Crew members managed to change one from panels installed in one from seven windows in the Cupola International Space Station module.
Cupola module remains attached to nadir-facing port of Tranquility ISS module since 2010. It is primary observation module of International Space Station mainly used for operating Canadarm2 robotic arm like undocking Dragon cargo spacecraft, controlling docking and undocking of Soyuz spacecrafts, taking pictures of Earth and performing various scientific experiments. Cupola is basically observatory connected to number of subsystems of ISS and remaining one of the most important places inside ISS. It is equipped with seven windows each with individual cover which can be closed to protect windows from micrometeoroids. Central window is 80 cm in diameter and is surrounded with six trapezoidal shaped windows. Each window is designed as modular construction. Two main glasses (thick for 25 mm) are covered with thinner panels - from the outside to protect from micrometeoroids and from inside of the module to protect from direct hits or scratches. Glass utilized for construction of the windows is fused silica and borosilicate glass, individual outer covers are made from Kevlar and Nextel.
On April 15 crew members managed to replace inner panel of largest central window in Cupola. Panel showed no damage and there was no risk of decompression, but during years of service it just accumulated number of scratches. It is one of the most used window in Cupola, most pictures are taken through it and due the low gravity it is hard to avoid scratching panel with outer trim of the lens of the camera. Newly installed panel will surely help in getting better quality on images taken from Cupola and also improve comfort of observation - it is worth to remind, that sunlight in space is multiple times stronger and even small scratches can generate flashes and blinks.