diff --git a/01.curriculum/01.physics-chemistry-biology/03.Niv3/02.Geometrical-optics/05.paraxial-optics/03.physical-system-versus-optical-system/04.new-course-parallel-1/default.en.md b/01.curriculum/01.physics-chemistry-biology/03.Niv3/02.Geometrical-optics/05.paraxial-optics/03.physical-system-versus-optical-system/04.new-course-parallel-1/default.en.md
index 7d736aa1b..668ed1482 100644
--- a/01.curriculum/01.physics-chemistry-biology/03.Niv3/02.Geometrical-optics/05.paraxial-optics/03.physical-system-versus-optical-system/04.new-course-parallel-1/default.en.md
+++ b/01.curriculum/01.physics-chemistry-biology/03.Niv3/02.Geometrical-optics/05.paraxial-optics/03.physical-system-versus-optical-system/04.new-course-parallel-1/default.en.md
@@ -4,17 +4,21 @@ published : true
visible : true
---
-!!!! *DIFFICULT POINT* : What is a virtual object? What is the difference of the
-image focal plane of a simple optical component part of an optical instrument, and the image focal plane
+!!!! *DIFFICULT POINT* :
+What is a virtual object?
+What is the difference between the image focal plane of a simple optical component part of an optical instrument, and the image focal plane
of the instrument itself?
!!!!
!!!! 
!!!!
!!!!
-!!!! I choose it
+!!!! I test my understanding
!!!!
!!!!
-!!!! PILOT is a telescope
+!!!! PILOT es un telescopio incrustado en un globo estratosférico, estacionado a una altitud de 40 km sobre la atmósfera,
+para observar la luz emitida por nuestra galaxia en un rango de longitud de onda (infrarrojo lejano) que no alcanza al suelo.
+El telescopio es un instrumento óptico que comienza con un gran espejo primario que mira al cielo, hasta detectores
+de imágenes que convierten la luz recibida por cada uno de sus píxeles en una señal eléctrica.
!
!