Next train to Meudon
Next train from Montparnasse to Meudon: http://www.transilien.com/gare/pagegare/filterListeTrains?codeTR3A=PMP&destination=MEUDON&ligne=&nomGare=GARE+DE+PARIS+MONTPARNASSE&x=39&y=14
Next train from Saint Michel Notre Dame (RER C) to Meudon Val Fleury: http://www.transilien.com/gare/pagegare/filterListeTrains?codeTR3A=SHL&destination=MEUDON+VAL+FLEURY&ligne=&nomGare=SAINT-MICHEL+NOTRE+DAME&x=22&y=14
Once arrived to a Meudon station, for the direction to take, remember that an observatory is always at the top of the hill. Just choose the correct hill.