The crisp, stunning images from the Hubble Space Telescope are a wonder to behold. Every bit i tin can see in the image comparison below, Hubble's views are significantly higher resolution than like images obtained by basis-based observatories.

m51_comparison_detail-wiyn_hst-1000x1000
Ground-based (left) vs space-based (correct) images of star-forming regions in the Whirlpool Galaxy. On the left is the view from the WIYN telescope at Kitt Peak in Arizona. On the correct is the view from the Hubble Infinite Telescope.

WIYN Image Credit: Thousand. Rhode, Thousand. Young and WIYN/NOAO/Aureola/NSF
HST Image Credit: NASA, ESA, S. Beckwith (STScI), and The Hubble Heritage Team (STScI/AURA)

Terrestrial telescopes must look through Globe's atmosphere, which blurs the view and limits their resolution. Orbiting to a higher place Earth'southward atmosphere, Hubble avoids that trouble and can get a clearer view of the universe.

While Hubble provides the highest resolution of any visible-light telescope, that resolution has a limit. At that place are many things in the universe that Hubble can't resolve, and the public is constantly curious about that boundary.

One question that we often hear is whether Hubble tin encounter the lunar landers left behind by the Apollo missions (short answer: no). We also get questions asking why Hubble has such poor views of nearby Pluto, when it can get almost 100 million pixels of the much more distant Whirlpool Milky way. To respond questions similar these fully, ane must delve into a combination of size and distance called "athwart resolution".

Reading the Signs

Let'due south showtime with an example from everyday life. When driving forth a highway, 1 tin can oft see signs far in the distance downward the road. At first, but the shape and color of the sign are recognizable. Then, one can tell that in that location is writing on the sign, but it is not possible to make out the words. Eventually, the words become clear enough to read.

content_highway_signs_near_far-crop-650x429
The signs on the virtually bridge are readable, while those on the far bridge are not.

The physical sizes of the sign and its lettering exercise not change. The major change is the distance between the observer and the sign. At a large altitude, the sign covers a very small bending in one'southward field of view and cannot exist read. When close, it covers a large plenty angle to be readable. Nosotros say that an object "subtends" an angle that depends on its size and distance from the observer. That "athwart size" is the important characteristic of the object in this scenario.

For the observer, the of import characteristic is called "angular resolution". The athwart resolution is a mensurate of the smallest bending at which the observer can distinguish betwixt two objects (or details within an object). Equally you probably know, there are 360 degrees of arc in a circle. For measuring small angles, we carve up each caste into threescore arcminutes, and each arcminute into 60 arcseconds. The angular resolution of the human middle is most 1 arcminute.

The result is that, for the sign along the highway, the words become readable when the letters accept an angular size that is several times larger than the athwart resolution of the human eye. Hence, the angular size of the messages needs to exist several arcminutes.

Hubble's Angle on the Universe

These same ideas apply to observations with the Hubble Space Telescope. Hubble has an athwart resolution of well-nigh 1/20th of an arcsecond. That is a very small-scale angle, but things in the universe can exist very, very far abroad. Objects whose angular size is less than this value are not resolved by Hubble. They are like a cosmic highway sign that is too small and also distant for fifty-fifty Hubble to read.

content_angular_sizes_comparison-blog-960x800
A comparison of the relative angular size, equally seen from Earth, for the Moon, 4 planets, and 2 Hubble images. For calibration, the Moon is about half a degree in angular size.

Allow's address whether Hubble can encounter the Apollo landers on the Moon. To be seen by Hubble, an object would need to subtend an bending greater than 0.05 arcseconds. The Moon is, on average, most 384,400 km away. At that distance, 0.05 arcseconds is equal to a size of 93 meters (101 yards), or the length of a football field.  An object on the Moon must exist a few football fields in size, or Hubble cannot resolve it. The Apollo landers are much smaller than a football field, and likewise minor for Hubble to see.

Now, what about the images of Pluto and the Whirlpool Milky way?

content_pluto_m51_resolution_comp-hst
Hubble images of Pluto (left) and the Whirlpool Galaxy (right).
Note: these images are at wildly dissimilar physical and angular scales.

At its closest point to the Sun, Pluto is about thirty times further from the Sun than Earth, which is a distance of virtually 4.5 billion km. At that distance, an angular resolution of 0.05 arcseconds corresponds to a physical size of only over 1,000 km. Pluto's diameter is a little less than 2,400 km, making information technology a little more than 2 pixels in a standard Hubble image. The paradigm above shows near 15 pixels across the diameter of Pluto. 1 should not be asking why the resolution is so bad, but, instead, why the resolution is and then good!

The extra resolution in the Pluto epitome is from the Faint Object Camera (FOC), which was part of Hubble's instruments from 1990 to 2002 (Information technology was removed during Servicing Mission 3B). Designed to see small, faint objects like Pluto, the FOC musical instrument had a high-resolution mode that provided 7 times the resolution of the standard Hubble cameras. The limitation of FOC was that it could provide such resolution over a very small-scale field of view, and at shorter wavelengths (green to ultraviolet). As such, FOC was non suited to general purpose imaging, and could not take images similar the one of the Whirlpool Galaxy higher up.

The Whirlpool Galaxy is not only much, much bigger than Pluto, but as well much, much farther away. Let's see how the size and distance factors play out in terms of angular resolution.

The Whirlpool is about 60,000 light-years across, making information technology medium-sized compared to the 100,000 lite-year diameter of our Milky way. At that size, the galaxy is around 250 trillion times larger than Pluto. The galaxy'due south distance is nearly 23 million lite-years, or near 50 billion times more than distant than Pluto. The size difference (250 trillion) is larger than the altitude difference (50 billion) by a cistron of 5,000. Therefore, Hubble should get effectually v,000 ten 2 = 10,000 pixels across an image of the Whirlpool. The total resolution of the above image is 11,477 pixels by 7,965 pixels.

Hubble's athwart resolution at the distance of the Whirlpool Galaxy corresponds to a large concrete altitude: over 5 light-years. However, the galaxy is roughly 10,000 times larger than that, and is extremely well-resolved.

Size, Distance, and Resolution

Concrete size of the object is important, but only part of the story. Distance to the object is as well a gene, but not enough for the full calculation. The combination of concrete size and distance, as expressed by angular size and athwart resolution, is the of import criterion for determining how well Hubble, other telescopes, or even the homo eye will be able to run across an object. Using these measures, one tin tell that Hubble has no hope of seeing the lunar landers, volition but barely discern Pluto, and can view the Whirlpool Galaxy in gorgeous detail. I promise we can at present consider these questions resolved.

  • Frank Summers is an astrophysicist at Hubble'southward Space Telescope Science Constitute, where he specializes in bringing astronomy discoveries to the public. He helps produce news, education, and outreach materials, gives educational and public presentations, and creates science visualizations and animations.

    View all posts