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Common ways to clean optics include drag wiping the surface with various cloths and solvents; blowing off the optics with ionized gaseous nitrogen (GN2); blowing off the optics with carbon dioxide (CO2) (snow gun). With these techniques the surface could be damaged if hard particles such as metallic pieces, are pushed along with the gas or wipe. Drag wiping also had the potential to smear acrylic adhesive particles and other molecular contaminates that may be present on the surface. Ionized GN2 at low velocity does not remove enough particles to make it effective at cleaning. At high velocity GN2 can damage thin light-weighted mirror substrate as well as dislodge beryllium dust from the exposed machined surface causing a potential health hazard. It also dislodges particles in a chaotic fashion potentially causing them to be forced into other sensitive regions of the optics or system. CO2 snow similarly causes all dislodged particles to move in an uncontrolled fashion. CO2 snow also significantly changes the temperature of the optic, and in the case of light-weighted space telescope optics with a thin face it can cool the surface enough to cause condensation, which in turn can leave difficult-to-remove evaporation marks on the optical surface. A new technique utilizing a fine bristle brush was developed for JWST to safely and effectively remove the majority of the particles. This technique removes particles in a controlled fashion making sure they do not migrate to other parts of the telescope, and uses very light pressure, which avoids damage to the coating from metallic particles. Not all particles are removed but the surface is left in a much cleaner state without loose particles present. Several of the individual types such as adhesive and large fibers can be removed later with a spot cleaning technique. Potential damage from the brush to the surface was evaluated utilizing several different techniques. |
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