There are several good sites on the web (sorry that I don't have any URLs at hand to give to you but a quick search on Google will find them) that describe the correct way to clean optics.
I will note that compressed air really isn't a good idea (the output from a typical air compressor) as the force of the blast will tend to make the dirt bounce along the surface and make scratches and dings that will actually make the surface worse than before. Light blasts from cans that are used for cleaning optics for the camera trade are marginal at best ss they also tend to have a higher than desired pressure.

• personal experience on cleaning - sellors - June 24, 2003 - 15:59 UTC
  • Cleaning optics - bobmay - June 24, 2003 - 20:02 UTC