Up to three emission lines can be superimposed on the input spectrum by the ETC. For a pure emission line spectrum, select the “No continuum” option.
In spectroscopic and spectroscopic acquisition modes, any specified emission lines will be broadened, if necessary, to match the minimum instrumental resolution.
Note: Extinction, redshift, and renormalization are applied to the continuum spectrum, but are not applied to emission lines. Attempting to apply extinction or redshift to a spectrum with “no continuum” selected will result in an error.
However, it is possible to use the pysynphot software directly to construct emission lines, apply extinction to them, and save the result as a FITS file which can then be uploaded to the ETC.
Then, after invoking the Python interpreter, you can create a spectrum including one or more emission lines; define the desired extinction; apply it to the spectrum; and save the result.:
# Import the pysynphot package import pysynphot as psyn # Create an emission line. Specify flux, central wavelength, and FWHM # - note this is not the same order that appears on the ETC form. sp=psyn.GaussianSource(1e-15,7000,10,fluxunits='flam') # Or, you can create several lines and add them to a flat spectrum contnm = psyn.FlatSpectrum(1,fluxunits='flam') line1 = psyn.GaussianSource(1e-15,7000,10,fluxunits='flam') line2 = psyn.GaussianSource(1e-14,5000,15,fluxunits='flam') line3 = psyn.GaussianSource(1e-13,3000,50,fluxunits='flam') sp = contnm + line1 + line2 + line3 # See what you have created print sp # Will produce output something like this for a single emission line #Gaussian: mu=7000 angstrom,fwhm=10 angstrom, total flux=1e-15 flam # See what extinction laws are available psyn.print_red_laws() # Will produce output something like this #name reference #-------- -------------------------------------------------------------- #None Cardelli, Clayton, & Mathis (1989, ApJ, 345, 245) R_V = 3.10. #gal3 Cardelli, Clayton, & Mathis (1989, ApJ, 345, 245) R_V = 3.10. #lmc30dor Gordon et al. (2003, ApJ, 594, 279) R_V = 2.76. #lmcavg Gordon et al. (2003, ApJ, 594, 279) R_V = 3.41. #mwavg Cardelli, Clayton, & Mathis (1989, ApJ, 345, 245) R_V = 3.10. #mwdense Cardelli, Clayton, & Mathis (1989, ApJ, 345, 245) R_V = 5.00. #smcbar Gordon et al. (2003, ApJ, 594, 279) R_V=2.74. #xgalsb Calzetti et al. (2000. ApJ, 533, 682) # Define the extinction specifying the preferred extinction law by name, # and the desired value of the extinction ext=psyn.Extinction(1.3,'lmcavg') # Apply the extinction to the emission line new_sp=ext*sp # Save the resulting spectrum to a FITS file new_sp.writefits('emline_plus_extinction.fits')
This FITS file can then be uploaded to the ETC with the “user-supplied spectrum” option. (Note that line broadening cannot be applied to the emission lines in such a spectrum; consult the relevant instrument handbook to determine the minimum line widths you should use.)
Pysynphot can be used to construct quite complex spectral distributions; for more details, see the pysynphot documentation at http://stsdas.stsci.edu/pysynphot/.
In spectroscopic and spectroscopic acquisition modes, emission lines that are narrower than the instrumental resolution are broadened by the ETC to be at least one resel (resolution element) wide. This is a flux-conserving operation performed by pysynphot when preparing the input target spectrum.
However, the photometric extraction calculation makes use of a simplifying assumption. Instead of integrating over the wavelength range corresponding to a resel, the target count rate per pixel at the specified wavelength is simply multiplied by the number of pixels per resolution element.