NAME

impars -- Edit the image data parameters

USAGE

impars

PARAMETERS

iscale = 1.(units per pixel)

The scale of the image e.g. 0.46 arc-seconds per pixel. Most xguiphot length parameters are defined in units of iscale, pixels if iscale = 1.0, arc-seconds etc. otherwise. Most xguiphot users should leave iscale set to 1.0 unless they need to compare their aperture photometry directly with results in the literature.

ihwhmpsf = 1.(iscale units)

The half-width at half-maximum of the psf in scale units, e.g. 2.0 pixels, or 0.92 arc-seconds if iscale = 0.46 arc-seconds per pixel. Most xguiphot users should set ihwhmpsf to a reasonable value in order to optimize the performance of the object detection algorithms and the "gauss1d" and "ofilter1d" centering algorithms.

iemission = yes

Are the objects to be measured emission objects, iemission = "yes", or absorption "objects", iemission = no ? Most xguiphot users should leave iemission set to "yes".

iskysigma = INDEF (ADU)

The default standard deviation of the sky pixels in ADU. Xguiphot users should set iskysigma to a reasonable value if they plan to set the cenpars.threshold parameter to a value other than INDEF or 0.0, or to use the sky fitting algorithms "zero" or "constant".

imindata = INDEF (ADU)

The minimum good pixel value. Xguiphot users should set this parameter if they wish to, automatically remove low valued bad data from the object detection analysis, remove low valued bad data from the sky pixel distribution before sky fitting, flag object measurements with low valued bad data in the centering and photometry apertures. To avoid biasing the sky estimates xguiphot users should avoid setting imindata to a single value for image sets with varying average background levels.

imaxdata = INDEF (ADU)

The maximum good pixel value. Xguiphot users should set this parameter if they wish to, automatically remove high valued bad data from the object detection analysis, remove high valued bad data from the sky pixel distribution before sky fitting, or to automatically flag saturated objects.

inoisemodel = "poisson"

The noise model used to estimate the xguiphot magnitude errors. Currently the only option is:

poisson

Poisson statistics in the object and the sky background as well as readout noise are used to estimate the error in the object magnitude measurement.

Xguiphot users should be aware that the parameters ikgain or igain and ikreadnoise or ireadnoise must be correctly set in order to obtain valid magnitude error estimates from xguiphot.

ikgain = "GAIN"

The image header keyword defining the gain value whose units are assumed to be electrons per ADU. If ikgain is undefined or does not exist then the gain defaults to the value of igain.

ikreadnoise = "RDNOISE"

The image header keyword defining the readout noise value whose units are assumed to be electrons. If ikreadnoise is undefined or does not exist the readout noise defaults to the value of ireadnoise.

igain = 1.(electrons / ADU)

The default gain in electrons per ADU if ikgain is undefined or does not exist. Xguiphot users should set the ikgain or igain parameter before making measurements in order to obtain magnitude error estimates.

ireadnoise = 0.(electrons)

The default readout noise in electrons if ikreadnoise is undefined or does not exist. Xguiphot users should set ikreadnoise or ireadnoise before making measurements in order to obtain valid magnitude error estimates.

ikexptime = "EXPTIME"

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. If ikexptime is undefined or does not exist the default exposure is the value of ietime. Xguiphot users should set this parameter in order to normalize the magnitude scale and simplify future reduction steps.

ikairmass = "AIRMASS"

The image header airmass keyword. This parameter does not affect the xguiphot computations in any way, but the airmass values stored in the output files can simplify future reductions steps. If ikairmass is undefined or does not exist, the value of iairmass is used.

ikfilter = "FILTERS"

The image header filter id keyword. This parameter does not affect the xguiphot computations in any way, but the filter id values stored in the output files can simplify future reductions steps. If ikfilter is undefined or does not exist, the value of ifilter is used.

ikobstime = "UT"

The image header time of observation keyword. This parameter does not affect the computations in any way, but the time of observation values stored in the output files can simplify future reductions steps. If ikobtime is undefined or does not exist, the value of iotime is used.

ietime = 1.0

The default exposure time for the image in arbitrary units if ikexptime is undefined or does not exist. The computed magnitudes are normalized to 1 timeunit using the value of ietime.

xairmass = INDEF

The default value of the airmass if ikairmass does not exist or is undefined.

ifilter = "INDEF"

The default value of the filter id string if ikfilter does not exisit or is undefined.

iotime = "INDEF"

The default value of the time of observation string if ikitime does not exist or is undefined.

DESCRIPTION

Impars defines the xguiphot parameters which are image data dependent. Impars parameters are functions of the telescope and instrument optics, the detector noise and linearity characteristics, and the observing conditions. Parameters in the findpars , cenpars , skypars and photpars parameter sets are defined in units of the impars parameters.

