Abstract

A near-infrared, two-beam interferometer has been built for astronomical observations by Fourier transform spectroscopy. Various improvements, especially a highly accurate interferometrically controlled stepping drive, have resulted in the production of laboratory spectra with 0.1-cm<sup>-1</sup> resolution and unusually clean instrumental line shape, and spectra of Venus and Mars with about 1-cm<sup>-1</sup> resolution.

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  1. P. Fellgett, thesis, Cambridge 1951, and J. Physique (France) 19, 187 (1958).
  2. P. Jacquinot, 17e Congrès GAMS 25 (1954).
  3. P. L. Richards, J. Opt. Soc. Am. 54, 1474 (1964) gives a complete review of the situation together with the most illustrative results published so far.
  4. J. Connes and P. Gush, J. Physique (France) 20, 915 (1959), and 21, 645 (1960).
  5. E. Archbold and H. A. Gebbie, Proc. Phys. Soc. 80, 793 (1962).
  6. L. Mertz, Mem. Soc. Roy. Sci. de Liège 9, 120 (1964).
  7. H. C. Bowers, Appl. Opt. 3, 627 (1964); N. J. Woolf, Appl. Opt. 3, 1195 (1964); J. F. James, J. Quant. Spectry. Radiative Transfer 4, 793 (1964).
  8. H. A. Gebbie, G. Roland, and L. Delbouille, Monthly Notices Roy. Astron. Soc. 123, 497 (1962); Astron. J. 69, 334 (1964).
  9. W. M. Sinton, J. Quant. Spectry. Radiative Transfer 2 (1964).
  10. G. Kuiper, Comm. Lun. and Plan. Lab., Univ. of Arizona 1, 83 (1962).
  11. J. Connes, Rev. Opt. 40, 45, 116, 171, 231 (1961).
  12. Another method of estimating noise is of course to compare spectra computed from different interferograms.
  13. P. L. Richards (Ref. 3) has recently built a stepping drive for his far-infrared interferometer and has used it with very good success, for reasons which are comparable to our own. The main differences are that he does not use true integration nor interferometric control. L. Delbouille at the Institut d'Astrophysique Université de Liège, has also been independently developing a stepping drive.
  14. J. Pinard, Echantillonnage des interferogrammes en spectroscopie par transformation de Fourier—These de 3eme cycle, Université de Paris, 1963—Preliminary results were also briefly described by J. Connes at the Liège Colloquium on Infrared Astronomical Spectra, June 1963 [Mem. Soc. Roy. Sci. de Liège 9, 81 (1964)].
  15. While the interferometer itself remained basically unchanged throughout the experiments, the fore optics suffered considerable modifications; only the latest version need be fully described here.
  16. This is by far the greatest part of the time, if we can believe most astronomers.
  17. The automatic guider was not available during the early part of the experiments (Mount Wilson and Kitt Peak).
  18. In fact, this system was used only for Mars at Observatoire de Saint Michel and the dimensions were then Φ = 3 mm, l = 100 mm. For Venus and Jupiter at Mount Wilson and Kitt Peak a different device was tried, because it could be improvised more quickly: a rapidly rotating dove prism was placed right after the coudé focus and made the whole beam rotate at 300 revolutions/sec. The results were about as good as those with the light pipe, but the system was very noisy and cumbersome; moreover, since the speed of revolution was not very stable it was difficult to avoid beats with harmonics of the chopper frequency.
  19. L. Mertz, J. Phys. (France) 1, 233 (1958).
  20. M. Cuisenier, Interferomètre de Michelson pour spectroscopic par transformation de Fourier, These de 3eme cycle, Université de Paris, 1964.
  21. We have also made a CaF2 beam splitter with Ge coating for use at longer wavelengths. Since the absorption of Ge would be too large in the visible, the quadrants corresponding to C and D are covered with a TiO layer.
