How EDI works |
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Conventional method requires high resolution For many observatories which cannot afford the cost or space of a high resolution spectrograph, they would only be able to measure the low resolution spectrum depicted in the right panel. The low resolution seriously degrades the strength of the Doppler signal, both because it reduces the measured depth of the absorption line, but also because it increases its width which decreases the slope of the spectrum on the side of the line, which is where the Doppler signal is strongest. The result is that the Doppler signal decreases very strongly with decreasing spectral resolution (R), as R to the 3/2 power. |
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EDI Method tolerates low resolution |
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The phase shifts of the iodine (reference) and stellar moire patterns are subtracted from each other to eliminate instrumental effects. This relative phase shift is independent of the detailed value of the interferometer delay. Multiplication by 15,000 m/s per fringe then yields the Doppler velocity. |
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Section of a solar fringing spectrum at low resolution, showing that the moire fringes persist even though the underlying absorption lines cannot be resolved, nor can the interferometer comb be resolved. |
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At high Res the EDI signal adds to the conventional |
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because the multiplicative interaction between the interferometer transmission function and the stellar spectrum can be thought of as occuring prior to, or independent of, the blurring. It can be shown that the EDI Doppler signal strength varies more slowly versus R (for a fixed bandwidth) as the conventional, as R to the 1/2 power for low R instead of R to the 3/2 power. Hence for low R, the EDI has a dramatic advantage. |
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The EDI is a differential technique robust to apparatus distortions The 2nd interferometer arm is used too |
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Easy dot-product calculation found from the dot product |
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www.SpectralFringe.org site maintained by |
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