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.help findpars May00 noao.digiphot.daophot
.ih
NAME
findpars -- edit the object detection parameters
.ih
USAGE
findpars
.ih
PARAMETERS
.ls threshold = 4.0 (sigma)
The object detection threshold above local background in units of
\fIdatapars.sigma\fR.
.le
.ls nsigma = 1.5
The semi-major axis of the Gaussian convolution kernel used to computed the
density enhancement and mean density images in Gaussian sigma. This semi-
major axis is equal to min (2.0, 0.42466 * \fInsigma\fR *
\fIdatapars.fwhmpsf\fR / \fIdatapars.scale\fR) pixels.
.le
.ls ratio = 1.0
The ratio of the sigma of the Gaussian convolution kernel along the minor axis
direction to the sigma along the major axis direction. \fIRatio\fR defaults
to 1.0 in which case the image is convolved with a circular Gaussian.
.le
.ls theta = 0.0
The position of the major axis of the elliptical Gaussian. \fITheta\fR is
measured counter-clockwise from the x axis.
.le
.ls sharplo = .2, sharphi = 1.0
\fISharplo\fR and \fIsharphi\fR are numerical cutoffs on the image sharpness
statistic chosen to eliminate brightness maxima which are due to bad pixels
rather than to astronomical objects.
.le
.ls roundlo = -1.0 roundhi = 1.0
\fIRoundlo\fR and \fIroundhi\fR are numerical cutoffs on the image roundness
statistic chosen to eliminate brightness maxima which are due to bad rows or
columns rather than to astronomical objects.
.le
.ls mkdetections = no
Mark the positions of the detected objects on the displayed image ?
.le
.ih
DESCRIPTION
DAOFIND approximates the stellar point spread function with an elliptical
Gaussian function, whose sigma along the semi-major axis is 0.42466 *
\fIdatapars.fwhmpsf\fR / \fIdatapars.scale\fR pixels, semi-minor to semi-major
axis ratio is \fIratio\fR, and major axis position angle is \fItheta\fR.
Using this model, a convolution kernel, truncated at \fInsigma\fR sigma,
and normalized to sum to zero, is constructed.
The density enhancement image \fIstarmap\fR is computed by convolving the input
image with the Gaussian kernel. This operation is mathematically equivalent to
fitting, in the least-squares sense, the image data at each point with a
truncated, lowered elliptical Gaussian function. After convolution each point
in \fIstarmap\fR contains as estimate of the amplitude of the best fitting
Gaussian function at that point. Each point in \fIskymap\fR, if the user
chooses to compute it, contains an estimate of the best fitting sky value
at that point.
After image convolution DAOFIND steps through \fIstarmap\fR searching
for density enhancements greater than \fIfindpars.threshold\fR *
\fIdatapars.sigma\fR, and brighter than all other density enhancements
within a semi-major axis of 0.42466 \fIfindpars.nsigma\fR *
\fIdatapars.fwhmpsf\fR. As the program selects candidates, it computes two
shape characteristics sharpness and roundness. The sharpness statistic
measures the ratio of the difference between the height of the central pixel
and the mean of the surrounding non-bad pixels, to the height of the best
fitting Gaussian function at that point. The roundness statistics measures
the ratio of, the difference in the height of the best fitting Gaussian
function in x minus the best fitting Gaussian function in y, over the average
of the best fitting Gaussian functions in x and y. The limits on these
parameters \fIfindpars.sharplo\fR, \fIfindpars.sharphi\fR,
\fIfindpars.roundlo\fR, and \fIfindpars.roundhi\fR, are set to weed out
non-astronomical objects and brightness enhancements that are elongated in
x and y respectively.
Lastly the x and y centroids of the detected objects are computed by
estimating the x and y positions of the best fitting 1D Gaussian
functions in x and y respectively, a rough magnitude is estimated
by computing the ratio of the amplitude of the best fitting Gaussian at
the object position to \fIfindpars.threshold\fR * \fIdatapars.sigma\fR,
and the object is added to the output coordinate file.
.ih
EXAMPLES
1. List the object detection parameters.
.nf
da> lpar findpars
.fi
2. Edit the object detection parameters.
.nf
da> findpars
.fi
3. Edit the FINDPARS parameters from within the DAOFIND task.
.nf
da> epar daofind
... edit a few daofind parameters
... move to the findpars parameter and type :e
... edit the findpars parameter and type :wq
... finish editing the daofind parameters and type :wq
.fi
4. Save the current FINDPARS parameter set in a text file fndnite1.par.
This can also be done from inside a higher level task as in the previous
example.
.nf
da> findpars
... edit the parameters
... type ":w fndnite1.par" from within epar
.fi
.ih
BUGS
daofind
.ih
SEE ALSO
epar,lpar,daofind,datapars
.endhelp
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