SAO/NASA JOINT INVESTIGATION OF ASTRONOMICAL VIEWING QUALITY AT MT. HOPKINS -&- OBSERVATORY: 1969-1971 0:, C - n c M. R. PEARLMAN, J. L. BUFTON, D. HOGAN, " " 01 r D. KURTENBACH, and K. GOODWIN - ;.,4s ** * * . - t .o 11- - -*, * *. is. C1 L rs .. .L . , -. 0ta . - .'. ". " . .... .... ' -Smithsonian Astrophysical Observatory TABLE OF CONTENTS Page ABSTRACT ...... .... ................... ........... ix 1 INTRODUCTION...................................... 1 2 THEORETICAL BACKGROUND AND EXPERIMENT RATIONALE .... 3 2. 1 Relationship of Atmospheric Turbulence to Optical Effects ..... 3 2.2 Scintillation ........................... .......... 7 2.3 Image Motion ................................... 10 2.4 Image Blurring........ ............... .......... 11 2. 5 Turbulence Profiles and Relationship of Meteorology to Turbu- lence Structure ........ ......_ ............. .......... 13 3 EXPERIMENT APPARATUS AND SITE LOCATION ............. 23 3.1 Stellar-Image Monitor System ............ .. .......... 23 3.2 Temperature-Fluctuation Measurement System.............. 26 3.3 Temperature-Difference Measurement System .............. 27 3.4 Radiosonde ................... ............... 27 3. 5 Mt. Hopkins Observatory Site and Location of Experiment Equip- ment ........................ .................. 28 4 IMAGE-QUALITY MEASUREMENTS ...................... 33 4.1 Introduction .................................... 33 4.2 Image Motion ............................ ......... 34 4.3 Scintillation ................................... 45 5 CORRELATION OF IMAGE QUALITY WITH METEOROLOGICAL PARAMETERS. ......................... .... ....... 57 5. 1 Image Quality vs. Local Meteorological Conditions .......... 57 5.2 Image Motion vs. Temperature Fluctuations. ............ 57 5.3 Image Motion vs. Temperature Gradient ................. 60 5.4 Scintillation Spectral Density vs. High-Altitude Winds ........ 64 5. 5 Image Motion vs. Temperature and Wind Profiles. .......... 66 PPECEDING PAGE BLANK NOT FILMED TABLE OF CONTENTS (Cont.) Page 6 COMMENTS AND APPLICATIONS........................... 69 6. 1 Comments on the Seeing Conditions at the Mt. Hopkins Site.... 69 6.2 Comments on Site Testing ...................... .... 71 6. 3 Direct Detection of Pulsed Laser Radiation . .............. 73 6.4 Heterodyne Detection of Continuous-Wave Laser Radiation ..... 76 7 ACKNOWLEDGMENTS ................................. 79 8 REFERENCES...................................... 81 APPENDIX A: COLLECTION AND REDUCTION OF SIM DATA A-i APPENDIX B: DATA B-1 APPENDIX C: COMMENTS ON CORRELATION ANALYSIS C-i iv ILLUSTRATIONS Page 1 Model of turbulent profile (Hufnagel, 1966b) .................. 15 2 The dimensionless temperature-structure parameter f 3 (Ri) versus the Richardson number R. (Wyngaard et al., 1971) ............... 20 1 3 Optical system and detector package for the stellar-image monitor. .. 24 4 Mt. Hopkins ridge and peak areas shown with access road. ........ 29 5 View of knoll 2 from the south ............................ 30 6 Detailed diagram of the ridge area showing the location of the equip- ment .................. ................... ....... 30 7 Cumulative probability distribution of one-dimensional image motion for short-term averages during Phase I. The values have been cor- rected to zenith ..................................... 36 8 Cumulative probability distribution of one-dimensional image motion for nightly averages during Phase I. The values have been corrected to zenith .......................................... 37 9 Three examples of short-term variations in ,mfrom Phase II data corrected to zenith ................................... 38 10 Cumulative probability distribution of one-dimensional image motion for short-term averages during Phase II. The values have been cor- rected to zenith ..................................... 40 11 Cumulative probability distribution of one-dimensional image motion for long-term averages during Phase II. The values have been cor- rected to zenith ..................................... 41 12 Experimental dependence of T, on zenith angle. The sec 1/2() line is intended to show the anticipated trend. Its intercept was chosen for convenience only .................................. 42 13 Typical image-motion spectral density with 8-Hz noise component introduced by the SIM equipment ......................... 44 14 Cumulative probability distribution of CIV for short-term averages during Phase I. The values have been corrected to zenith ......... 46 15 Cumulative probability distribution of CIV for long-term averages during Phase I. The values have been corrected to zenith. ........ 47 16 Two examples of short-term variations in CIV. The values have been corrected to zenith ................................... 