  22. These were cut from a single paraboloid of 20-mm focal length and 120-mm aperture by the Atelier de 1'Institut d'Optique Paris under the direction of J. Demarcq and were found, even when used at full aperture, to give an image definition better than 0.1 mm.
  23. On the other hand, balancing does not depend on the sensitivity of the receivers and it is not necessary to match them.
  24. 198Hg with high-frequency excitation is used for D>5 cm.
  25. Sanborn Company, Waltham, Massachusetts.
  26. Prior to the speed servo, an oil damper was used to stabilize the velocity. It gave reasonably good results, but the speed was still subject to large, slow variations and had to be readjusted by hand control; also, transportation of the instrument was made difficult. On the other hand, it gave better damping of high-frequency external vibrations.
  27. Bell Telephone Company.
  28. Even airplane transmission is not error proof. The tapes sometimes went to London.
  29. Several reasons for preferring pure cosine transforms have been given earlier (Ref. 11, p. 133): Optimum signal-to-noise ratio and perfect linearity in the computed spectrum; reduction in the scanning length and the computing time. Another important reason is given by L. Delbouille, G. Roland, H. A. Gebbie [Mem. Soc. Roy. Sci. de Liège 9, 125 (1964)]; it is the only method which does not rectify negative portions of the spectrum. It has, however, the disadvantage that an extremely accurate value of e is needed.
  30. The unapodized profile from the same interferogram shows a resolving power R0 = 105.
  31. For this reason apodization is never used with gratings. However, it has been shown [P. Jacquinot and B. Roizen Dossier, in Progress in Optics III, edited by E. Wolf (North-Holland Publishing Co., Amsterdam, 1964), p. 31.], that it is actually possible to improve the diffraction pattern of an extremely good grating. But the device used for the demonstration, a diamond-shaped diaphragm, would be inconvenient in any practical spectrometer or spectrograph because it needs a point source. Also, the making of an apodizing screen of the size of a large modern grating would no doubt be a delicate operation.
  32. To a physicist turned astronomer some of the environmental difficulties involved appeared rather more severe than anticipated. For instance, the usually reliable servo once refused to operate. A check of the electrical, then of the optical system, was made, and revealed the unexpected combination of a "cat's eye" with a mouse's nest inside. On a different occasion, the paper punch appeared to be making a slightly irregular noise; upon investigation a rattlesnake was found to be responsible.
  33. We thank W. E. Mitchell for the loan of this record which shows increased resolving power when compared with the Mc-Math-Hulbert—University of Michigan atlas. The latest improvements in the instrument are described by W. E. Mitchell and O. C. Mohler, Appl. Opt. 3, 467 (1964).
  34. For instance, the PbS cells were then much too large, because the system had originally been designed with the hope of using it with a bigger telescope.
  35. Defined by taking the center of the half-width on special plots, using 50 spectral points per resolution width. Since the resolving power is not large enough to separate these lines completely, their profiles are slightly asymmetrical and there would be no point in giving the absolute positions.
  36. G. P. Kuiper, Mem. Soc. Roy. Sci. Liège 9, 365 (1964).
  37. Furthermore, the 1965 opposition was not a near one; the energy available in 1971 should be larger by a factor of 3.

