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Red and Blue shift 
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#1
Mar2510, 02:32 PM

P: 344

How accurate is it possible to measure the EM spectre ?



#2
Apr310, 03:07 AM

P: 31

It depends on your tool...



#3
Apr310, 03:47 AM

P: 344




#4
Apr310, 08:29 AM

Mentor
P: 22,281

Red and Blue shift
You're asking about the red and blue shift of stars and galaxies? Very accurately, since you have spectral absorption lines to compare with a reference.



#5
Apr310, 08:33 AM

P: 344

What is "very accurate" ? 1 millionth of a meter? Which devise is the best ? 


#6
Apr410, 01:07 PM

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P: 22,281

I've done some googling and haven't found too much, but this paper implies an accuracy of 0.1% to 1%.
http://www8.nationalacademies.org/as...ay.aspx?id=208 Redshift is measured with a spectrograph: http://en.wikipedia.org/wiki/Faint_Object_Spectrograph Redshift doesn't directly measure speed or distance, it directly measures the shift in a spectrum of light. 


#7
Jul1212, 01:29 AM

P: 344

What about absorbed photons, is it only these that has very certain frequencies that are absorbed?
For example here http://www.astro.ucla.edu/~wright/doppler.htm is mentioned that those at 393 nm are absorbed. My question is;  how accurate is that? Is it only these that have the exact wavelength 393nm that are absorbed What when one is 394 nm or 392nm ,  will noting happen ? If so it must be possible to measure much more accurate as 0.1% to 1%. I mean the difference between 393nm and 392 nm is not much. 


#8
Jul1212, 05:48 AM

P: 1,212

The spectrograph gives you signal over a range of wavelengths, so you should see characteristic peaks on a graph.
393 nm/392 nm = 0.26% which seems nicely in the range 0.11%. 


#9
Jul1212, 09:34 AM

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Sci Advisor
PF Gold
P: 2,361

http://wwwstar.stand.ac.uk/~kw25/t...linewidths.pdf 


#10
Jul1212, 06:43 PM

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#11
Jul1312, 08:40 AM

PF Gold
P: 513

http://www.sciencedirect.com/science...30402608000089 Respectfully submitted, Steve 


#12
Jul1312, 10:52 AM

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P: 11,815

There is no theoretic limit on the precision, and the technical limits depend on the instruments, measurement time and brightness of the source. Exoplanet searches can measure the radial velocity of nearby stars with a precision of ~1m/s which is equivalent to a relative precision of ~3*10^(9). The key point here: While the actual lines are broader than this, the center of those lines can be measured with a precision better than the line widths. 


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