reply to post by VoidHawk
It's all about a telescope's "resolving power", known as
Angular Resolution
The angular resolution of a telescope can be found by:
R= Wavelength/Diameter
Where R is radians, Wavelength is the wavelength of the light from the object and Diameter is the diameter of the telescope lens.
Hubble's Angular Resolution is around 0.1 arcseconds, which is 0.000027 degrees
Sounds impressive........however, you then have to consider not only the size of an object you wish to view in the telescope, but how far away it is.
Because the further away an object is, the smaller the amount of degrees or radians in the sky it will cover.
To determine if a telescope can "resolve" something or not (be able to see clearly), we figure out it's angular resolution, and we also need to know
the object that we want to see, what it's
Angular Diameter is.
In astronomy the sizes of objects in the sky are often given in terms of their angular diameter as seen from Earth, rather than their actual sizes.
Since these angular diameters are typically small, it is common to present them in arcseconds. An arcsecond is 1/3600th of one degree, and a radian is
180/pi degrees, so one radian equals 3600*180/pi arcseconds, which is about 206265 arcseconds. Therefore, the angular diameter of an object with
visual diameter d at a distance D, expressed in arcseconds, is given by:
Ad = 206265 d / D arcseconds.
Let's say we are looking at a star the same size as the Sun, and it is 1 lightyear away. It would cover 0.03 arcseconds in the sky, so while Hubble
would see it as a point of light, because it's angular resolution is only 0.1 arcseconds, it will not be able to resolve the star to show any
detail.
Let's say it's a Earth sized planet at the same distance (1 light year). The angular diameter will be about 0.0003 arcseconds, so again.....Hubble can
not resolve any details (IE show what it looks like, land masses, water bodies, etc).
"So why can Hubble show us such detailed images from objects that are so far away then?"
What people either forget, or do not know is: many of those objects that Hubble shows you are HUGE in size.
Take the Orion Nebula, Hubble gives us beautiful detailed images of it, even though it's over 1,300 lightyears away.
Yep, it does. However, as far away as that is, the Orion Nebula is over 12 lightyears across. That's huge.
It covers 65 x 60 arcminutes of the sky.
That's 3900 x 3600 arcseconds. Hubble's angular resolution is 0.1 arcseconds. So yes, it can resolve it.
You can even figure out what the largest objects on the moon that Hubble can see:
Hubble's resolution = 0.1 arcseconds.
So:
(0.1 / 20265) * 384400 (km to the moon) = 0.186 km or 186 meters. So the smallest craters that it can see are about 186 meters across. Anything
smaller is going to be blurred, distorted or just not visible.
edit on 5-3-2014 by eriktheawful because: (no reason given)