**PURPOSE**- Determines orientations of N=>3 projections in terms of three Eulerian angles.

**USAGE**- .OPERATION: OP

- .IMAGES: IMG***

[Enter template for 2D image file series.].FILE NUMBERS: 1-11

[Enter file numbers.].LENGTH OF LINE PROJECTION: 79

[Enter length of 1D line projection to be used internally by the program. The length has to be less than NX.].MINIMUM, MAXIMUM FREQUENCY: 0.0, 0.25

[For each line projection only frequencies between minimum and maximum will be used. The default is 0.0, 0.5, which is the full range.].ACCURACY OF THETA: 5

[Enter accuracy of search in terms of theta angle. Theta accuracy will define a set of quasi-evenly distributed projections directions to be used in the program. It is the same set as generated by command 'VO EA' for the same delta theta.].MAXIMUM NUMBER OF CYCLES: 200

[Enter maximum number of cycles the program should perform. In some cases program may fail to terminate, so to prevent an endless loop the maximum number of cycles has to set.].DOCUMENT FILE WITH INITIAL ANGLES: initial_angles

[Enter the name of the document file containing initial Eulerian angles for 2D images (psi, theta, phi). The angles can be either random, or can be estimated by other means.].DOCUMENT FILE: angles002

[Enter the name of the output document file. For each cycle resulting in a decrease of the discrepancy measure, new results will be appended to this document file. Key "-1" will have two columns: the current cycle number and the current value of the discrepancy measure. Keys from one to the number of 2D input images specified will contain Eulerian angles (psi, theta, phi) found.]

**NOTES**

- Input projections have to be centered.
- Eulerian angles can be used directly in 3D reconstruction
programs to calculate a 3D structure.
- Implemented by P. Penczek.
- Command 'OP' implements orientation search strategy described in: Penczek P., J. Zhun, J. Frank. A common-lines based method for determining orientations for N>3 particle projections simultaneously. Ultramicroscopy, 63, 205-218 (1996).

**SUBROUTINES**- POLQS

**CALLER**- UTIL3

; Example DE angsett ; Generate an initial set of random Eulerian angles [dum]=0.0 DO [i]=1,16 [a] = RAN([dum])*360. [b] = RAN([dum])*180. [c] = RAN([dum])*360. SD [i],[a],[b],[c] angsett ENDDO SD E angsett OP ahr*** 1-16 80 0.0, 0.156 5 20 angsett rangles001 ; Calculate 3D structure for the solution found. DOC CREATE select 1 1-16 BP RP ahr*** select 40 rangles001 N rvol001 1.0e-4,0.0 60,0 0,0 0.5 EN

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