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

2. Contents • 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 • The Red Horizon • Analysis of the spectrum of the Red Rectangle nebula III.1 Broad-band analysis III.2 Fine structure: the Red Rectangle bands • Diffuse Interstellar Bands • Conclusion

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

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

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

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

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

9. 30 ’’ 10’’

10. II The Red Horizon

11. Å 6000 5500 5000 4700 7000 Rayleigh extinction ozone Why is the horizon red? Spectrum of a red horizon at sunrise (Observed spectrum)/(spectrum of the sun) Fit: Cte x exp(-2.3 1020oz) x exp(-b/4)/4

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

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

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

15. 30 ’’ 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)

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

17. Ground-based spectrum of galaxy UGC 11917

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

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

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

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

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

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

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

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

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

27. IV. Diffuse Interstellar Bands (DIBS)

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

29. The 6200 Å Diffuse Interstellar Band

30. Interstellar cloud Atmosphere Telescope

31. 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 V. CONCLUSION

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