8. Wavelength Calibration Guide for GNIRS LS

This is a quick reference table to guide you towards the wavelength calibration scenario that most likely applies based on the instrument configuration.

Below the table, we discuss the various ways to do the wavelength solution calculation.

Bandpass	Grating	Central Wavelength	Advise
X-band J-band H-band	10/mm 32/mm	Any	most likely The arcs will normally have enough lines and reasonable coverage. possible improvement If the OH and O₂ emission lines are visible, they can be used instead of the arc. unlikely If emission lines are not visible (short exposures), and the arc is not sufficient, the telluric absorption lines can be used.
X-band J-band H-band	111/mm	Any	most likely The arc lines will be very few, 1 to 5, and will not offer a good coverage of the spectral range. OH and O₂ lines should be visible, use them. less likely If the exposure times are short, the OH and O₂ lines might not be visible, the only solution will be to use the telluric absorption features.
K-band	10/mm 32/mm	Any	most likely The arcs will normally have enough lines and reasonable coverage. possible improvement If the OH and O₂ emission lines are visible, they can be used instead of the arc. unlikely If emission lines are not visible (short exposures), and the arc is not sufficient, the telluric absorption lines can be used.
	111/mm	< 2.3 ${\mu}$ m	most likely The arc lines will be very few, 1 to 5, and will not offer a good coverage of the spectral range. OH and O₂ lines should be visible, use them. less likely If the exposure times are short, the OH and O₂ lines might not be visible, the only solution will be to use the telluric absorption features.
	111/mm	> 2.3 ${\mu}$ m	most likely The arc lines will be very few, 1 to 5, and will not offer a good coverage of the spectral range. There will be no OH or O₂ lines available. The telluric absorption features must be used.
L-band M-band	Any	Any	No arc lamp observations are taken for the thermal bands. The emission features in the sky spectrum must be used for wavelength calibration. Note that for L-band > 3.8 ${\mu}$ m, the automatic line identification is often wrong and the use of the interactive mode is required.

8.1. Usage

8.1.1. Need for processed flat

While optional it is highly recommended to use processed flat when producing a wavelength solution. When using an arc, the flat needs to be passed manually (there are no calibration association rules yet between the arcs and the flats). When using the sky lines, emission or absorption, the flat will be retrieved automatically.

The processed flat stores a mask that defines the illuminated region. That’s the information that helps calculating the wavelength solution.

8.1.2. From the Arc Lamp

Producing a wavelength solution from the arc observations is fairly straightforward. Just call reduce on the raw arcs.

The use of the interactive mode is recommended to verify the solution and ensure that the lines offer a good coverage the entire spectral range.

reduce @arcs.lis -p interactive=True

8.1.3. From the Emission Lines

When OH and O₂ lines are present in the science data, or, in the thermal bands, when emission features in the sky spectrum are present, it is possible to use those to calculate the wavelength solution.

This is done by running reduce on the science frames and specifying the use of the makeWavecalFromSkyEmission recipe. The interactive mode is recommended to verify and ensure correct line identification.

reduce @sci.lis -r makeWavecalFromSkyEmission -p interactive=True

If the OH and O₂ sky lines are bright, it is possible to use only one science observation instead of the stack. This could lead to higher precision as the stack might make the lines a touch thicker.

8.1.4. From the Absorption Lines

When the arc lamp offers very few lines, or poor coverage (eg. all the lines at one end of the spectrum), and there are no emission lines, one has to resort to using the telluric absorption features to measure the wavelength solution.

This is done by running reduce on the science frames and specifying the use of the makeWavecalFromSkyAbsorption recipe.

This recipe requires a solution from the arc lamp to serve as initial condition. So in this case, there are two steps to the process.

reduce @arcs.lis -p interactive=True
caldb remove N20210407S0181_arc.fits

reduce @sci.lis -r makeWavecalFromSkyAbsorption --user_cal processed_arc:N20210407S0181_arc.fits -p interactive=True

The solution from the arc is really poor and used only to help calculate the solution from the telluric absorption. Therefore it is removed from the calibration manager to ensure that no science reduction ever uses it.

The interactive mode is recommended to verify and ensure correct line identification.