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+.help datapars May00 noao.digiphot.daophot 
+.ih
+NAME
+datapars -- edit the data dependent parameters
+.ih
+USAGE
+datapars
+.ih
+PARAMETERS
+.ls scale = 1.0
+The scale of the image in user units, e.g. arcseconds per pixel.  All DAOPHOT
+distance dependent parameters are assumed to be in units of scale. If
+\fIscale\fR = 1.0 these parameters are assumed to be in units of pixels. Most
+DAOPHOT users should leave \fIscale\fR set to 1.0 unless they intend to compare
+their aperture photometry results directly with data in the literature.
+.le
+.ls fwhmpsf = 2.5 (scale units)
+The full-width half-maximum of the point spread function in scale units.
+The DAOFIND task and the PHOT task  "gauss" and "ofilter" centering algorithms
+depend on the value of fwhmpsf. DAOPHOT users can either determine a value
+for fwhmpsf using an external task such as IMEXAMINE, or make use of the
+interactive capabilities of the DAOPHOT tasks to set and store it.
+.le
+.ls emission = yes
+The features to be measured are above sky. By default the DAOPHOT package
+considers all features to be emission features. DAOPHOT users should
+leave this parameter set to "yes". 
+.le
+.ls sigma = 0.0
+The standard deviation of the sky pixels.  The DAOFIND task and the PHOT task
+"constant" sky fitting algorithm error estimate depend on the value of sigma. 
+DAOPHOT users should set sigma to a value representative of the noise in
+the sky background.
+.le
+.ls datamin = INDEF
+The minimum good pixel value. Datamin defaults to -MAX_REAL the minimum
+floating point number supported by the host computer. Datamin is used
+to detect and remove bad data from the sky aperture, detect and flag
+bad data in the aperture photometry aperture, and detect and remove  bad
+data from the PSF fitting aperture.  DAOPHOT users should either leave
+datamin set to INDEF or set it to a number between 5-7 sigma below the
+sky background value.
+.le
+.ls datamax = INDEF
+The maximum good pixel value. Datamax defaults to MAX_REAL the maximum
+floating point number supported by the host computer.  Datamax is used
+to detect and remove bad data from the sky aperture, detect and flag
+bad data in the aperture photometry aperture, and detect and remove  bad
+data from the PSF fitting aperture.  DAOPHOT users should either leave
+datamax set to INDEF or set it to the linearity or saturation
+limit of the detector.
+.le
+.ls noise = "poisson"
+The noise model used to estimate the uncertainties in the computed
+magnitudes. DAOPHOT users must leave noise set to "poisson".
+.le
+.ls ccdread = ""
+The image header keyword defining the readout noise parameter whose units
+are assumed to be electrons.
+.le
+.ls gain = ""
+The image header keyword defining the gain parameter whose units are assumed to
+be electrons per adu.
+.le
+.ls readnoise = 0.0
+The readout noise of the detector in electrons. DAOPHOT users should set
+readnoise or ccdread to its correct value before running any of the DAOPHOT
+package tasks in order to ensure that the PSF fitting weights, magnitude
+error estimates, and chi values are correct.
+.le
+.ls epadu = 1.0
+The gain of the detector in electrons per adu. DAOPHOT users should set this
+epadu or gain to its correct value before running any of the DAOPHOT package
+tasks in order to ensure that the PSF fitting weights, magnitude error 
+estimates, and chi values are correct.
+.le
+.ls exposure = ""
+The image header exposure time keyword. The time units are arbitrary but
+must be consistent for any list of images whose magnitudes are to be compared.
+The computed magnitudes are normalized to  one timeunit by the PHOT task.
+As the magnitude scale of the DAOPHOT package is set by the PHOT task,
+setting exposure can save DAOPHOT users a lot of unnecessary zero point
+corrections in future analysis and calibration steps.
+.le
+.ls airmass = ""
+The image header airmass keyword.  The airmass parameter is not used
+directly by DAOPHOT but the airmass value is stored in the output file
+and its presence there will simplify future calibration steps.
+.le
+.ls filter = ""
+The image header filter id keyword.  The filter parameter is not used
+directly by DAOPHOT but the filter id is stored in the output file
+and its presence there will simplify future calibration steps.
