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snapccd - top view integrated velocity moment ccd-like image
snapccd
in=snapshot out=image [parameter=value]
snapccd makes a ccd
frame from an N-body snapshot, by binning the (projected) x-y coordinates
of all stars on a regular rectangular (square) grid and assigning a mean
integrated 'surface density' (integrated along z-axis, i.e. the line of sight,
The ccd frame is written to a standard Image(5NEMO)
output file, and can
be accessed by various other programs for smoothing, display etc.
snapccd
can also create channel maps, where only signal near a specified velocity
is copied. See the vrange and moment keywords below. One can use either an
exact velocity window, or a gaussian beam in velocity space.
Note that the
radial velocity has the astronomical convention. The observer is located
at infinity at the positive Z-axis: a positive z velocity (vz) means stars
are approaching, i.e. negative radial velocity.
snaprotate(1NEMO)
has to be
used first to do any other than a top-view, probably the more general version
snapgrid(1NEMO)
should be used.
The following parameters are recognized
in any order if the keyword is also given:
- in=in-file
- input file, must be
in SnapShot(5NEMO)
format [no default].
- out=out-file
- output file, will be
in Image(5NEMO)
format [no default].
- origin=x,y
- The origin (x,y) of the
center of the ccd [default: 0,0].
- size=value
- Full size of the CCD frame
in 'physical' units. Note that the picture is forced to be square [default:
4.0].
- cell=value
- Cell- or pixel size. Again they will be square. For a 2D matrix
this value will be set by the header of that dataset. [default: 0.1].
- vrange=vmin:vmax
| vmean,vsig
- Range in (astronomical) radial velocity (-vz) to be used only
while binning the data. One can also give a mean and dispersion in velocity
space to weigh the data with. In this case the range in velocity space is
somewhat arbitrarely taken from vmean-3*vsig:vmean+3*vsig. This last procedure
is somewhat more realistic for astronomical applications, unless the package
where the data will be transported to has decent smoothing utilities in
velocity space also. The default of this keyword is to take all data along
the vz axis. It does not make sense to select data along the z-axis when
the moment (see below) is non-zero. [default: -infinity:infinity].
- moment=number
- Order of the velocity moment: must be a non-negative number. Most commonly
choosen are: 0 (total intensity), 1 (velocity weighted intensity) and
2 (velocity square weighted intensity), where 'intensity' should really be
read as surface density per square unit length. [default: 0].
The
following example makes (three) CCD frames from an N-body snapshot, smoothes
the three moment maps and combines them into an 'intensity' (int), 'mean velocity'
(vel) and 'velocity dispersion' (sig) map.
Note that the moment maps must
be smoothed before they can be combined to the proper velocity and dispersion
maps.
% snapccd in=nbody.dat out=map0 moment=0
% snapccd in=nbody.dat out=map1 moment=1
% snapccd in=nbody.dat out=map2 moment=2
% ccdsmooth in=map0 out=map00 gauss=0.1
% ccdsmooth in=map1 out=map11 gauss=0.1
% ccdsmooth in=map2 out=map22 gauss=0.1
% mv map00 int
% ccdmath in=int,map11 out=vel fie=%2/%1
% ccdmath in=int,vel,map22 out=sig fie="sqrt(%3/%1-%2*%2)"
% rm map11 map22
Units are maintained in the same way as in snapshots, they don't have
a specific name, but carry their normal meaning 'length', 'velocity' and 'mass'.
Since snapccd calculates surface densities, its units are formally 'mass'
per square 'length' times 'velocity' to the power moment.
When channel maps
are produced (moment=0), the data are not normalized w.r.t. the convolving
velocity beam. For a rectangular beam (vrange=vmin:vmax) the data should
formally be divided by (vmax-vmin), for a gaussian beam (vrange=vmean,vsig)
by vsig*sqrt(2*pi). Also remember that a gaussian beam has FWHM = 2.355*sigma.
snapccd(1NEMO)
's function is rather similar to that of snapplot(1NEMO)
,
except of plotting the position of the stars, its bins the data. It would
be a nice idea to combine both programs.
Large snapshots (of order 4k
or more) will have problems when also large images (512 square) or so are
used. Need larger memory machine.
The program gracefully allows the user
to use nonzero moment and a non-infinite vrange. No warnings here.
snapgrid(1NEMO)
,
snaprotate(1NEMO)
, snapslit(1NEMO)
, snapsmooth(1NEMO)
, snapaxsym(1NEMO)
,
tsf(1NEMO)
, ccdgrid(1NEMO)
Peter Teuben
src/pjt/image snapccd.c, snapccd.1
17-Jun-87 V1.0: Created PJT
25-jun-87 V1.1: grayscale possibility added PJT
29-jun-87 V2.0: image-format + three programs: ccd, smooth, look PJT
30-Jun-87 V2.1: improved 'struct' filestructure PJT
1-Jul-87 V2.2: added velocity-moments option PJT
8-jul-87 V2.4: proper defn. of cell position PJT
9-Mar-88 V3.0: added data history PJT
1-jun-88 V4.0: new filestruct, renamed programname PJT
22-dec-88 V4.1: channel maps can be produces, keyword vrange PJT
30-jan-89 V4.2: vel is now Zmin, also proper dimensions PJT
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