The parameter iscale sets the length scale used by the object detection, centering, sky fitting and photometry algorithms. Iscale converts distance measurements in iscale units to distance measurements in pixel units. The impars parameters ihwhmpsf , the cenpars parameters cradius and cxyshift , the skypars parameters srannulus , swannulus , and srgrow , and the photpars parameter papertures are defined in units of iscale. The iscale parameter is most useful in cases where the measurements are to be compared to published aperture photometry measurements in the literature.

If the parameter iscale is defined in units of the half-width at half-maximum of the psf per pixel, then the remaining iscale dependent object detection, centering, sky fitting, and photometry algorithm parameters may be defined n units of the half-width at half-maxima of the psf. Obviously ihwhmpsf should be set to 1.0 in that case. Once an optimal set of iscale dependent algorithm parameters is chosen, the user need only edit the iscale parameter before executing xguiphot on a new set of images.

The parameter ihwhmpsf defines the half-width at half-maximum of the image psf. Most xguiphot algorithms do not use ihwhmpsf. The exceptions are the object detection algorithms and the the centering algorithms "gauss1d" and "ofilter1d".

If iemission is "yes" the measured objects are assumed to be emission objects; if it is "no" the measured objects are assumed to be absorption objects. In most cases xguiphot users should leave iemission set to "yes".

The parameter iskysigma provides an estimate of the standard deviation of the background pixels. The centering algorithms use iskysigma to estimate the signal to noise ratio in the centering box, and in combination with the cenpars.cthreshold parameter to define the lower intensity limit for good pixels in the centering aperture. If iskysigma is INDEF or <= 0.0 the background noise in the centering aperture is assumed to be 0 and the default lower intensity limit is used. If the sky fitting algorithm is "zero" or "constant" iskysigma is used to estimate the contribution of the background noise to the magnitude error.

The imindata and imaxdata parameters define the good data range. Bad pixels in the centering subraster and in the photometry apertures are noted, the measurements tagged with an error code, and the resulting magnitudes set to INDEF. Bad data in the sky aperture is removed before the sky fit is performed. Xguiphot users should set imaxdata before making measurements. Normally imindata should be set to a value 5-7 standard deviations below the average sky value. However if sky values vary a lot within a set of images it may be safer to leave imindata set to INDEF.

The two parameters ikgain and igain define the image gain. The ikgain parameter specifies which keyword in the image header contains the gain value. If ikgain is undefined or not present in the image header, the value of igain is used. Igain must be in units of electrons per ADU. Xguiphot users should set either ikgain or igain before making measurements in order to insure that the computed magnitude errors are correct.

The two parameters ikreadnoise and ireadnoise define the image readout noise. The ikreadnoise parameter specifies which keyword in the image header contains the readout noise value. If ikreadnoise is undefined or not present in the image header the value of ireadnoise is used. Ireadnoise is assumed to be in units of electrons. Xguiphot users should set either ikreadnoise or ireadnoise before making any measurements in order to insure the computed magnitude errors are correct.

The xguiphot magnitudes are normalized to an exposure time of 1 timeunit using the value of the exposure time in the image header parameter ikexptime or ietime . If ikexptime is undefined or not present in the image header the value of ietime is used. Exposure times may be in arbitrary units but the units must be the same for image sets which are analyzed together. Xguiphot users should set ikexptime in order to simplify future reduction steps.

The two parameters ikairmass and iairmass define the airmass of the observation. The ikairmass parameter specifies which keyword in the image header contains the airmass value. If ikairmass is undefined or not present in the image header the value of iairmass is used. The airmass values are not used in any xguiphot computations. Xguiphot users should set ikairmass in order to simplify future reduction steps.

The two parameters ikfilter and ifilter define the filter of the observation. The ikfilter parameter specifies which keyword in the image header contains the filter id. If ikfilter is undefined or not present in the image header the value of ifilter is used. The filter id values are not used in any xguiphot computations. Xguiphot users should set ikfilter in order to simplify future reduction steps.

The two parameters ikobstime and iotime define the time of the observation, e.g. UT. The ikobstime parameter specifies which keyword in the image header contains the time stamp of the observation. If ikobstime is undefined or not present in the image header the value of iotime is used. The time of observations values are not used in any xguiphot computations computations. Xguiphot users should set ikobstime in order to simplify future reduction steps.

EXAMPLES

1. List the image data dependent parameters.

xp> lpar impars

2. Edit the image data dependent parameters.

xp> impars

3. Edit the image data parameters and run xguiphot.

xp> epar xguiphot

... edit a few parameters

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

... edit the impars parameters and type :q

... finish editing the other pset and task parameters

... type :go to startup the task

4. Save the current image data dependent parameter set in a text file call imnite1.par.

xp> impars

... edit a few parameters

... type ":w imnite1.par"  from within epar

TIME REQUIREMENTS

BUGS

SEE ALSO

epar,lpar,xguiphot,findpars,cenpars,skypars,photpars