+.le
+.ls obstime = ""
+The image header time of observation keyword. The obstime parameter is not used
+directly by DAOPHOT but the obstime value is stored in the output file
+and its presence there will simplify future calibration steps.
+.le
+.ls itime = 1.0
+The exposure time for the image in arbitrary units. The DAOPHOT magnitudes are
+normalized to 1 timeunit by the PHOT task using the value of exposure in the
+image header if exposure is defined or the value of itime.
+.le
+.ls xairmass = INDEF
+The airmass value.  The airmass is read from the image header if airmass
+is defined  or from xairmass. The airmass value is stored in the DAOPHOT
+output files.
+.le
+.ls ifilter = "INDEF"
+The filter id string. The filter id is read from the image header if filter
+is defined otherwise from ifilter. The filter id is stored in the DAOPHOT
+output files.
+.le
+.ls otime = "INDEF"
+The value of the time of observation. The time of observation is read from
+the image header if obstime is defined otherwise from otime. The time of
+observation is stored in the DAOPHOT output files.
+.le
+
+.ih
+DESCRIPTION
+
+\fIDatapars\fR sets the image data dependent parameters. These parameters are
+functions, of the instrument optics, the noise characteristics and range of
+linearity of the detector, and the observing conditions. Many of the
+centering, sky fitting, and photometry algorithm parameters in the CENTERPARS,
+FITSKYPARS, PHOTPARS, and DAOPARS  parameter sets scale with the data dependent
+parameters.
+
+The parameter \fIscale\fR sets the scale of the apertures used by the
+centering, sky fitting, aperture photometry, and psf fitting  algorithms.
+Scale converts radial distance measurements in pixels to radial distance
+measurements in scale units. The DAOPHOT parameters cbox, maxshift, rclean
+and rclip in the CENTERPARS parameter set; annulus, dannulus, and rgrow in
+FITSKYPARS parameter set; apertures in the PHOTPARS parameter set; and psfrad,
+fitrad, sannulus, wsannulus, and matchrad in the DAOPARS parameter set are
+expressed in units of the scale. The scale parameter is useful in  cases where
+the observations are to be compared to published aperture photometry
+measurements in the literature.
+
+The parameter \fIfwhmpsf\fR defines the full-width at half-maximum of the
+stellar point spread function. The DAOFIND task, the PHOT task centering
+algorithms "gauss" and "ofilt", and the PSF modeling task PSF all require
+an accurate estimate for this parameter.
+
+By setting the \fIscale\fR and \fIfwhmpsf\fR appropriately the aperture
+sizes and radial distances may be  expressed in terms of the half-width
+at half-maximum of the stellar point spread function.  The way to do this
+is to define the scale parameter in units of the number of half-width at
+half-maximum per pixel, set the fwhmpsf parameter to 2.0, and then
+set the remaining scale dependent centering, sky fitting, aperture photometry,
+and psf fitting algorithm parameters in CENTERPARS, FITSKYPARS, PHOTPARS,
+and DAOPARS to appropriate values in units of the half-width at half-maximum
+of the point-spread function. Once an optimum set of algorithm parameters is
+chosen, the user need only alter the DATAPARS scale parameter before
+executing a DAOPHOT task on a new image.
+
+If \fIemission\fR is "yes", the features to be measured are assumed to
+be above sky. By default the DAOPHOT package considers all features to be
+emission features. DAOPHOT users should leave this parameter set to "yes".
+Although the DAOFIND and PHOT tasks can detect and measure absorption features
+the PSF fitting tasks currently cannot.
+
+The parameter \fIsigma\fR estimates the standard deviation of the sky
+background pixels. The star finding algorithm in DAOFIND uses sigma
+and the \fIfindpars.threshold\fR parameter to define the stellar
+detection threshold in adu. The PHOT task centering algorithms use sigma,
+1) with the \fIcenterpars.kclean\fR parameter to define deviant pixels
+if \fIcenterpars.clean\fR is enabled; 2) to estimate the signal to
+noise ratio in the centering box; 3) and with the \fIcenterpars.cthreshold\fR
+parameter to define a lower intensity limit for the pixels to be used
+for centering.  If sigma is undefined or <= 0.0 1) no cleaning is performed
+regardless of the value of centerpars.clean; 2) the background noise in the
+centering box is assumed to be 0.0; and 3) default cutoff intensity is used
+for centering.
