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.