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diff --git a/noao/digiphot/apphot/doc/centerpars.hlp b/noao/digiphot/apphot/doc/centerpars.hlp new file mode 100644 index 00000000..008869f4 --- /dev/null +++ b/noao/digiphot/apphot/doc/centerpars.hlp @@ -0,0 +1,206 @@ +.help centerpars May00 noao.digiphot.apphot +.ih +NAME +centerpars -- edit the centering algorithm parameters +.ih +USAGE +centerpars +.ih +PARAMETERS +.ls 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: +.ls 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. +.le +.ls 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. +.le +.ls 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. +.le +.ls 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. +.le +.le +.ls cbox = 5.0 (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. +.le +.ls cthreshold = 0.0 (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. +.le +.ls 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. +.le +.ls cmaxiter = 10 +The maximum number of iterations performed by the centering algorithm. +All the centering algorithms use this parameter. +.le +.ls maxshift = 1.0 (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. +.le +.ls clean = no +Symmetry-clean the centering subrater before centering? APPHOT users should +leave clean set to "no". +.le +.ls rclean = 1.0 (scale units) +The cleaning radius for the symmetry-clean algorithm in units of the scale +parameter. +.le +.ls rclip = 2.0 (scale units) +The clipping radius for the symmetry-clean algorithm in units of the scale +parameter. +.le +.ls kclean = 3.0 (sigma) +The number of sky background standard deviations for the symmetry-clean +algorithm where sigma is the value of the DATAPARS parameter sigma. +.le +.ls mkcenter = no +Mark the fitted object centers on the displayed image ? +.le +.ih +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 \fIscale\fR, +the scale of the image, and \fIsigma\fR the standard deviation of +the sky pixels. + +For each object to be measured a subraster of data \fIcbox\fR / \fIscale\fR +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 \fIclean\fR 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 \fImaxshift / +scale\fR 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 > \fIrclean\fR / \fIscale\fR +and r <= \fIrclip\fR / \fIscale\fR and diametrically opposed about the +center of symmetry are tested for equality. If the difference between the +pixels is greater than \fIkclean * sigma\fR, 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 > \fIrclip\fR / \fIscale\fR 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 +\fIcalgorithm\fR, the data specified by \fIcbox / scale\fR, and pixels +that are some threshold above (below) an estimate of the local minimum +(maximum). \fICthreshold\fR values of 0.0, a positive number, and INDEF +invoke the default thresholding algorithm, a threshold equal to the +local minimum (maximum) plus (minus) \fIdatapars.sigma * cthreshold\fR, +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 < \fIminsnratio\fR the new center is still computed but an error +flag is set. + +The default centering algorithm is \fIcentroid\fR. 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, \fIdatapars.sigma * cthreshold\fR +above (below) the local minimum (maximum) if \fIcthreshold\fR is greater +than zero, or zero above (below) the local minimum (maximum) if +\fIcthreshold\fR 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 +\fIdatapars.fwhmpsf\fR. + +The centering algorithm \fIgauss\fR computes the new centers by fitting a +1D Gaussian function to the marginal distributions in x and y using a +fixed fwhmpsf set by \fIdatapars.fwhmpsf\fR. 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 \fIofilter\fR employs a variation of the +optimal filtering technique in which the profile is simulated by a triangle +function of width \fIdatapars.fwhmpsf\fR. + +The default thresholding algorithm for all centering algorithms other +than "centroid" is no thresholding. + +If the computed shift in either coordinate > \fImaxshift\fR / \fIscale\fR, +the new center is returned but an error flag is set. + +.ih +EXAMPLES +1. List the centering parameters. + +.nf + ap> lpar centerpars +.fi + +2. Edit the centering parameters + +.nf + ap> centerpars +.fi + +3. Edit the CENTERPARS parameters from with the PHOT task. + +.nf + 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 +.fi + +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. + +.nf + da> centerpars + + ... edit the parameters + + ... type ":w ctrnite1.par" from within epar +.fi + +.ih +BUGS + +.ih +SEE ALSO +center,phot,wphot,polyphot,radprof +.endhelp |