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Goals:To monitor reflectivity and scattering performance of the HET primary mirror segments.Equipment and Conditions Required:- µScan Scatterometer and associated hardware and software- Aluminum and Silver reference mirrors - Safety harness with two lanyards - Radio - Backpack - Neck strap for scatterometer unit Frequency:Monthly (the day before and immediately after CO2 Cleaning – See Primary Mirror CO2 Cleaning procedure)Time Required:Approximately one hour each for pre and post cleaning measurements.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.Take six reflectivity measurements of both the aluminum and silver reference mirrors. Climb along the perimeter of the truss, and gain access to mirror segments 1, 6, 10, 14, 81, 84, 85, 86, 89 and 90. Take 10 measurements for each segment in the following approximate locations: - Six readings around the outer perimeter of each segment - Three readings in the interior area of each segment - One reading in the center of each segment 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 primary mirror array shall be reflectivity tested the day before (e.g. pre-cleaning data) and then again the same day as it is cleaned (e.g. post-cleaning data).- 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. - Mirrors 1, 6, 10, 14, 89 and 90 were selected because they were measured in June ’99. - Mirrors 81, 84, 85 and 86 were selected because they are at the top of the array and therefore lie more nearly parallel to the gravity vector. - 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 segment. Combine the 100 data points from all 10 mirrors and report the mean, standard deviation, maximum and minimum values for the pre and 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 |