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Re: IDL FFT (spec -> interferogram) [message #30122 is a reply to message #30034]

Mon, 08 April 2002 07:14

Paul van Delst
Messages: 364
Registered: March 1997

Senior Member

Robert Stockwell wrote:
>
> Randall Skelton wrote:
>
>> Hi all,
>>
>> Having read through all of the FFT posts that google groups keeps, I am no
>> closer to understanding why I am unable to transform a spectrum into an
>> interferogram using IDL. All of the data files, procedures, and pictures
>> of this are at http://tulip.atm.ox.ac.uk/~rhskelto/fft-help/
>>
>> Given two files:
>>
>> 1) 'spec.dat' contains 512 points of complex spectral data
>>
>> 2) 'igm.dat' contains 512 points of complex interferogram data that was
>> derived from 'spec.dat' using a prime factor FFT written in C. This is
>> the correct interferogram as far as I am concerned. The plot
>
> complex-valued interferrogram?
>
> hmmmmm
>
> to shed a little light on it [1], in interferometry, the interferrogram is
> the autocorrelation function of the electric field vector.
> The power spectrum is the fft of the autocorrelation function.
> (this is a well known theorem, and if I only had a brain, I'd remember
> the name of it)
> Note the real value-ed-ness of "autocorrelation" and "power".
> The interferrogram is an even function, the power spectrum is real-valued.

But one doesn't always want the power spectrum. Usually (in my field at least) one wants the
complex valued spectrum where the imaginary component is known and happily zero. (Don't know if
Randall wants that tho')

paulv

--
Paul van Delst Religious and cultural
CIMSS @ NOAA/NCEP purity is a fundamentalist
Ph: (301)763-8000 x7274 fantasy
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[Message index]

		IDL FFT (spec -> interferogram) By: Randall Skelton on Thu, 04 April 2002 07:27
		Re: IDL FFT (spec -> interferogram) By: Robert Stockwell on Tue, 09 April 2002 07:04
		Re: IDL FFT (spec -> interferogram) By: Randall Skelton on Mon, 08 April 2002 15:00
		Re: IDL FFT (spec -> interferogram) By: Paul van Delst on Mon, 08 April 2002 11:36
		Re: IDL FFT (spec -> interferogram) By: Robert Stockwell on Mon, 08 April 2002 10:34
		Re: IDL FFT (spec -> interferogram) By: Paul van Delst on Mon, 08 April 2002 07:14
		Re: IDL FFT (spec -> interferogram) By: Robert Stockwell on Sat, 06 April 2002 11:54
		Re: IDL FFT (spec -> interferogram) By: Paul van Delst on Fri, 05 April 2002 07:36

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