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snaprotate - rotate N-body snapshot file about principle axes.
in=snap-file out=snap-file [parameter=value] ...
the positions and velocities of an N-body snapshot file. A maximum of 3 rotations
around either the x, y or z axes can be applied in any order. An Eulerian
rotation is a rotation around the z, y and z axes respectively. Some common
examples for projections of barred and unbarred galaxies are given in the
The rotation is counter clock-wise (i.e., mathematically positive,
unless invert=f is used) as seen from the right-handed xyz-axes; a z rotation
will rotate the x axis towards the y axis; a y rotation will rotate the
z axis towards the x axis; a x rotation will rotate the y axis towards
the z axis.
An alternate way to specify rotations is using the spinvector=
keyword, where all points are rotated around this vector.
Although by default
all vectors are rotated by the same matrix, the keyword select= can be
used to select any set of Position, Velocity or Acceleration.
following parameters are recognized; they may be given in any order.
Suppose you have
a barred galaxy model, bar.dat, with the galactic plane at z=0 and the
bar along the y axis ("position angle=0"), and want to project this (such
that z=0 will be the sky plane, and the positive z-axis pointed towards
the observer) with given final position angle ($pa), inclination ($inc)
and angle from line of nodes to bar ($phi) the following rotations will
do the job:
- Input data is read from snap-file, which must be in snapshot format.
- Output data is written to snap-file.
- Angles (in degrees) to
rotate particles about the specified axes in the order= keyword. Up to 3
rotations can be applied. Default: none.
- Order to apply
the rotations about the three coordinate axes. An Eulerian rotation would
be order=zyz. Default is none. See also the spinvector keyword below for
an alternate way to specify rotations.
- If true, invert
specified transformation. Useful for rotating something back to original
coordinate system. Default is false.
- If used, this is
the degrees per time-unit that the theta angles are multiplied with in order
to calculate the actual rotation angle. This can give interesting effects
if used for simulations. Default not used.
- Use this vector
if you know the vector to rotate around instead of a set of euler angles.
Only one angle theta= is needed. Positive rotation is a counterclockwise
rotation as seen down the spin vector towards the origin. The vector does
not need to be normalized. **note this option still disables various other
options such as tscale, invert
- Vector variables in a
snapshot that should be rotated. Valid options are: pos, vel, and acc. Exact
match required for now. [Default: pos,vel,acc]
snaprotate bar.dat n7476.dat "atand(tand($phi)/cosd($inc)),$inc,$pa"
snapplot n7476 psize="z>0?0.03:0.0"
where psize was choosen such that ‘‘near’’ (z>0) particles were plotted a bit
bigger to accentuate the near side of the model. Flipping the sign if $inc
will flip the near and far side.
For ’normal’ spiral galaxies the following
example shows how to project them into their normaly sky view, taking the
sense of rotation into account:
set pa=170 # M51, use pa=-160 for M33
set inc=22 # M51, use inc=30 for M33
set sign=-1 # M51, use sign=1 for M33
set sinc=‘nemoinp "iflt($sign,0,$inc,-$inc)"‘
mkdisk - 1000 sign=$sign |\
snaprotate - - theta=$sinc,$pa order=yz |\
snapgrid - velfie.ccd mom=-1
nds9 velfie.ccd # view the velocity field using ’ds9’
With sign=-1 it will create a counter-clockwise rotating disk and project
it using $inc,$pa, For a clock-wise rotating disk the math is nearly the
same, except the sign for the inclination needs to be flipped, or -$inc,$pa,
hence the somewhat awkward looking expressing to compute sinc.
Joshua E. Barnes,
xx-jan-88 V1.0 original JEB
3-oct-88 V2.1 eulerian angles (order=zyz) PJT
17-feb-89: V2.1a new get+snap macros redef’s PJT
27-nov-89: V2.1b cosmetic defv PJT
15-nov-90 V3.0 theta is now an array keyword PJT
1-jun-92 documented the rotation manner bit more precise PJT
11-jun-92 V4.0 rotation now defined in math sense PJT
21-nov-98 V5.0 added tscale= keyword PJT
18-nov-05 V5.1a added select=, also implemented select=acc PJT
22-sep-08 V6.0 added spinvector= PJT
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