Leiden can be easily accessed from Schiphol International Airport by train. There are 2 to 4 trains leaving each hour that take about 20-25 minutes to Leiden. From Leiden Central Station it is a 20 minutes walk to Leiden Observatory in the Huygens Laboratory, or alternatively you can take a bus (Line 57, two times / hour, get out at stop Wassenaarseweg/Corpus). The Science buildings are visible from the stop (cross over the parking spot).
Please note that you do NOT walk to the Old Observatory downtown. This is a great building at a beautiful location, but not the place where you will find us !