How is Red Shift Measured Pecisely?

[skeptic]

How is red shift measured? I understand that astronomers compare the colors of spectral lines but how do they measure the color of the lines precisely in order to determine the amount of shift? <br /><br />How do astronomers compensate for red shift due to gravity wells such as those for quasars?

 

[doubletruncation]

There are many different ways to get spectra and determine redshifts. One such way might be as follows:<br /><br />Collect the light with a telescope and shine it at a grated metallic surface which diffracts the light into a spectrum (so light of different colors is comes off the grating at a different angle - you can see an example of this by looking at the reflection from the music side of a cd, you'll see a rainbow resoulting from the diffraction of light by the grooved surface of the cd). You then take a picture of the spectrum with a ccd (digital camera basically), if you line things up correctly each pixel in one direction along the camera corresponds to a different color, so you can measure how bright the light is at different colors. To actually figure out which pixels correspond to which wavelengths of light (or colors) you'll need to calibrate your spectrograph, to do that you can use a lamp with some known gas (something like neon) which emits light a very specific set of wavelengths. You shine that lamp at the grating and take a picture of its spectrum with the same camera, then you can figure out what pixels correspond to what wavelengths of light. So when you take a spectrum of some galaxy you will see either absorption lines (wavelengths where the spectrum is fainter then average) or emission lines (wavelengths where it is brighter than average) and from the pattern you can identify which lines are which, then you see how much the whole pattern is shifted to bluer or redder wavelengths and that gives you the redshift.<br /><br />Re. the gravity wells. It's true that light that comes out of a gravity well will be red-shifted. If all you had was the redshift to go on, I don't know that you could conclude that the light was due to an expansion of the universe vs. from coming out of a gravitational well. But you don't only measure redshifts - you can also measure the spectra of type Ia supernovae and find that the apparent brightness of these supernova <div class="Discussion_UserSignature"> </div>

 

[Saiph]

the more traditional measuring of redshift comes form using a prism or diffraction grating. Each wavelength of light is deflected from the original path by a specific angle. By carefully measuring the angle of deflection you can very precisely measure the wavelength of the light.<br /><br />To get the redshift, you compare the measured wavelengths of each portion of the spectral pattern of an element (say hydrogen) and compare that, to the position of the pattern when viewed in a labratory setting (i.e. you know it isn't moving). The difference between the two wavelength measurements (observation, and labratory) is related to the redshift.<br /><br />Now, the specific details on how this is done, depend on the device (old photographic plates, spectrographs, multi-spectrum analyzers, CCD cameras, etc). <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>