Ravensburger Escape Room Puzzle - The Observatory, 759st.

Everyone knows escape rooms - and now there is also Escape the puzzle!

Puzzle - Solve the riddles - Find the solution
The mysterious illustration immerses puzzlers and players into a mysterious world where an "exit" can be found! Contents: a puzzle with 759 mysterious pieces, a manual and an example of the solution.

Please note: puzzle motif differs from the motif of the box.

The observatory

During an evening walk you discover a strange light in the sky. Out of curiosity, you go to the nearby library and look for information in the Astronomy department. While you pick up a book with the title 'we discover space', a letter falls at your feet.
On the sheet it is written:
My name is Professor Dr. Fresenius Gamomeus,
I have been studying the stars and space for years. A few weeks ago I discovered a strange object in the sky. I have calculated its orbit and concluded that it is coming directly to Earth! I decided to design a laser to destroy the object. Unfortunately, this couldn't be done legally in such a short time, and now I'm afraid the Secret Service will get me. In case I get arrested, I have the necessary parts hidden in my lab.
Hopefully my message will be found in time, the laser assembled and used to save the world!
Yours faithfully,
Prof. Dr. F. Gamomäus

After reading the letter again, you look up in disbelief and discover a door. You open the door and follow the stairs upstairs...