bandpar: Calculate photometric parameters of a passband

Package: synphot

Usage

bandpar obsmode

Description

This task computes the photometric parameters of a passband. The passband can either be read from a file, calculated as the combined throughputs of the individual HST components, or calculated from certain functional forms, such as a Gaussian or a rectangular window. The passband is specified by an expression in the 'obsmode' task parameter. See the help file for calcband for a description of the syntax of this expression.

This task computes eleven different photometric parameters. These are:

 NAME           PARAMETER
=======         =========
URESP           Flux (in flam) of a star that produces a response
                of one photon per second in this passband
PIVWV           Passband pivot wavelength
BANDW           Passband rms width
FWHM            Full width half max of equivalent gaussian
WPEAK           Wavelength at peak throughput
TPEAK           Peak bandpass throughput
AVGLAM          Passband average wavelength
QTLAM           Dimensionless efficiency
EQUVW           Passband equivalent width
RECTW           Passband rectangular width
EMFLX           Equivalent monochromatic flux

In addition to computing these photometric parameters, this task computes two parameters used in the computation of the equivalent monochromatic flux:

 NAME           PARAMETER
=======         =========
REFWAVE         The reference wavelength
TLAMBDA         The throughput at the reference wavelength

Unless specified by the user, the reference wavelength is set to the average wavelength of the passband (AVGWV). The photometric parameters are defined by the equations:

URESP = H * C / (AREA * INT(THRU * LAM))

PIVWV = SQRT(INT(THRU * LAM) / INT (THRU / LAM))

BANDW = BARLAM * SQRT(INT(THRU * LN(LAM / BARLAM) ** 2 / LAM) /
                      INT(THRU / LAM)

FWHM = SQRT(8 * LOG(2)) * BANDW

BARLAM = INT(THRU * LN(LAM) / LAM) / INT(THRU / LAM)

TPEAK = MAX(THRU)

AVGWV = INT(THRU * LAM) / INT(THRU)

QTLAM = INT(THRU / LAM)

EQUVW = INT(THRU)

RECTW = INT(THRU) / MAX(THRU)

EMFLX = URESP * RECTW * (TPEAK / TLAMBDA)

In these formulas, INT() denotes the integral with repect to lambda, SQRT() denotes square root, LN() the natural logarithm, LOG() the common logairthm, and MAX() the maximum function. The variable LAM is wavelength, THRU is the total throughput, which is a function of wavelength, AREA is the telescope area, and H and C are the usual physical constants.

The photometric parameters are printed on STDOUT and optionally saved in a table. Which parameters are printed and saved is controlled by the hidden parameter 'photlist'. By default, all parameters will be printed.

Parameters

obsmode [string]: The command the specifies the synthetic passband. The command syntax is described in the help file for calcband. The photometric parameters of more than one passband can be computed by putting the commands in a file, one per line, and setting 'obsmode' to the name of the file preceded by an "@" character, e.g. "@filename". The keywords which form an obsmode are explained further in the obsmode task.

(output = "none") [string]: The name of the output table containing the photometric parameters. If this parameter is set to "none" or left blank, no output table will be created. But output will still be sent to STDOUT regardless of the value of this parameter.

(photlist = "all") [string]: A comma separated list of the photometric parameters to print. The value "all" prints all the photometric parameters. Placing a "~" in front of the list causes all the parameters except the named parameters to be printed. The two auxiliary parameters REFWAVE and TLAMBDA are printed by default if EMFLX is printed and not printed by default if EMFLX is not printed.

(refwave = INDEF) [real]: The reference wavelength used in the computation of EMFLX. If this parameter is set to INDEF, the average wavelength (AVGWAV) will be calculated and used in its place. The units of the wavelength must be angstroms.

(wavetab = "") [file name]: Name of an optional wavelength table or file. An appropriate table can be generated by using the 'genwave' task. If a table is used, the wavelength column name must be "WAVELENGTH". If an ASCII file is used the first column is taken to be the wavelength column. The subdirectory 'synphot$data has ASCII wavelength tables useful for specific HST passbands. If no wavelength table is specified, the task generates a wavelength set which covers the range where the passband is non-zero. Wavelengths are spaced logarithmically over this range. If there is more than one passband, the range is computed based on all the passbands.

(refdata = "") [pset name]

Parameter set for reference data used in calculations. This pset contains the following parameters:


area = 45238.93416:  HST telescope area in cm**2.

grtbl = "mtab$*.tmg":  HST graph table.  By default, this
        uses the more recent version.

cmptbl = "mtab$*.tmc":  Instrument component table.  By
        default, this uses the more recent version.

Examples

1. Calculate the photometric parameters for the wfpc and f555w filter.

sy> bandpar wfpc,f555w

2. Calculate the pivot wavelength and rms bandwidth for each of the Johnson passbands.

sy> bandpar @ubvri.lis output=johnson.tab phot=pivwv,bandw

The file ubvri.lis contains the lines:

band(u)
band(b)
band(v)
band(r)
band(i)

3. Calculate the photometric parameters for the wfpc and the f555w filter, setting the reference wavelength to 5500 angstroms. Don't print the reference wavelength.

sy> bandpar wfpc,f555w refwave=5500 phot=~refwave

References

Written by B.Simon