Transmission and opto-mechanical performance of the liquid lens coupling in the Robert Stobie Spectrograph on SALT

Liquid lens coupling provides excellent transmission efficiency when compared to multilayer coatings especially for applications where broadband transmission is required. However, long term reliability of liquid coupling is difficult to achieve. This is typically due to chemical compatibility issues affecting both the optical transmission and the integrity of the opto-mechanical support. As part of a recent service of the Robert Stobie Spectrograph on SALT we had the opportunity to study these problems further and in this paper we provide analysis of problems identified and some solutions to prevent them. We also present general guidelines which could aid future opto-mechanical designs for liquid coupling of lenses.