49 17 Cumulative probability distribution of CIV for short-term averages during Phase II. The values have been corrected to zenith ........ 51 v ILLUSTRATIONS (Cont.) Page 18 Cumulative probability distribution of CIV for long-term averages during Phase II. The values have been corrected to zenith ........ 52 19 Experimental dependence of CIV on zenith angle. The secl14 (6) line is intended to show the anticipated trend. Its intercept was chosen for convenience only.................................. 54 20 Three examples of the normalized spectral density for CIV showing noise contribution at 8 Hz from the SIM equipment .............. 55 21 Long-term-averaged image-motion and peak temperature fluctuation as a function of observation period during Phase II. The data from observation periods 6 and 27 were not available owing to equipment failure ........................................... 59 22 Long-term-averaged image-motion and temperature differences as a function of observation period during Phase II................. 61 23 Probability of error in the presence of log-normal fading (Titterton and Speck, 1973) .................................... 75 C1 Example of data distribution with corresponding correlation coeffi- cients ............................................ C-4 C2 Example of data distribution with corresponding correlation coeffi- cients ........................................... C-5 C3 Example of data distribution with corresponding correlation coeffi- cients ............................................ C-6 vi TABLES Page 1 Summary of rms one-dimensional image-motion statistics in Phase I..... ....................................... .. 34 2 Summary of rms one-dimensional image-motion statistics in Phase II ............................................. 43 3 Summary of scintillation statistics for Phase I .............. 48 4 Summary of scintillation statistics for Phase II............... 50 5 Correlation of rms image motion with average peak temperature fluctuation in Phase II .................................. 58 6 Correlation of rms image motion with temperature difference (AT) in Phase II ............................................. 63 7 Correlation of scintillation half-width frequency with wind speed/ correlation distance ratio (Phase II) ............... ....... 65 8 Scintillation: Power spectrum components vs. high-altitude winds ......................................... 66 vii ABSTRACT Quantitative measurements of the astronomical seeing conditions have been made with a stellar-image monitor system at the Mt. Hopkins Observatory in Arizona. The results of this joint SAO-NASA experiment indicate that for a 15-cm-diameter telescope, image motion is typically 1 arcsec or less and that intensity fluctuations due to scintillation have a coefficient of irradiance variance of less than 0. 12 on the average. Correlations between seeing quality and local meteorological conditions are investigated. Local temperature fluctuations and temperature gradients were found to be indicators of image-motion conditions, while high-altitude-wind conditions were shown to be somewhat correlated with scintillation-spectrum bandwidth. The theoretical basis for the relationship of atmospheric turbulence to optical effects is discussed in some detail, along with a description of the equipment used in the experiment. General site-testing comments and applications of the seeing-test results are also included. PRECEDING PAGE BLANK NOT FILMIFa ix RESUME Les mesures quantitatives des conditions astronomiques de visibilit6 ont td prises ' l'observatoire du Mont Hopkins, en Arizona, A l'aide d'un systhme de contrsle d'image stellaire. Les r6sultats de cette experience commune du SAO et de la NASA indiquent que, pour un t6l6scope de 15 cm de diambtre, le mouve- ment typique de I'image est de 1 seconde d'arc maximum et que les fluctuations d'intensite dues ' la scintillation ont un coefficient de variation d'irradience inferieur, en moyenne, ' 0,12. On 6tudie les corr1lations qui existent entre la qualit6 de visibilit6 et les conditions metiorologiques locales. On a trouv6 que les fluctuations de temperature locale et les gradients de temp&rature indiquaient les conditions de mouvement de l'image, alors que les conditions de vent en haute altitude 6taient, dans une certaine mesure, correlatifs de la largeur de bande du spectre de scintillation. On discute, de faon assez detaill6e, le fondement theorique du rapport entre la turbulence atmospherique et les effets optiques, et l'on decrit le materiel utilis6 pour l'exp~rience. Sont 4galement inclus, des commentaires generaux relatifs aux essais sur place et aux applications des r6sultats de l'essai de visibilite. X KOHCHEKT SbIRZ EPOBegeHbJ KOJIMLieCTBeH[ibie z3mepeHi-ig aCTPOHOMZqeCKHX