NAME

phay - Pulse height analysis for FOS observations at different thresholds.

USAGE

phay input intphd difphd fitphd table

DESCRIPTION

This routine does a pulse height analysis (PHA) on a set of Faint Object Spectrograph (FOS) observations taken at different discrimator thresholds. The input observations give the integral pulse height distribution (PHD) for each of the 512 channels. The integral PHD is differentiated using the negative of the slope of a least squares line through a user-specified number of threshold levels, centered at each threshold level. A least squares (i.e., a Guassian distribution with a polynomial baseline) is performed for each of the 512 channels. From the coefficients of the fits, appropriate threshold levels for each of the channels can be computed using the tcalc task in the tools package.

PARAMETERS

input [file name template]

Input files containing FOS observations at different discriminator thresholds. These observations must be in ascending threshold order with a constant delta threshold between observations. The data must be vectors of length 512---overscanning is not permitted for threshold data. If any input observation uses a different exposure time than was used for the other files, it must be normalized (by dividing by exposure time) before running the phay task.

intphd = "intphd" [file name]

Name of the output file that will contain the integral pulse height distributions for each of the 512 diodes. These distributions are organized as a two-dimensional image of size j,i where j is the threshold position and runs from 1 to the number of files in the input data, and i is the diode number running from 1 for diode 0 to 512 for diode 511.

difphd = "difphd" [file name]

Name of the output file that will contain the differential pulse height distribution for each of the 512 diodes. It is a two-dimensional image file having the same format as intphd.

fitphd = "fitphd" [file name]

Name of the output file that will contain the least squares fit to the differential pulse height distribution. It is a two-dimensional image file having the same format as intphd.

table = "phay" [file name]

Name of the output table containing results of the least squares fit for each of the diodes. This table has 512 rows (one per diode), and the following columns.

     'CENTER' - Center of the fitted Gaussian distribution (double).
     'PEAK'   - Peak value of the fitted Gaussian (double).
     'SIGMA'  - Sigma (width) of the fitted Gaussian (double).
     'A0'     - Constant term of the polynomial baseline (double).
     'A1'     - Linear term of the polynomial baseline (double).
     'A2'     - Quadratic term of the polynomial baseline (double).
     'RMS'    - RMS of the fit (i.e., root mean square of the difference
		of the differential PHD and its fit) (double).

t= 50.0 [real, min=1.0, max=255.0]

Threshold level of the first observation in input.

delt = 1.[real, min=1.0, max=200.0]

Increment in threshold levels between the files in input.

center = 175. [real, min=0.0, max=256.0]

Initial guess for the center of the Gaussian fits used by the non-linear least squares fitting process.

sigma = 15.[real, min=1.0, max=50.0]

Initial guess for the sigma of the Gaussian fits.

niter = [integer, min=1, max=100]

Number of non-linear least squares iterations.

tmin = 125.[real, min=0.0, max=255.0]

Minimum threshold level to fit. Only threshold levels between tmin and tmax are included in the fit process.

tmax = 255.[real, min=0.0, max=255.0]

Maximum threshold level to use in the fit.

order = [integer, min=0, max=2]

Order of the polynomial baseline of the least squares fit.

fwidth = [integer, min=3, max=29]

Width of the differential smoothing function. The negative of the slope of the line of fwidth thresholds, centered at each threshold, is used as the differential pulse height distribution value.

hmin = 50.[real, min=0.0]

Minimum pulse height distribution value for the peak. The peak is sought in a 25 point region around the guess for center. If no value higher than hmin is found, then no fit is attempted. The hmin parameter should be used to avoid fitting dead channels.

EXAMPLES

1. Perform a PHA on the data observations ybw001 through ybw100 using the default parameters, except for the minimum fit threshold, which is set to 140.0.

fo> phay ybw* intphd difphd fitphd outtab tmin=140.0

BUGS

REFERENCES

Howard Bushouse, STSDAS

HELP

For assistance using this or any other tasks, please contact help@stsci.edu or call the help desk at 410-338-1082.

SEE ALSO