Table of Contents
snapfour - fourier analyze a snapshot
snapfour in=snapshot_file
[parameters=values...]
snapfour computes a number of requested
fourier coefficients in a user determined part of the snapshot By default
the snapshot is divided in a set of concentric cylindrical rings, and a
fourier analysis is done in cylindrical coordinates.
The observable quantity
f is fitted as
f(r,theta) = sum_{cmi..}{A(r)cos(cmi*theta) + sum_{smi..}{B(r)sin(smi*theta)
where x, y as well as f r = sqrt(x*x+y*y) , theta=atan2(y,x)
can be any arbitrary bodytrans(3NEMO)
expression using the xvar, yvar
and fvar keywords. The least squares is done using normalized equations,
which are known for roundoff problem in case a large number of coefficients
or points are used.
The following parameters are recognized in
any order if the keyword is also given:
- in=in_file
- Input data file, must
be in standard snapshot(5NEMO)
format [No default].
- radii=rmin,r1,..,rmax
- Radii of the rings (but see xvar and yvar below) within which the fourier
coefficients are computed. [Default: 0:2:0.2].
- cos=cm1,cm2,..
- Set of integers
for which the cosine amplitudes need to be computed. [Default: 0:4].
- sin=sm1,sm2,..
- Set of integers for which the sine amplitudes need to be computed. [Default:
1:4].
- xvar=x-variable
- X-variable which is used to compute the radius and angle
used in the fourier decomposition. [Default: x].
- yvar=y-variable
- Y-variable
which is used to compute the radius and angle used in the fourier decomposition.
[Default: y].
- fvar=f-variable
- Fourier-variable which is the variable used
in in the fourier decomposition. [Default: vy].
- weight=w-variable
- Weight-variable
which is the weight applied to the observable. [Default: 1].
- amode=t|f
- Logical;
if true it displays the sin and cos terms, if false, the amplitudes and
phases (in degrees). [Default: t]
- times=t1,t2,...
- Times to select for analysis.
[Default: all].
% mkexpdisk disk1 10000
% snapfour disk1 amode=f > tab1
% tabplot tab1 1 4,3,6,8,10 line=1,1 color=2,3,4,5 ycoord=0
% tabplot tab1 1 5:11:2 line=1,1 color=2,3,4,5 ycoord=0
bodytyrans(1NEMO)
, orbfour(1NEMO)
, snapshot(5NEMO)
Binney, J.J. &
Spergel, D. (1982) ApJ 253, 308.
Peter Teuben
~/src/nbody/reduc snapfour.c
3-dec-90 V1.0: written PJT
17-feb-92 V1.1: added weight= PJT
10-nov-93 V1.2: added times= pjt