`vtpdetect' locates connected sets of photons above a threshold
density and groups each set into a source.
Events are read from the event list and reduced to unique X,Y
pixel locations (possibly with a "multiplicity" greater than
1, i.e. more than 1 event). An exposure time is assigned to
each pixel location.
If the input file is an image, then the image is read-in and
a pseudo event list is created; events are added with a
multiplicity equal to the pixel amplitude.
An area is assigned to each pixel location by computing the
Delaunay triangulation from which the Voronoi tessellation is
computed (the triangulation information is saved for the
percolation step).
The cumulative distribution (cumulative density function, CDF)
for non-source pixels is fitted on the low end (up to
maxbkgflux) with a Poisson distribution. For the first pass,
all pixels are considered to be background.
The minimum of the residual between the total (all pixel
locations) CDF and the estimated background CDF (from above)
is found between the mintotflux and maxtotflux ranges by two
methods: absolute minimum and a fitted quadratic function.
A flux cutoff is determined from these two values and the
values of the mincutoff and maxcutoff parameters and scaled by
the user-input scale factor.
Below this flux cutoff, pixel locations are assumed to be
background pixels. Above the flux cutoff, the pixels are
grouped into sources based on the percolation (friends of
friends) using the triangulation information to know which
pixel locations are adjacent to every other pixel.
Given the total area of the source region (sum of the area of
all pixel locations in source region), the minimum number of
events needed to exceed the tolerated false source limit of
background fluctuations is computed. If the number of events
in the source (correcting for expected background) exceeds
this minimum, then the source is significant. If not, then
the pixel location with the lowest flux is eliminated from the
source region and the percolation and significance testing is
recomputed. (The percolation needs to be recomputed since the
"trimmed" event may cause the source to split into more than 1
source region.)
After all the pixel locations have been searched and the
sources found significant or not (they revert back to
background events), the threshold is recomputed using newly
found background events and the source list is recomputed
from scratch. This process continues until the number of
background events converges or maxiter iteration have
occurred.
Super Voronoi Cell update: After the final
iteration, the Voronoi tessellation is computed from just the
set of source pixel locations and the edge pixel locations (to
constrain the tessellation). To better account for flux in
the wings of a source profile, the total flux is taken to be
the total flux contained in these "super" Voronoi cells around
each source. Then the background is estimated from this
larger source area.
This option is currently implemented; however, since it has
not been sufficiently tested, it does not produce output other
than to STDOUT at the end in lines that have "BEFORE" and
"AFTER" comments.
Finally the properties of the source regions are computed and
output to the FITS source list.
For a more complete description of the theory and operation
of vtpdetect, please see the vtpdetect sections of the
CIAO Detect Manual.
NOTES:
-
See H. Ebeling and G. Wiednmann, Phys. Rev E, 47, 704 (1993).
-
The output source list ("outfile" parameter) can be used
by ds9 as a region file if the block name is renamed to "REGION"
(e.g. dmcopy srclist.fits[srclist][region] regfile.fits).
-
NB: IF THE INPUT FILE IS AN EVENTS FILE, THE FILTER "[cols x,y]"
must follow the filename.
Input file name. This can either be an event list or an image
as specified by the datamodel syntax.
NB: IF THE INPUT FILE IS AN EVENTS FILE, THE FILTER "[cols x,y]"
must follow the filename.
Parameter=expfile (file filetype=input default=none)
There is preliminary support for exposure map files. This is
assumed to be a 2 D image with WCS matching the input file.
Filename for ASCII region output.
Size, in sigmas, to make the elliptical source regions.
ellsigma is a multiplicative factor applied to sigma, the standard deviation
of the distribution, to scale the major and minor axes of the ellipses for each
source. ellsigma affects both the outfile and the ASCII region file (regfile).
This feature is included so that the graphics overlay will be more visible
and under the user's control. Often a value greater than 3 is helpful.
Background flux cutoff scale.
`vtpdetect' determines the background flux cutoff. This parameter
allows the user to scale that value. Setting this to
something greater than 1 will tend to de-blend sources while
losing faint sources. Setting it to less than 1 will tend
to blend source while picking out fainter sources.
Parameter=limit (real default=1e-06 min=0 max=1)
Number of false source events per number of background events.
Minimum number of events to consider to be a real source.
Parameter=maxiter (integer default=10 min=0 max=100)
Maximum number of iterations to allow.
Maximum number of iterations to allow. `vtpdetect' may
converge before this. `vtpdetect' may also enter a state
where the background estimate cycles between 2 or 3
values. Setting this to different values in this case
will allow the user to control the exit level.
The Voronoi tessellation at the edges is unbounded.
This parameter controls how deep into the field to
ignore events.
Perform the super Voronoi cell update algorithm at the
end of the program. Since this is a time consuming
and uncalibrated routine this option is optional.
Maximum normalized background flux level to fit with
the cumulative Poisson distribution.
Minimum normalized total flux level to fit with
parabola.
Maximum normalized total flux level to fit with
a parabola.
Minimum absolute normalized flux level to find background
cutoff.
Maximum absolute normalized flux level to find
background cutoff.
Fit tolerance on Poisson fit.
Initial guess at Poisson fit.
Remove output file if it already exists?
Level of informative output as the program is running.
WARNING: verbose=4 and verbose=5 can produce hundreds of pages
of output. Use only if needed to diagnose a problem during
the percolation.
Parameter=logfile (file filetype=output default=stderr)
File where all messages will be directed. There are two
special cases "STDOUT" and "STDERR" to indicate to use
the C standard output (terminal) and standard error file.
Selects format of output file (e.g. FITS, IRAF). "default"
sets output kernel to be the same as that for the input file.
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