Absolute frequency units `f_a`

• `f_a=0` corresponds to the zero Fourier term (zero frequency), while `f_a=0.5` corresponds to Nyquist frequency `f_N`.

• Nyquist frequency is the maximum spatial frequency in an image scanned with a pixel size p [Å] and in pixels `f_N=1/2`, while in Å `f_N=1/(2p) [1/Å]`.

For the pixel size p [Å], spatial frequency `f_s` [1/Å] is related to absolute frequency `f_a` by:

• `f_s=f_a/p`

• `f_a=f_s**p`

For an image size n pixels sampled at p Å, k'th Fourier pixel (0<=k<=n/2) is related to frequency by:

• `f_s=k/n/p`

• `f_a=k/n`

Resolution r [Å] (defined as inverse of spatial frequency) is:

• `r=1/f_s=p**n/k`

For examples of code in which Fourier pixels are handled check sparx/filter.cpp.