Started adding some FAQs to the documentation.

docs/index.rst

@@ -16,6 +16,7 @@ Welcome to PampelMuse's documentation!
    data
    psf
    coordinates
+   troubleshooting
    gui
    routines
    output

docs/troubleshooting.rst

+
+.. _troubleshooting:
+
+Frequently asked questions
+==========================
+
+How to increase the number of PSF stars?
+----------------------------------------
+
+PampelMuse requires a certain number of PSF stars in order to determine the coordinate transformation from reference to
+spaxel coordinates. In some cases, the number of PSF stars found by the :ref:`INITFIT routine <initfit>` is too low,
+resulting in the error message `OSError: Need at least 4 sources to fit coordinate transformation.` The following
+parameters can be tweaked to increase the number of PSF stars found by :ref:`INITFIT <initfit>`.
+
+* `initfit|apernois`: The default setting of $0.1$ will result in only the most isolated stars to be considered as
+  PSF stars. Increasing this value to something like $0.4$ often strongly increases the number of PSF stars found
+  without significantly affecting the accuracy of the fitted PSF
+
+* `initfit|psfmag`: By default, only stars that are 3 magnitudes brighter than the faintest star that is extracted from
+  the integral field data are considered as PSF stars. This can lead to problems in cases where the field of view is
+  only moderately crowded, but most visible stars have similar magnitudes. Consider setting `initfit|psfmag` explicitly,
+  to the magnitude of the faintest star you consider useful for a PSF determination.