## NAME

centerpars -- edit the centering algorithm parameters

## USAGE

`centerpars`

## PARAMETERS

- calgorithm = "centroid"
- The centering algorithm. The "gauss" and "ofilter" options depend critically
on the value of the fwhmpsf parameter in the DATAPARS task. The centering
options are:
- none
- The initial positions are assumed to be the true centers. Users may select this option if the initial centers are know to be accurate, e.g. they were computed by DAOFIND task.

- centroid
- The object centers are determined by computing the intensity weighted means of the marginal profiles in x and y. This is the recommended default algorithm for APPHOT users.

- gauss
- The object centers are computed by fitting a Gaussian of fixed fwhmpsf, specified by the DATAPARS fwhmpsf parameter, to the marginal profiles in x and y using non-linear least squares techniques.

- ofilter
- The object centers are computed using optimal filtering techniques, a triangular weighting function of half width equal to fwhmpsf as specified by the DATAPARS fwhmpsf parameter, and the marginal distributions in x and y.

- cbox = 5.(scale units)
- The width of the subraster used for object centering in units of the DATAPARS scale parameter. Cbox must be big enough to include a reasonable number of pixels for center determination but not so large so as to include a lot of noise. Recommended initial values are 2.5-4.0 * the FWHM of the PSF value.

- cthreshold = 0.(sigma units)
- Pixels cthreshold * sigma above (emission features) or below (absorption features) the data minimum or maximum respectively are used by the centering algorithms where sigma is equal to the value of the DATAPARS sigma parameter. Most APPHOT users should leave this value at 0.0 which invokes the appropriate default thresholding technique for each centering algorithm. Setting cthreshold to INDEF turns off thresholding altogether for all the centering algorithms.

- minsnratio = 1.0
- The minimum signal to noise ratio for object centering. If the estimated signal to noise ratio is less than minsnratio the computed center will be returned with an error flag.

- cmaxiter = 10
- The maximum number of iterations performed by the centering algorithm. All the centering algorithms use this parameter.

- maxshift = 1.(scale units)
- The maximum permissible shift of the center with respect to the initial coordinates in units of the scale parameter. If the shift produced by the centering algorithms is larger than maxshift, the computed center is returned with an error flag.

- clean = no
- Symmetry-clean the centering subrater before centering? APPHOT users should leave clean set to "no".

- rclean = 1.(scale units)
- The cleaning radius for the symmetry-clean algorithm in units of the scale parameter.

- rclip = 2.(scale units)
- The clipping radius for the symmetry-clean algorithm in units of the scale parameter.

- kclean = 3.(sigma)
- The number of sky background standard deviations for the symmetry-clean algorithm where sigma is the value of the DATAPARS parameter sigma.

- mkcenter = no
- Mark the fitted object centers on the displayed image ?

## DESCRIPTION

The centering algorithm parameters control the action of the centering
algorithms. The default parameters values have been proven to produce
reasonable results in the majority of cases. Several of the centering
parameters are defined in terms of the DATAPARS parameter *scale*
,
the scale of the image, and *sigma*
the standard deviation of
the sky pixels.

For each object to be measured a subraster of data *cbox*
/ *scale*
pixels wide around the initial position supplied by the user is extracted
from the IRAF image. If scale is defined in units of the number
the half-width half-maximum of the psf per pixel, then a single value of
cbox can be used for centering objects in images with different psfs.

If *clean*
is "yes" the symmetry-clean algorithm is applied to the
centering subraster prior to centering. The cleaning algorithm attempts
to correct defects in the centering subraster by assuming that the image
is radially symmetric and comparing pixels on opposite sides of the center
of symmetry. The center of symmetry is assumed to be the maximum pixel
in the subraster, unless the maximum pixel is more than *maxshift /
scale*
from the initial center, in which case the initial center is used
as the center of symmetry. Pixels inside the cleaning radius are not edited.
Pairs of pixels in the cleaning region, r > *rclean*
/ *scale*
and r <= *rclip*
/ *scale*
and diametrically opposed about the
center of symmetry are tested for equality. If the difference between the
pixels is greater than *kclean * sigma*
, the larger value is replaced
by the smaller. In the cleaning region the sigma is determined by the
noise model assumed for the data. Pairs of pixels in the clipping region,
r > *rclip*
/ *scale*
are tested in the same manner as those in
the cleaning region. However the sigma employed is the sigma of the
sky background. Most APPHOT users should leave clean set to "no".

New centers are computed using the centering algorithm specified by
*calgorithm*
, the data specified by *cbox / scale*
, and pixels
that are some threshold above (below) an estimate of the local minimum
(maximum). *Cthreshold*
values of 0.0, a positive number, and INDEF
invoke the default thresholding algorithm, a threshold equal to the
local minimum (maximum) plus (minus) *datapars.sigma * cthreshold*
,
and a threshold exactly equal to the local minimum (maximum) respectively.

After thresholding the signal to noise ratio of the subraster is estimated.
If the SNR < *minsnratio*
the new center is still computed but an error
flag is set.

The default centering algorithm is *centroid*
. Centroid computes the
intensity weighted mean and mean error of the centering box x and y marginal
distributions using points in the marginal arrays above (below) the minimum
(maximum) data pixel plus (minus) a threshold value.

The threshold value is either the mean, *datapars.sigma * cthreshold*
above (below) the local minimum (maximum) if *cthreshold*
is greater
than zero, or zero above (below) the local minimum (maximum) if
*cthreshold*
is INDEF. The centroid algorithm is similar to that
by the old KPNO Mountain Photometry Code. Note that centroid is the only
centering algorithm which does not depend on the value of
*datapars.fwhmpsf*
.

The centering algorithm *gauss*
computes the new centers by fitting a
1D Gaussian function to the marginal distributions in x and y using a
fixed fwhmpsf set by *datapars.fwhmpsf*
. Initial guesses for the fit
parameters are derived from the data. The gauss algorithm iterates until
a best fit solution is achieved.

The final centering algorithm choice *ofilter*
employs a variation of the
optimal filtering technique in which the profile is simulated by a triangle
function of width *datapars.fwhmpsf*
.

The default thresholding algorithm for all centering algorithms other than "centroid" is no thresholding.

If the computed shift in either coordinate > *maxshift*
/ *scale*
,
the new center is returned but an error flag is set.

## EXAMPLES

1. List the centering parameters.

ap> lpar centerpars

2. Edit the centering parameters

ap> centerpars

3. Edit the CENTERPARS parameters from with the PHOT task.

da> epar phot ... edit a few phot parameters ... move to the centerpars parameter and type :e ... edit the centerpars parameters and type :wq ... finish editing the phot parameters and type :wq

4. Save the current CENTERPARS parameter set in a text file ctrnite1.par. This can also be done from inside a higher level task as in the previous example.

da> centerpars ... edit the parameters ... type ":w ctrnite1.par" from within epar

## BUGS

## SEE ALSO

center,phot,wphot,polyphot,radprof