1. 1 Non-linear Effects of Atmospheric Extinction on Observations in Astronomy Frédéric Zagury Institut Louis de Broglie Paris - France

2. 2 Contents I.Modifications of the night-sky spectrum in the vicinity of a star I.1 The night-sky spectrum I.2 Diffracted and scattered starlight I.3 Intermediate distances II.The Red Horizon III.Analysis of the spectrum of the Red Rectangle nebula III.1 Broad-band analysis III.2 Fine structure: the Red Rectangle bands IV.Diffuse Interstellar Bands V.Conclusion

3. 3 I. Modifications of the night-sky spectrum in the vicinity of a star

4. 4 5700-6000 Å region Night-sky spectrum at M t Wilson, USA I.1 The night-sky spectrum  > a few arc-minute (3 10 -4 rd) Massey et al., 1990, PASP, 112, 566 Massey & Foltz, 2000, PASP, 102, 1046

5. 5 Night skies at La Silla-Paranal and Mount Wilson Spectrum of the night sky at Mount Wilson (USA) Spectrum of the night sky at La Silla-Paranal (Chili)

6. 6 I.2 Diffracted and scattered starlight  < a few arcsecond (5 10 -6 rd) 5’’ 10’’

7. 7 I.3 Spectra at intermediate distances from the star (  ~ a few 10’’) 10’’

8. 8 The Red Rectangle nebula 30 ’’ HST image of the RR nebula with the WFPC2 camera

9. 9 30 ’’ 10’’

10. 10 II The Red Horizon

11. 11 Why is the horizon red? Spectrum of a red horizon at sunrise (Observed spectrum)/(spectrum of the sun) Fit: Cte x exp(-2.3 10 20  oz ) x exp(-b/ 4 )/ 4 Rayleigh extinction ozone 7000 600050005500 Å 4700

12. 12 III. Analysis of the spectrum of the Red Rectangle nebula

13. 13 The spectrum of the Red Rectangle nebula  = 5’’ 30 ’’  > 10’’

14. 14 Spectrum of the Red Rectangle nebula close to HD44179 (  = 5’’) nebula Diffracted light from HD44179 30 ’’

15. 15 Spectrum of the Red Rectangle nebula away from HD44179 (  > 10’’) nebula Diffracted light from HD44179 Nebula (  = 14’’) Background (x 6) Scattered starlight (1/ x HD44179) 30 ’’

16. 16 Red slope of the Red Rectangle nebula spectrum Nebula/(non reddened A0 star) 1/ 4

17. 17 Ground-based spectrum of galaxy UGC 11917

18. 18 Atmosphere Telescope star galaxy, nebula ozone Rayleigh ozone Rayleigh Non-linearity of atmospheric extinction

19. 19 Fine structure of the Red Rectangle spectrum: the Red Rectangle bands Spectrum of the nebula Background x 7

20. 20 Comparison of the Red Rectangle and background spectra in the 5800 Å region

21. 21 Comparison of the Red Rectangle and the background spectra in the 6100 Å region

22. 22 Comparison of the Red Rectangle and the background spectra in the 6200 Å region

23. 23 Temptative identification of the carriers for the atmospheric absorption in the Red Rectangle nebula - 1 Sun occultation (SAOZ experiment) Red Rectangle spectrum

24. 24 Temptative identification of the carriers for the atmospheric absorption in the Red Rectangle nebula - 2

25. 25 Temptative identification of the carriers for the atmospheric absorption in the Red Rectangle nebula - 3

26. 26 Temptative identification of the carriers for the atmospheric absorption in the Red Rectangle nebula - 3

27. 27 IV. Diffuse Interstellar Bands (DIBS)

28. 28 Telescope Interstellar cloud From Jenniskens & Desert, 1994, A&ASS, 106, 39 Star

29. 29 The 6200 Å Diffuse Interstellar Band

30. 30 Atmosphere Interstellar cloud Telescope

31. 31 V. CONCLUSION 1. Consequences of the observed non- linearity of atmospheric extinction - Need for a review of the data reduction process used in Astronomy -The atmosphere may impose the most serious limits to future deep observations of the Universe from Earth

32. 32 CONCLUSION 2. Synergies between Astronomy and studies of the atmosphere 3. DIBs and Red Rectangle bands: A need for observations with HST