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?

1 Answer
Nov 24, 2015

The Hubble telescope has an angular resolution of #.05 " arc-seconds"#. Galaxies are much larger than that, but Pluto is not.


While researching this answer, I came across this article which provides a more in depth answer to the question.

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 " arc-seconds"#. That means that each of these pixels is #.05 " arc-seconds"# wide. A single arc-second is #1"/"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 " arc-seconds"#.

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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 " billion km"#. Comparing that with Pluto's diameter of #2372 " 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 " arc-seconds"#

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