European Southern Observatory
LIN - Optics
With the progress of the detector technology as well as the telescope optical quality, the star image decrease in size and all "secondary" effects will reduce the performance of the telescope.
Dust influence on telescope PSF is expected as a widening of the PSF function on the wing background by diffusion. Of course the reflectivity is also decreasing. For IR observations the dust increase the background emissivity reducing the telescope sensitivity.
A regular cleaning operation will reduce this critical effects degrading the telescope performances.
The methodology applied must be a simple and fast operation to be repeated as soon is required to maintain the optical surface performances.
E.S.O has been testing several technics within the past ten years to finally select the best operation by projecting CO2 snow flake on an inclined mirror surface. The CO2 particles trap and drag the dust by gravity out the mirror surface. More often the cleaning is performed and less is the dust particle adherence. Of course this method has the inconvenient of the CO2 snow low temperature increasing the risk of humidity condensation. For high humidity conditions (H%>60%) the snow/dust particles will stick on the surface producing small stains.
The experience of several year cleaning shows the importance of the periodicity which in case of the NTT seems optimal on a week basis.
After longer period of high humidity, specially during the Chilean summer period, the mirror surface is not restored with the CO2 cleaning. A deeper operation of washing is required. Small dust particle adherence is high and can be only overcome by mechanical intervention perfomed with wet sponge patting operation. Only one telescope on la Silla has been designed to perform an on line washing with the mirror still mounted within its cell. The NTT main mirror is now regularly washed once the reflectivity survey gave a value below 88%. This operation enlarged the aluminization period to a total of 3 years with a telescope efficiency close the optimum.
The 3m60 main mirror washing is also performed with more difficulties to ensure the watertight.
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Last Update: March 29, 2000