Question about rectification in spectral profiles

[AndreeaAV]

Hello ! 

I am quite new to the field of actually capturing spectra of different objects. However, there is a question which really bothers me: is it necessary to actually remove the continuum (flatten it) from the spectrum of a planetary nebulae? Of course, a spectrum which has already been wavelength calibrated and radiometrically corrected with the instrument response curve. Maybe in some cases, as for stars, it may help view the spectral features better. But what about planetary nebulae? Is just a normalisation (setting to 1 the continuum) better ? Or performing both has advantages?

Thank you!

[robin_astro]

Hi,

You would not normally rectify (ie normalise the continuum to 1 at all wavelengths ) a spectrum of a nebula as the continuum (if it is even present) will be faint and will not be related to the emission line strengths.  You do need to correct for instrument response though (and atmospheric extinction)  if you want to compare emission line strengths. In fact you would also need to take into account interstellar extinction,though this can also be calculated from the spectrum.  There is a nice writeup by Francois Teyssier on what you can measure in nebulae spectra here

http://www.astronomie-amateur.fr/feuilles/Spectroscopie/NGC2392.html.

In general if you have response corrected a spectrum you would not then rectify it, except perhaps to make a direct comparison of absorption line strengths at different wavelengths, which are normally a proportion of the continuum.  (note that emission line strengths at different wavelengths, even in stars, can only be compared in response corrected spectra)

In certain circumstances, rectifying a spectrum can be used as a shortcut to avoiding having to do a response correction, for example when looking at the  profile of a single line at high resolution where the wavelength range is limited or even in low resolution spectra if you are only interested in the presence or absence of particular features  or the relative strength of absorption lines.

Robin

[AndreeaAV]

Hi,

You would not normally rectify (ie normalise the continuum to 1 at all wavelengths ) a spectrum of a nebula as the continuum (if it is even present) will be faint and will not be related to the emission line strengths.  You do need to correct for instrument response though (and atmospheric extinction)  if you want to compare emission line strengths. In fact you would also need to take into account interstellar extinction,though this can also be calculated from the spectrum.  There is a nice writeup by Francois Teyssier on what you can measure in nebulae spectra here

http://www.astronomie-amateur.fr/feuilles/Spectroscopie/NGC2392.html.

In general if you have response corrected a spectrum you would not then rectify it, except perhaps to make a direct comparison of absorption line strengths at different wavelengths, which are normally a proportion of the continuum.  (note that emission line strengths at different wavelengths, even in stars, can only be compared in response corrected spectra)

In certain circumstances, rectifying a spectrum can be used as a shortcut to avoiding having to do a response correction, for example when looking at the  profile of a single line at high resolution where the wavelength range is limited or even in low resolution spectra if you are only interested in the presence or absence of particular features  or the relative strength of absorption lines.

Robin

Hi Robin! 

Thank you very much. Well, in my case where I have a wavelength calibrated and instrument response corrected spectrum I guess it is not necessary at all to perform the rectification. Maybe only the normalisation of the continuum to 1/ rescaling to 1 as to obtain some relative intensities on the Y axis of the graph, right?

[AndreeaAV]

I used an Alpy 600 for taking the spectrum