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Goals:To monitor reflectivity and scattering performance of the HET SAC mirrors.Equipment and Conditions Required:- µScan Scatterometer and associated hardware and software- Aluminum and Silver reference mirrors - JLG with either small or large basket - JLG safety harnesses for two people - Tracker tie-down strap - Radio - Backpack - Neck strap for scatterometer unit Frequency:Monthly (immediately after CO2 Cleaning – see SAC Mirror CO2 Cleaning procedure)Time Required:Approximately one hour.Responsible Party:Optical TechProcedure:Secure a neck strap to the µScan Scatterometer. Place scatterometer in the dome thirty minutes prior taking data to taking data to allow the device to reach thermal equilibrium.Position the structure facing south and the rho ring at 0 degrees. Take six reflectivity measurements of both the aluminum and silver reference mirrors. Drive the JLG to the tracker and secure with tie-down strap. Drive the JLG basket to a position underneath the PFIP. Unzip the light baffle for M-2 and M-3, and remove tie-wraps as needed to gain access to the mirrors. Take 10 reflectivity measurements around the surface for each mirror. Zip up the baffle and install new tie-wraps. Climb up and unzip the light baffle for M-4 and M-5. Remove tie-wraps as needed to gain access to the mirrors. Take 10 reflectivity measurements around the surface for of M-4. Gently move the moving baffle to its limiting position on one side and take five measurements for M-5. Move the moving baffle to the extreme position on the opposite side and take five additional measurements of M-5. Carefully re-center the moving baffle. Zip up the light baffle and install new tie-wraps. Remove the tracker tie-down strap and drive the JLG to the floor. Take another six reflectivity measurements of both the aluminum and silver reference mirrors. Safety Precautions:All personnel shall observe HET safety precautions as described in the HET Safety Document.Notes:- The SAC shall be reflectivity tested the same day as it is cleaned (e.g. post-cleaning data).- The µScan Scatterometer is a surface contact device; therefore, in order to minimize damage to the SAC mirror coatings, reflectivity measurements shall only be taken after the SAC is cleaned (e.g. no pre-cleaning measurements will be taken). - Be extremely careful not to drag the scatterometer on the surface of a mirror while taking data. Lateral movement of the scatterometer while in contact with a mirror may result in scratching of the mirror coating. - The aluminum and silver reference mirrors are measured both before and after the HET segments for calibration of the scatterometer reflectivity values. - Download all data onto a computer. Use the reference mirror measurements to calibrate the measured reflectivity of each SAC mirror. Report the mean, standard deviation, maximum and minimum values for each of the four SAC mirrors (in the post-cleaned condition) for the following: Calibrated Reflectivity History:This procedure was first written up on 22 October 2002 by Richard Schuler.Return to Policy & Procedures |
Created: 2-May-2002 Send comments to: webmaster@het.as.utexas.edu |