RESOL.DOC 12/11/85 jf CONVERSION OF SPATIAL FREQUENCY UNITS FROM PHYSICAL TO DIMENSIONLESS QUANTITIES Most of the SPIDER operations use dimensionless quantities: e.g. image shift, mask radius, etc. The same is true for operations on Fourier transforms. Filter radius or R-factor radius specifications are in reciprocal Fourier units. For conversion into reciprocal Angstroms note the following: If the image has N x N points, and has been scanned with sampling distance D, then the Fourier unit has the size 1/(DxN). Filtration to a given physical resolution limit of R is therefore accomplished by using a filter radius of R x D x N. Example: N = 64, D = 5A and R = 1/20A-1 Corresponding Fourier units: R*D*N = 16 Rectangular formats: all Fourier unit specifications in 'FF' refer to the sample direction. For instance, an image with dimensions 64 x 128 with D = 5A as above is filtered to R = 1/20A-1 if 16 units is specified as above. The actual resolution cutoff follows an elliptical outline in terms of Fourier sampling units, because R*D*N = 32 in the vertical direction. The program underlying operation 'FF' is smart enough to keep track of this.