# Doppler

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V

##### Guest
what is the fastest speed that can be picked up using the doppler method?

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#### yruc

##### Guest
v = c x (wavelength shift) / (wavelength) <br /><br />where v is the relative velocity (speed) of the star, c is the velocity of light, and {wavelength} the known wavelength of the line as measured in the laboratory. We can use this equation until the relative speed becomes a significant fraction of the speed of light -- and then it's back to Einstein's Theory of Relativity, for a more detailed formula. <br /><br />Link: http://www.astrocappella.com/background/doppler_background.shtml

S

##### Guest
You've not specified the direction. When recessional & the shift produces zero frequency, making it undetectable.

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#### Saiph

##### Guest
there is no theoretical limit. The problem is if it shifts to far (especially to longer wavelengths) we may not be able to detect it anymore. So that's a practical limit.<br /><br />That practical limit is approached only when objects are basically going the speed of light. But even a little slower (like .999 C) should easily be detectable. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

V

##### Guest
Thank you all, I was wondering about moving bodies such as asteroids and comets and how they can be detected? Here is a site that has used ccd imaging to carry out these tests..........<br />http://spacewatch.lpl.arizona.edu/<br /><br />

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#### zenith

##### Guest
There is a robotic optical telescope in remote NSW Australia, which uses the ccd imaging, where the computer maps all objects found in the image, overlays images taken over a time period, and traces moving objects... though it is only funded to track those over 1 kilometre in diametre which is heading toward earth..

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#### nexium

##### Guest
I'm not an expert, but when the dopplar shift goes from gamma to infrared = 99.9%? of c the signal keeps getting weaker, and harder to separate from other signals. The technology is still improving, so we should be able to get readings with confidence even closer to the speed of light soon. Please embellish, refute and/or comment. Neil

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#### nexium

##### Guest
I suspect the one km limit applies to poor conditions and objects smaller than 100 meters are occassionally recorded. Nearly all objects have at least a tiny direction component toward or away from Earth. <br /> Perhaps the computer solves equations in multiple unknowns, which produce equations for the orbit of each objects plus it's distance, obedo, and dimentions. I think that is possible, and the computer could likely also predict the margin of error of each bit of data. Neil

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#### newtonian

##### Guest
shuttle_guy - you posted:<br /><br />Everything outside of our solar system and all other star systems is receeding from every thing else.<br /><br />All other star systems? You lost me!<br /><br />However, there are a large number of galaxies which are not red-shifted, but rather blue-shifted, I believe.<br /><br />Certainly Andromeda is blue-shifted.<br /><br />Galaxies in the local group and the supercluster are gravitatioanlly bound, more or less, to a Great Attractor - they are not expanding from everything else.<br /><br />Also, as I posted on another thread, astronomer Wendy Freidman has questioned the value of the Hubble constant - see 1998 Scientific American Presents, pp. 92-97.<br /><br />You all - the obvious upper limit for recessional speed is the speed of light.<br /><br />I believe that is also the limit for blue-shift measurements. We would not be able to measure the Doppler effect of an object heading towards us faster than light.<br /><br />Search under tachyons, for example.<br />

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