+
+The \fIdatamin\fR and \fIdatamax\fR parameters define the good data range.
+If datamin or datamax are defined bad data is removed from the sky pixel
+distribution before the sky is fit, data containing bad pixels in the
+photometry apertures is flagged and the corresponding aperture photometry
+magnitudes are set to INDEF, and bad data removed from the PSF fitting
+aperture. DAOPHOT users should set datamin and datamax to appropriate values
+before running the DAOPHOT tasks.
+
+DAOPHOT users must leave \fInoise\fR set to "poisson".  This model includes
+Poisson noise from the object and both Poisson and readout noise in the sky
+background.
+
+The parameters \fIgain\fR and \fIepadu\fR define the image gain.
+The gain parameter specifies which keyword in the image header contains
+the gain value. If gain is undefined or not present in the image header
+the value of epadu is used.  Epadu must be in units of electrons per adu.
+DAOPHOT users should set either gain or epadu to a correct value before
+running any of the DAOPHOT package tasks to ensure that the aperture
+photometry magnitude error estimates, and the PSF fitting weights, chis, and
+magnitude error estimates are computed correctly.
+
+The two parameters \fIccdread\fR and \fIreadnoise\fR define the image
+readout noise.  The ccdread parameter specifies which keyword in the
+image header contains the readout noise value. If ccdread is undefined or
+not present in the image header the value of readnoise is used.
+Readnoise is assumed to be in units of electrons.
+DAOPHOT users should set either ccdread or readnoise before running any
+DAOPHOT tasks to insure that the PSF fitting weights, chis, and magnitude
+error estimates are computed correctly.
+
+The magnitudes computed by PHOT are normalized to an exposure time of 1 
+timeunit using the value of the exposure time in the image header parameter 
+\fIexposure\fR or \fIitime\fR. If exposure is undefined or not present
+in the image header a warning message is issued and the value of itime
+is used. The itime units are arbitrary but must be consistent for images
+analyzed together. As the magnitude scale in DAOPHOT is determined by the
+PHOT task setting either exposure or itime can save DAOPHOT users a lot
+of unnecessary zero point corrections in future analysis and calibration
+steps.
+
+The parameters \fIairmass\fR and \fIxairmass\fR define the airmass
+of the observation. The airmass parameter specifies which keyword in the
+image header contains the airmass value. If airmass is undefined or
+not present in the image header the value of xairmass is used.
+The airmass values are not used in any DAOPHOT computations, however their
+presence in the DAOPHOT output files will simplify future reduction steps.
+
+The parameters \fIfilter\fR and \fIifilter\fR define the filter
+of the observation. The filter parameter specifies which keyword in the
+image header contains the filter id. If filter is undefined or not present
+in the image header the value of ifilter is used. The filter id values are
+not used in any DAOPHOT computations, however their presence in the DAOPHOT
+output files can will simplify future reduction steps.
+
+The parameters \fIobstime\fR and \fIotime\fR define the time
+of the observation (e.g. UT). The obstime parameter specifies which keyword
+in the image header contains the time stamp of the observation. If obstime is
+undefined or not present in the image header the value of otime is used.
+The time of observations values are not used in any DAOPHOT
+computations, however their presence in the DAOPHOT output files can
+greatly simplify future reduction steps.
+
+
+.ih
+EXAMPLES
+
+1. List the data dependent parameters.
+
+.nf
+	da> lpar datapars
+.fi
+
+2. Edit the data dependent parameters.
+
+.nf
+	da> datapars
+.fi
+
+3. Edit the data dependent parameters from within the PSF task.
+
+.nf
+    da> epar psf
+
+	... edit a few parameters
+
+	... move to the datapars parameter and type :e
+
+	... edit the datapars parameters and type :wq
+
+	... finish editing the psf parameter and type :wq
+.fi
+
+4. Save the current DATAPARS parameter set in a text file datnite1.par.
+This can also be done from inside a higher level task as in the previous
+example.
+
+.nf
+    da> epar datapars
+
+	... edit a few parameters
+
+	... type ":w datnite1.par"  from within epar
+.fi
+.ih
+TIME REQUIREMENTS
+.ih
+BUGS
+
+.ih
+SEE ALSO
+epar,lpar,daofind,phot,pstselect,psf,group,peak,nstar,allstar,substar,addstar
+.endhelp