What is approximately the greatest distance out to which it is possible for ground-based telescopes to measure

Question by lapis: What is approximately the greatest distance out to which it is possible for ground-based telescopes to measure
What is approximately the greatest distance out to which it is possible for ground-based telescopes to measure parallax? Also, why does such a limit on parallax measurements exist?

Best answer:

Answer by Mmkeita K
i dont no

Know better? Leave your own answer in the comments!

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2 Comments

1.   KBW3   |  Monday, 19 July 2010 at 10:24 pm

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   There are different estimates. Pessimists might say we really can’t do better than about 50 light years reliably. Others may claim two or three hundred. Our ability will increase with more orbiting measuring devices planned in the future.

   Parallax works by shooting an angle to a star from two positions which form the bottom two angles on a very long triangle. We use trigonometry to figure out how far away the other vertex of the triangle is. Our ability to do this is limited over long distances. Here’s an example. Suppose you could see and point at some point in California from your position in Maryland. Now you shoot an angle to it. Now you move over a half an inch and shoot another angle. You can see how close these two lines are to being parallel. And it is very difficult to tell where they intersect, or how far off of 90 degrees they are to the 1/2-inch baseline. It is actually about these same proportions that we are trying to measure stars with. The earth’s orbital diameter would be the half inch. Mind boggling that we can do even a fraction of this.
   
2.   mathematician   |  Monday, 19 July 2010 at 10:50 pm

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   The biggest problem for ground-based telescopes is the atmosphere. It is always moving around and making the images dance. because of this, it is difficult to measure very small angles, like those needed for paralax determinantions. In addition, the paralax measurements have to be made up to half a year apart so that the earth is on opposite sides of its orbit.

   Modern telescopes use ‘adaptive optics’, whihc is a trick to cut down on the effects ofatmospheric turbulence. If this is well controlled, the biggest effect limiting paralax measurements are due to diffraction in the telescope. This will blur the image because of different paths that the light takes through the telescope. A telescope with larger diameter has smaller interference effects, so would allow smaller paralax to be measured.

   There is also a difficulty stemming from how bright a star is. Dimmer stars turn out to be much more dififcult to measure the paralax of, so good ground-based measurements were mostly for the brighter stars.

   Because of these effects, ground based measurements are limited to distances of around 50 parsecs (around 160 light years), although occasionally farther distances can be done.

   Since the Hipparchos probe, which was an orbiting telescope, we have very good paralax measurements to an accuracy of 1 milliarc second. This gives accuracies of 25% out to about 800 light years.