Basics of FFT 1d
http://hesperia.gsfc.nasa.gov/~schmahl/fourier_tutorial/node6.html
https://idldatapoint.wordpress.com/2012/11/08/calculating-the-period-of-the-sunspot-cycle/
2d FFT
http://homepages.inf.ed.ac.uk/rbf/HIPR2/fourier.htm
NFT - very useful
https://www-user.tu-chemnitz.de/~potts/nfft/guide3/html/index.html
Bispectrum
http://www.cosmostat.org/software/fastlens/
http://irfu.cea.fr/Images/astImg/1800/doc_fastlens.pdf