Telluric Line Cancellation (Step=9)

Relevant code

XDpiped.csh TelluricXD.py

Relevant options

1extras:          yes/no (no) [extract full-slit spectrum and in steps along the slit?]
2telluric2_inter: yes/no (no) [run 'telluric' interactively when dividing by standard star?]
3continuum_inter: yes/no (no) [interactive continuum fitting?]

What it does

The code corrects the science target for telluric absorption using the line-free standard star spectrum created in H Line Removal (Step=8).

The first step in the telluric line removal is to fit a continuum to each order of the line-free standard star spectrum created in the previous step and divide the spectra by the fits. This gives better results than using the spectra directly. The continuum fitting seems to work well without intervention, but the continuum_inter=yes option can be used to do it interactively if desired.

The telluric task then takes the continuum-divided spectra of the standard star and the science target spectra and finds the shift and scaling that (hopefully) best take out the telluric lines in the science target. This can be done interactively by setting telluric2_inter=yes. The shift and scale values are written to LISTS/telluric_telluric.txt so that any manual adjustment can be reproduced later if desired.

At this stage, the telluric-corrected science target spectrum has some arbitrary overall shape dictated by the continuum fit to the standard star and the telluric task have introduced a normalization factor that differs from order to order. The normalization is taken out and the science target spectra are divided by the standard star continuum fit.

What to look for

It is good to check that telluric has chosen reasonable shift and scale values for your spectra. These values are written into the PRODUCTS/Checks_and_Warnings.txt file.

Things to most likely go wrong

Nothing, exactly, but the results of the non-interactive reduction can often be improved.

Example: NGC 3031

Order 5/extension 3 of the NGC3031 spectrum is shown before and after telluric cancellation in Fig. 12. The atmospheric transmission near 1.4 \(\mu\)m is low, and it is difficult to remove these lines except when the atmospheric water column is very low. The telluric correction in the rest of the spectrum is quite good.

../_images/telluric3.png

Fig. 12 Extension 3/order 5 of the NGC 3031 spectrum, before and after (green) the automatic telluric cancellation.