**PURPOSE**- Calculates the Fourier Shell Correlation (FSC) curve between two volumes. Stores the FSC curve in a document file and determines the resolution at which FSC falls below requested value (default is 0.5). Can apply a soft mask to real input before FSC calculation. Takes real or Fourier input volumes. This obsolete operation is superceeded by 'FSC NEW'. Example.

**SEE ALSO****FSC NEW**[Fourier shell correlation and resolution determination ||] **FRC**[Fourier ring correlation and resolution determination ||] **RF 3**[Phase Residual & Fourier shell correlation, 3D ||] **FT**[Fourier Transform ||] **MO 3**[Create Model volume]

**USAGE**- .OPERATION: FSC [v],[sf],[resol]

- [This operation can return up to three optional register variables:
Variable Example Receives First [v] Interpolated voxel where the FSC falls below requested cutoff value Second [sf] Approximate resolution (in spatial frequency units) at this cutoff level Third [resol] Resolution (in Angstroms) at this cutoff level If there are multiple downward crossings of the FSC cutoff line then the values are reported for the

**LAST**crossing. If there is no such crossing the**minimal**values from the FSC curve are reported.].FIRST INPUT VOLUME: SUB001

[Enter name of the first input volume. File can be real or Fourier format.].SECOND INPUT VOLUME: SUB002

[Enter name of the second input volume. File must be same size as first input file.] - If both input files are real format then SPIDER asks:

.SHELL WIDTH (RECIPROCAL SPACE UNITS) & MASKING RADIUS (PIXELS) : 0.5, 66

[Enter the shell width in reciprocal space sampling units i.e. (1/number of pixels). Enter radius for second order super-Gaussian masking. See MO 3 for details. If radius is zero or missing then masking is skipped.]- Instead, if both input files not real format then SPIDER asks:

.SHELL WIDTH (RECIPROCAL SPACE UNITS): 0.5

[Enter the shell width in reciprocal space sampling units.].VOXEL SIZE (A) & RESOLUTION CUTOFF: 0.41, .5

[Enter the voxel size in Angstroms. If unknown, enter 1.0 and resolution column will be reported in voxel units. Enter resolution cutoff (if missing the default is 0.5). ].FSC OUTPUT DOCUMENT FILE: DOC001

[Enter name for the output document file containing the FSC curve.].GNUPLOT FILE: res-plot.gn

[Enter name for file which will contain Gnuplot commands which can plot the FSC curve. Enter '*' if you do not want a plotting file.]

**NOTES**

- The following measures are reported for each shell:

a) Normalized Frequency. |NORM-FREQ| ---- Doc. file register: #1

a) Resolution in Angstroms. |Resolution| ---- Doc. file register: #2

c) Fourier Shell Correlation |FSC| ---- Doc. file register: #3

[SUM(F1 * CONJ(F2))] / [SQRT{SUM(ABS(F1)**2) * SUM(ABS(F2)**2)}]

where 'CONJ' denotes complex conjugate.

d) Square root of Fourier Shell Correlation |SQRT-FSC| ---- Doc. file register: #4

[SQRT(FSC)]

e) Square root (2 * Fourier Shell Correlation / (FSC + 1)) |SQRT-2FSC/(FSC+1)| ---- Doc. file register: #5

[SQRT(2*FSC/(FSC+1)] - If you do not want a results file listing of the FSC values use
'MD' operation with option 'VERBOSE OFF'.
- References:

Saxton and Baumeister,*J. of Microscopy*, 127,(1982) 127-138.

M. van Heel,*Ultramicroscopy*, 21, (1987) 95-100.

Unser, et. al.,*Ultramicroscopy*, 23, (1987) 39-52.

- Pawel Penczek has notes on interpreting FRC/FSC curves.

**SUBROUTINES**- PR3D, PR3DB, RFACTSD2, FMRS_3

**CALLER**- FOUR1

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