The Hubble Space Telescope is pretty amazing but does it still produce the best pictures? I’ve heard about these massive telecopes which also countenance the Earth’s atmospheric disturbance. So are these now providing better science?
And a follow up question is: Has it ever been pointed at the Earth? What would it see if it was?
The Hubble 'scope wouldn’t see anything at all if it was pointed at Earth, because it would not be able to focus on it. Ever take a picture of something that was too close to the lens? Same idea; the Hubble is a really really really really big camera with telephoto lens. AFAIK, there is no good way to counteract the effect of atmosphere on telescope images. There are certainly bigger telescopes on Earth, but there are things so dim they could never see inside the atmosphere.
There are actually a couple of ways to control for turbulence in the atmosphere, without using a space-based telescope. This month’s Scientific American gives details.
http://www.sciam.com/2001/0301issue/0301armstrong.html
Emphasis added.
friedo wrote:
I don’t think this is true. With any lens, if you focus it to “infinity” so that incoming parallel rays are focused to the same point, there is a distance that will be far enough so that for all practical purposes, it can be considered infinity.
If the Hubble’s lens is about one meter across, and it’s orbit around 300 km above the surface, then the blur due to being out-of-focus would correspond to a resolving power of around 0.7 arc-seconds. The Hubble can do 0.1 arc-seconds, so it wouldn’t be as clear as the Hubble could do, but the best Earth-based telescopes resolve about 1.0 arc-seconds, so it would be clearer than that.
According to my bar-napkin calculations, anything greater than about 2000 km from the Hubble would be practically infinity.
minor nitpick…
According to NASA, the altitude of the HST ranges from 586.47 km to 610.44 km.
ARG! First, Hubble has a mirror, not a lens.
Second, it used to point at the Earth all the time. It did this to take what are called ‘flat fields’; images that are evenly illuminated. They are used to calibrate the camera: if one pixel is twice as sensitive as another, that will be seen in the flat field and can be accounted for when processing an image. The flats were also called ‘streak flats’ because you could see blurred trails of things in them. Trees, houses, whatever. They’re actually pretty nifty.
Third, Hubble is considered a small 'scope. The primary mirror is 2.4 meters across, while the biggest are the Keck twins in Hawaii with enormous 10 meter mirrors. 8 and 6 meter 'scope are getting to be fairly common, actually. Hubble’s advantage is being over the atmosphere, so there is no distortion in the images.
Here are some places to read about all this:
http://www.badastronomy.com/bad/misc/badhst.html
http://www.badastronomy.com/bitesize/twinkle.html
http://www.badastronomy.com/bitesize/adaptive.html