Explain the use of vacuum wavelengths for wavelength calibration

tutorials/tutorial_mos/mos_example.rst

@@ -208,6 +208,12 @@ slitlets in two groups: slitlets closed (widths < 1.475 mm) and opened (width >
 Preliminary rectification and wavelength calibration using OH lines
 ===================================================================
 
+.. warning::
+
+   The wavelength calibration performed using OH lines makes use of the vacuum
+   wavelengths published by `Oliva et al. (2013)
+   <https://www.aanda.org/articles/aa/pdf/2013/07/aa21366-13.pdf>`_.
+
 As explained in :ref:`simple_example`, the rectification and wavelength
 calibration of any EMIR spectroscopic image can be obtained with two levels of
 quality:

tutorials/tutorial_mos/simple_example.rst

@@ -4,6 +4,15 @@
 Simple example: arc exposure
 ****************************
 
+.. warning::
+
+   The wavelength calibration is performed using vacuum wavelengths. Some of
+   the arc lines were identified using the `NIST Atomic Spectra Database
+   <https://physics.nist.gov/PhysRefData/ASD/lines_form.html>`_, which provides
+   wavelengths in vacuum above 2000 nm, and in air between 200 and 2000 nm. The
+   latter were accordingly transformed to vacuum following `Ciddor (1996)
+   <https://doi.org/10.1364/AO.35.001566>`_.
+
 .. warning::
 
    All the commands are assumed to be executed in a terminal running the **bash