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omdetect
November 22, 2004
Abstract
Program for detecting sources on OM images, parameterising and performing
aperture photometry on those detected.
1 Instruments/Modes
Instrument Mode
OM FAST
OM IMAGING
2 Use
pipeline processing yes
interactive analysis yes
3 Description
This task determines point and extended sources in 2 separate operations. In order to detect sources
the program computed the mode of the background, and then any pixel with a value greater than or
equal to nsigma * backgound mode is considered to be likely associated with a source. In detecting point
sources, each such pixel in the image is analaysed to see if it may be centred on a point-source. If a
likely point-source is found, the pixels assigned to the source are set equal to the source number and its
parameters calculated and stored. All other pixels above the detection threshold are then assumed to be
associated with \extended" sources.
Having obtained a source list, comprising their positions, sizes, counts and background counts, aperture-
photometry is then performed on each source to produce a count value for each source that is consistent
with the CAL calibration procedure.
The output le is a source list comprising source positions and error (in pixels), parameters measuring
the FWHM of the major and minor axes of an ellipse characterising the source shape, source count-
rate and error, psf1 correction (point-spread function correction applied to counts to extrapolate from
source extraction area to coincidence-loss area), aperture radius, no of counts associated with an extended
source, coincidence-loss correction, signicance of the source and ags re ecting whether the source is
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aected by quality, confusion or extension problems. A count-rate limit is written to the source list le
header (keyword CRLIMIT) re ecting the mean nsigma above background detection threshold for the
image, where nsigma is a parameter to the task.
4 Parameters
This section documents the parameters recognized by this task (if any).
Parameter Mand Type Default Constraints
set yes string none
name of OM OSW FITS image le
outset yes string none
name of output OM OSW source list le
wdxset no string \"
name of wdx le - needed for images produced by pre 5.4 sas
nsigma no real 2.0 1.0{
number of  above background required for a detection
minsignicance no real 1.0 0.0{
minimum signicance of sources to be included in OM OSW le. if 0 then no checking done.
levelimage no string none
name of image of island detections
regionle no string
name of saoimage region le
detectextended no bool true
Determines whether algorithm for detecting extended sources should be used
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5 Errors
This section documents warnings and errors generated by this task (if any). Note that warnings and
errors can also be generated in the SAS infrastructure libraries, in which case they would not be docu-
mented here. Refer to the index of all errors and warnings available in the HTML version of the SAS
documentation.
Source detection problem- unreliable source list (warning)
corrective action: A problem was encountered when trying to deblend a source
image le is not 2D (fatal)
dierent binning in x and y axes (fatal)
Could not allocate memory for: name (fatal)
Memory allocation for the named allocatable array failed
WrongFilter (fatal)
Filter in not one of White, WHITE, V, B, U, UVW1, UVW2, UVM2, or MAGNIF
Failed to release memory for: name (warning)
corrective action: Memory deallocation for the named allocatable array failed
6 Input Files
1. Intermediate FITS image le (output from ommodmap)
7 Output Files
1. Intermediate OM OSW source list (input to omwavelet)
2. Source number image (optional))
3. Saoimage region le of source list (optional)
8 Algorithm
subroutine omdetect
read data for selected OSW image
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get mode of image by sigma clipping to get average
background level
Loop though all pixels on the image- for each pixel with a value >=nsigma *
background mode analyse surrounding pixels to ascertain if these pixels may
constitute a ``point'' source. If so, set the pixels to an assigned source
number, compute the source position and shape by moments, and store the
parameters. Repeat until all pixels have been checked.
Recompute the background mode, ignoring those pixels assigned to a point-source.
Check all the remaining pixels and those with a value >=nsigma *
background mode assign a number. Analyse these pixels for individual extended
sources and assign a number to each one. Compute the source position and shapes of
each extended source by moments, and store the parameters.
Perform aperture photometry on each source.
Examine each point-source and discard any with a significance less than the
minimum allowed value.
Examine each extended source to attempt to see if it is just scattered light
around a point-source- reject this source if it is.
write output sourcelist containing source parameters. Also output
detection count rate threshold.
end subroutine omdetect
9 SourceListFile
 COLUMN NO NAME UNITS DESCRIPTION
 1 SRCNUM none
 2 XPOS pixels x-coordinate of centroided source centre
 3 YPOS pixels y-coordinate of centroided source centre
 4 POSERR pixels error in x and y centroided position
 5 RATE counts/sec Total measured source counts within measurement aperture divided by
exposure time
 6 RATE ERR counts/sec
 7 BACKGROUND RATE counts/sec Total measured background counts within measure-
ment aperture divided by exposure time
 8 SIGNIFICANCE sigma source counts / sqrt(background counts) (ie signal-to-noise ratio)
 9 CORR RATE counts/sec RATE after extrapolating (if neccesary) to an aperture radius
of 12 unbinned pixels and correcting for coincidence losses.
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 10 CORR RATE ERR counts/sec
 11 CORR BACKGROUND RATE counts/sec BACKGROUND RATE after extrapolating
(if neccesary) to an aperture radius of 12 unbinned pixels and correcting for coincidence
losses.
 12 PSF1 CORR percentage Point-spread function correction applied to extrapolate RATE
from measurement aperture area to area of 12 unbinned pixels
 13 COINCLOSS CORR percentage Coincidence-loss correction applied - (RATE - CORR RATE)
* 100.0 / RATE
 14 NOPIXELS none Number of pixels contained by an extended source (EFLAG=1). Point-
sources (EFLAG=1) will have this value set to zero.
 15 APERTURE unbinned pixels For point sources (EFLAG=0), the aperture radius (un-
binned pixels) used in the photometry. For extended-sources (EFLAG=1) this value will be
set to NULL.
 16 FWHM MAJ arcsecs The full-width-at-half-maximum of the semi-major of the ellipse
describing the shape of the source (computed by moments)
 17 FWHM MIN arcsecs The full-width-at-half-maximum of the semi-major of the ellipse
describing the shape of the source (computed by moments)
 18 PA degrees The postion-angle (referred to the x-axis) of the semi-major of the ellipse
describing the shape of the source (computed from moments)
 19 QFLAG none (source quality ag) Can be either 0 or 1. Set to 0 if all the image pixels
used to obtain the counts for the source were good, 1 otherwise.
 20 CFLAG none (source confusion ag) Can be either 1, 2, 3 or 4. 1 means that the nearest
source is at a distance of greater than twice the aperture radius. 2 means that there are
one or more sources within a distance of twice the aperture radius, but at a distance greater
than the aperture radius. 3 means that there are one or more sources within a distance of
the aperture radius. 4 means that the source lies near an edge of the image
 21 EFALG none (source extension ag) Either 0 point-source (none-extended) or 1 extended.
10 Deciencies
 If two point-sources are close together so that their images overlap, the detection algorithim
for detecting point-sources will either fail to detect one or both. However, the detection
algorithm for detecting extended sources will either pick up the missing object or detect
both as a single extended source.
11 Comments
 We have seen images that have been generated by OMFLATFIELD that contain pixel values
that have been set to the BLANK keyword value. This occurrs due to the Flateld being
zero for these pixels. The quality array has been agged to the value of 16 for these cases.
The image has been modied to contain the value zero as a temporary x.
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12 Future developments
References
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