.. _Bright_line_tails:

******************************
Problem with bright line tails
******************************

It has been detected that in some MEGARA observations where spectra with very
bright emission lines are obtained, a spurious signal may appear near the tails
of those lines. It is believed that the issue may be due to moisture
condensation that, when present, results in a extended low-surface-brightness
spherical halo (at ~0.5% of the peak level) in the LSF.

This is an example of a raw Th-Ne arc exposure, using the HR-R VPH:

.. image:: _static/bright_lines/bright_lines_image1.png
   :width: 100%
   :alt: raw HR-R exposure of ThNe lamp

.. image:: _static/bright_lines/bright_lines_cut1.png
   :width: 100%
   :alt: single spectrum of raw HR-R exposure of ThNe lamp

This is another example of a raw Th-Ne arc exposure, using the LR-R VPH:

.. image:: _static/bright_lines/bright_lines_image2.png
   :width: 100%
   :alt: raw LR-R exposure of ThNe lamp

.. image:: _static/bright_lines/bright_lines_cut2.png
   :width: 100%
   :alt: single spectrum of raw LR-R exposure of ThNe lamp

**Note that in the spectral cut (in both examples), a logarithmic vertical 
scale is being used.**

Although the effect has a very low signal compared to the center of the
lines, it is important to examine the raw images of the arc exposures to check
whether the effect is present. This effect can also be detected in exposures of
scientific objects with intense emission lines. 

**MEGARA DRP does not include any functionality to address this issue.** It is
recommended to avoid those regions near the tails of very bright lines when
extracting relevant scientific information.