$.

Pluto's angular size changes as it orbits the sun. The closer it is to the Earth, the larger it appears. At its closest, Pluto is $4.28 \text{ billion km}$. Comparing that with Pluto's diameter of $2372 \text{ km}$ tells us that Pluto's largest angular size is;

"Angular Size" = "Size"/"Distance" = (2372 "km")/(4.28 * 10^9 "km") = 5.54*10^-7

This is the angular size in radians. If we convert to arc-seconds, we get;

$.11 \text{ arc-seconds}$

Which is about $22$ pixels across for Hubble. Now lets look at a galaxy. NGC 1300 is a spiral galaxy $23.7 \text{ million light years}$ away, which has an angular size of $1116 \text{ arc-seconds}$. That's about $10 , 000$ times the angular size as Pluto, meaning that we can see much more detail.