sbands: Bandpass spectrophotometry of spectra

Package: onedspec

Usage

sbands input output bands

Parameters

input: Input list of spectra to be measured. These may be one dimensional spectra in individual or "multispec" format or calibrated spatial spectra such as long slit or Fabry-Perot images. The dispersion axis and summing parameters are specified by package parameters for the spatial spectra.

output: Output file for the results. This may be a filename or "STDOUT" to write to the terminal.

bands: Bandpass file consisting of lines with one, two, or three bandpasses per line. A bandpass is specified by an identification string (quoted if it is null or contains whitespace), the central wavelength, the width of the bandpass in wavelength, and a filter filename with the special value "none" if there is no filter (a flat unit response). This format is described further in the description section.

apertures = "": List of apertures to select from the input spectra. For one dimensional spectra this is the aperture number and for spatial spectra it is the column or line. If the null string is specified all apertures are selected. The aperture list syntax is a range list which includes intervals and steps (see ranges).

normalize = yes: Normalize the bandpass fluxes by the bandpass response? If no then the results will depend on the bandpass widths and filter function values. If yes then fluxes will be comparable to an average pixel value. When computing indices and equivalent widths the flux must either be normalized or the bandpasses and filter response functions must be the same.

mag = no, magzero = 0.: Output the bandpass fluxes as magnitudes with specified magnitude zero point?

verbose = yes: Include a verbose header giving a banner, the parameters used, the bandpasses, and column headings?

Description

Sbands performs bandpass spectrophotometry with one or more bandpasses on one or more spectra. A list of input spectra is specified. The spectra may be of any type acceptable in the noao.onedspec package including multispec format with nonlinear dispersion, long slit spectra, and even 3D cubes with one dispersion axis. The apertures parameter allows selecting a subset of the spectra by aperture number.

The bandpasses are specified in a text file. A bandpass consists of four fields; an identification name, the wavelength of the bandpass center, a bandpass width, and a filename for a filter. The identification is a string which must be quoted if a null name or a name with whitespace is desired. The identification could be given as the central wavelength if nothing else is appropriate. The filter field is a filename for a text file containing the filter values. A filter file consists of a wavelength ordered list of wavelength and relative response. Extrapolation uses the end point values and interpolation is linear. The special name "none" is used if there is no filter. This is equivalent to unit response at all wavelengths.

In the bandpass file there may be one, two, or three bandpasses on a line. Below are some examples of the three cases:

alpha 5000 10 myalpha.dat
beta1 4000 100 none       beta2 4100 100 none
line  4500 100 none       red   4000 200 none blue 5000 200 none

The flux in each bandpass is measured by summing each pixel in the interval multiplied by the interpolated filter response at that pixel. At the edges of the bandpass the fraction of the pixel in the bandpass is used. If the bandpass goes outside the range of the data an INDEF value will be reported. If the normalize option is yes then the total flux is divided by the sum of the filter response values. If the mag option is yes the flux will be converted to a magnitude (provided it is positive) using the formula

magnitude = magzero - 2.5 * log10 (flux)

where magzero is a parameter for the zero point magnitude and log10 is the base 10 logarithm. Note that there is no attempt to deal with the pixel flux units. This is the responsibility of the user.

If there is only one bandpass (on one line of the band file) then only the band flux or magnitude is reported. If there are two bandpasses the fluxes or magnitudes for the two bands are reported as well as a band index, the flux ratio or magnitude difference (depending on the mag) flag, and an equivalent width using the second band as the continuum. If there are three bandpasses then a continuum bandpass flux is computed as the interpolation between the bandpass centers to the center of the first bandpass. The special bandpass identification "cont" will be reported.

The equivalent width is obtained from the two bandpasses by the formula

eq. width = (1 - flux1 / flux2) * width1

where flux1 and flux2 are the two bandpass fluxes and width1 is the width of the first bandpass. Note that for this to be meaningful the bandpasses should be normalized or have the same width/response.

The results of measuring each bandpass in each spectrum are written to the specified output file. This file may be given as "STDOUT" to write the results to the terminal. The output file contains lines with the spectrum name and aperture, the band identifications and fluxes or magnitudes, and the band index and equivalent width (if appropriate). The verbose option allows creating a more documented output by including a commented header with the task name and parameters, the bandpass definitions, and column labels. The examples below show the form of the output.

Examples

The following examples use artificial data and arbitrary bands.

1. Show example results with one, two, and three bandpass entries in the bandpass file.

cl> type bands
test 6125 50 none red 6025 100 none blue 6225 100 none
test 6125 50 none red 6025 100 none
test 6125 50 none blue 6225 100 none
test 6125 50 none
cl> sbands oned STDOUT bands

# SBANDS: NOAO/IRAF IRAFX valdes@puppis Mon 15:31:45 01-Nov-93
#   bands = bands, norm = yes, mag = no
#       band     filter wavelength      width
#       test       none      6125.        50.
#        red       none      6025.       100.
#       blue       none      6225.       100.
#       test       none      6125.        50.
#        red       none      6025.       100.
#       test       none      6125.        50.
#       blue       none      6225.       100.
#       test       none      6125.        50.
#
#       spectrum    band    flux    band    flux   index eqwidth
         oned(1)    test   44.33    cont   97.97    0.45   27.37
         oned(1)    test   44.33     red   95.89    0.46   26.89
         oned(1)    test   44.33    blue  100.04    0.44   27.84
         oned(1)    test   44.33

2. This example shows measurements on a long slit spectrum with an aperture selection and magnitude output.

cl> type lsbands.dat
band1 4500 40 none
band2 4600 40 none
band3 4700 40 none
cl> nsum=5
cl> sbands ls STDOUT lsbands.dat apertures=40-60x5 mag+ magzero=10.1

# SBANDS: NOAO/IRAF IRAFX valdes@puppis Mon 15:37:18 01-Nov-93
#   bands = lsbands.dat, norm = yes, mag = yes, magzero = 10.10
#       band     filter wavelength      width
#      band1       none      4500.        40.
#      band2       none      4600.        40.
#      band3       none      4700.        40.
#
#       spectrum    band     mag
 ls[38:42,*](40)   band1    3.14
 ls[38:42,*](40)   band2    3.19
 ls[38:42,*](40)   band3    3.15
 ls[43:47,*](45)   band1    3.13
 ls[43:47,*](45)   band2    3.15
 ls[43:47,*](45)   band3    3.14
 ls[48:52,*](50)   band1    2.34
 ls[48:52,*](50)   band2    2.43
 ls[48:52,*](50)   band3    2.43
 ls[53:57,*](55)   band1    3.10
 ls[53:57,*](55)   band2    3.15
 ls[53:57,*](55)   band3    3.12
 ls[58:62,*](60)   band1    3.14
 ls[58:62,*](60)   band2    3.19
 ls[58:62,*](60)   band3    3.15

Revisions

SBANDS V2.10.4: The flux column is now printed to 6 digits of precision with possible exponential format to permit flux calibrated spectra to print properly.

SBANDS V2.10.3: The task is new in this release