datapars: Edit the data dependent parameters

Package: irred

Usage

datapars

Parameters

scale = 1.0

The scale of the image in user units, e.g arcseconds per pixel. All APPHOT distance dependent parameters are assumed to be in units of scale. If scale = 1.0 these parameters are assumed to be in units of pixels. Most APPHOT users should leave scale set to 1.0 unless they intend to compare their aperture photometry results directly with data in the literature.

fwhmpsf = 2.5 (scale units)

The full-width at half-maximum of the point spread function in scale units. The DAOFIND, FITPSF and WPHOT tasks and the "gauss" and "ofilter" centering algorithms depend on the value of fwhmpsf. APPHOT users can either determine a value for fwhmpsf using an external task such as IMEXAMINE, or make use of the interactive capabilities of the APPHOT tasks to set and store it.

emission = yes

The features to be measured are above sky. By default the APPHOT package considers all features to be emission features. However all the package tasks measure absorption features if emission is set to no.

sigma = INDEF

The standard deviation of the sky pixels. The DAOFIND task and the "constant" sky fitting algorithm error estimate depend on the value of sigma. APPHOT users should set sigma to a value which is representative of the noise in the sky background.

datamin = INDEF

The minimum good pixel value. Datamin defaults to -MAX_REAL, the minimum floating point number supported by the host computer. APPHOT users should set this parameter if they wish to remove bad data from the sky pixel distribution before the sky is fit or if they wish to flag stars with bad data in the centering and / or photometry apertures.

datamax = INDEF

The maximum good pixel value. Datamax defaults to MAX_REAL the maximum floating point number supported by the host computer. APPHOT users should set this parameter if they wish to flag saturated stars or stars with bad data in the centering and / or photometry apertures.

noise = "poisson"

The noise model used to estimate the uncertainties in the computed APPHOT magnitudes. The options are the following:

poisson

Poisson statistics in the object and the sky background are used to estimate the error in the object measurement. There are two components to the sky noise measurement the sky noise in the object aperture and the mean error in the estimated sky value.

constant

The standard deviation of the sky background is used to estimate the error in the object measurement. There are two components to the error estimate the sky noise in the object aperture and the mean error in the estimated sky value.

Most APPHOT users should use the Poisson model appropriate for CCD detectors. APPHOT users should also be aware that one or other of the parameters gain or epadu must be set correctly in order to compute the magnitude errors correctly.

ccdread = ""

The image header keyword defining the readout noise parameter whose units are assumed to be electrons.

gain = ""

The image header keyword defining the gain parameter whose units are assumed to be electrons per adu.

readnoise = 0.0

The readout noise of the image in electrons. APPHOT users should set this parameter or the ccdread parameter to its correct value before running any of the APPHOT tasks.

epadu = 1.0

The gain in electrons per adu. APPHOT users should set epadu or ain to its correct value before running any of the APPHOT tasks in order to insure that the magnitude error estimates are correct.

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 1 timeunit. Setting the exposure parameter will greatly simplify future reduction steps. The value of exposure is recorded in the APPHOT output file.

airmass = ""

The image header airmass keyword. The airmass parameter is not used directly by APPHOT but the airmass value is stored in the output file and its presence there will simplify future calibration steps.

filter = ""

The image header filter id keyword. The filter parameter is not used directly by APPHOT but the filter id is stored in the output file and its presence there will simplify future calibration steps.

obstime = ""

The image header time of observation keyword. The obstime parameter is not used directly by APPHOT but the obstime value is stored in the output file and its presence there will simplify future calibration steps.

itime = 1.0

The exposure time for the image in arbitrary units. The APPHOT magnitudes are normalized to 1 timeunit using the value of exposure in the image header if exposure is defined or the value of itime.

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 APPHOT output files.

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 APPHOT output files.

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 APPHOT output files.

Description

Datapars 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 and PHOTPARS parameter sets scale with the data dependent parameters.

The parameter scale sets the scale of the apertures used by the centering, sky fitting and photometry algorithms. Scale converts radial distance measurements in pixel units to radial distance measurements in scale units. The APPHOT parameters, cbox, maxshift, rclean and rclip in the CENTERPARS parameter set; annulus, dannulus, and rgrow in the FITSKYPARS parameter set; and apertures in the PHOTPARS 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 fwhmpsf defines the full-width at half-maximum of the stellar point spread function. Most APPHOT tasks and algorithms do not require this parameter. The exceptions are the DAOFIND task, the centering algorithms "gauss" and "ofilter", the FITPSF task, and the WPHOT task.

By setting the scale and fwhmpsf 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 and photometry algorithm parameters in CENTERPARS, FITSKYPARS and PHOTPARS 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 an APPHOT task on a new image.

If emission is "yes", the features to be measured are assumed to be above sky. By default the APPHOT package considers all measurements to be measurements of emission features. In most cases APPHOT users should leave emission set to "yes".

The parameter sigma estimates the standard deviation of the sky background pixels. The star finding algorithm in DAOFIND uses sigma and the findpars.threshold parameter to define the stellar detection threshold in adu. The centering algorithms uses sigma, 1) with the centerpars.kclean parameter to define deviant pixels if centerpars.clean is enabled; 2) to estimate the signal to noise ratio in the centering box; 3) and with the centerpars.cthreshold parameter to define the 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; and 3) default cutoff intensity intensity is used for centering.

The datamin and datamax 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. APPHOT users should set datamin and datamax to appropriate values before running the APPHOT tasks.

Two noise models are available "constant" and "poisson". If noise = constant, the total noise is assumed to be due to noise in the sky background alone. If noise = poisson, the total noise includes Poisson noise from the object and the sky noise.

The parameters gain and epadu 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. APPHOT users should set either gain or epadu before running any APPHOT tasks to insure the magnitude error computations are correct.

The two parameters ccdread and readnoise 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. APPHOT users should set either ccdread or readnoise before running any APPHOT tasks to insure the magnitude error computations are correct.

The magnitudes are normalized to an exposure time of 1 timeunit using the value of the exposure time in the image header parameter exposure or itime. If exposure is undefined or not present in the image header the value of itime is used. Itime can be in arbitrary units. Setting either exposure or itime will simplify future analysis steps.

The parameters airmass and xairmass 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 APPHOT computations, however their presence in the APPHOT output files will simplify future reduction steps.

The parameters filter and ifilter 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 APPHOT computations, however their presence in the APPHOT output files can will simplify future reduction steps.

The parameters obstime and otime 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 APPHOT computations, however their presence in the APPHOT output files can greatly simplify future reduction steps.

Examples

1. List the data dependent parameters.

ap> lpar datapars

2. Edit the data dependent parameters.

ap> datapars

3. Edit the DATAPARS parameters from within the PHOT task.

da> epar phot

    ... edit a few parameters

    ... move to the datapars parameter and type :e

    ... edit the datapars parameters and type :wq

    ... finish editing the phot parameters and type :wq

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.

da> datapars

    ... edit a few parameters

    ... type ":w datnite1.par"  from within epar

Time requirements

Bugs

See also

epar,lpar,daofind,center,fitsky,phot,wphot,polyphot,radprof,fitpsf