Archbold, E.

E. Archbold and H. A. Gebbie, Proc. Phys. Soc. 80, 793 (1962).

Bowers, H. C.

H. C. Bowers, Appl. Opt. 3, 627 (1964); N. J. Woolf, Appl. Opt. 3, 1195 (1964); J. F. James, J. Quant. Spectry. Radiative Transfer 4, 793 (1964).

Connes, J.

J. Connes and P. Gush, J. Physique (France) 20, 915 (1959), and 21, 645 (1960).

J. Connes, Rev. Opt. 40, 45, 116, 171, 231 (1961).

Cuisenier, M.

M. Cuisenier, Interferomètre de Michelson pour spectroscopic par transformation de Fourier, These de 3eme cycle, Université de Paris, 1964.

Delbouille, L.

H. A. Gebbie, G. Roland, and L. Delbouille, Monthly Notices Roy. Astron. Soc. 123, 497 (1962); Astron. J. 69, 334 (1964).

Fellgett, P.

P. Fellgett, thesis, Cambridge 1951, and J. Physique (France) 19, 187 (1958).

Gebbie, H. A.

H. A. Gebbie, G. Roland, and L. Delbouille, Monthly Notices Roy. Astron. Soc. 123, 497 (1962); Astron. J. 69, 334 (1964).

E. Archbold and H. A. Gebbie, Proc. Phys. Soc. 80, 793 (1962).

Gush, P.

J. Connes and P. Gush, J. Physique (France) 20, 915 (1959), and 21, 645 (1960).

Jacquinot, P.

P. Jacquinot, 17e Congrès GAMS 25 (1954).

Kuiper, G.

G. Kuiper, Comm. Lun. and Plan. Lab., Univ. of Arizona 1, 83 (1962).

Kuiper, G. P.

G. P. Kuiper, Mem. Soc. Roy. Sci. Liège 9, 365 (1964).

Mertz, L.

L. Mertz, Mem. Soc. Roy. Sci. de Liège 9, 120 (1964).

L. Mertz, J. Phys. (France) 1, 233 (1958).

Pinard, J.

J. Pinard, Echantillonnage des interferogrammes en spectroscopie par transformation de Fourier—These de 3eme cycle, Université de Paris, 1963—Preliminary results were also briefly described by J. Connes at the Liège Colloquium on Infrared Astronomical Spectra, June 1963 [Mem. Soc. Roy. Sci. de Liège 9, 81 (1964)].

Richards, P. L.

P. L. Richards (Ref. 3) has recently built a stepping drive for his far-infrared interferometer and has used it with very good success, for reasons which are comparable to our own. The main differences are that he does not use true integration nor interferometric control. L. Delbouille at the Institut d'Astrophysique Université de Liège, has also been independently developing a stepping drive.

P. L. Richards, J. Opt. Soc. Am. 54, 1474 (1964) gives a complete review of the situation together with the most illustrative results published so far.

Roland, G.

H. A. Gebbie, G. Roland, and L. Delbouille, Monthly Notices Roy. Astron. Soc. 123, 497 (1962); Astron. J. 69, 334 (1964).

Sinton, W. M.

W. M. Sinton, J. Quant. Spectry. Radiative Transfer 2 (1964).

Other (37)

P. Fellgett, thesis, Cambridge 1951, and J. Physique (France) 19, 187 (1958).

P. Jacquinot, 17e Congrès GAMS 25 (1954).

P. L. Richards, J. Opt. Soc. Am. 54, 1474 (1964) gives a complete review of the situation together with the most illustrative results published so far.

J. Connes and P. Gush, J. Physique (France) 20, 915 (1959), and 21, 645 (1960).

E. Archbold and H. A. Gebbie, Proc. Phys. Soc. 80, 793 (1962).

L. Mertz, Mem. Soc. Roy. Sci. de Liège 9, 120 (1964).

H. C. Bowers, Appl. Opt. 3, 627 (1964); N. J. Woolf, Appl. Opt. 3, 1195 (1964); J. F. James, J. Quant. Spectry. Radiative Transfer 4, 793 (1964).

H. A. Gebbie, G. Roland, and L. Delbouille, Monthly Notices Roy. Astron. Soc. 123, 497 (1962); Astron. J. 69, 334 (1964).

W. M. Sinton, J. Quant. Spectry. Radiative Transfer 2 (1964).

G. Kuiper, Comm. Lun. and Plan. Lab., Univ. of Arizona 1, 83 (1962).

J. Connes, Rev. Opt. 40, 45, 116, 171, 231 (1961).

Another method of estimating noise is of course to compare spectra computed from different interferograms.

P. L. Richards (Ref. 3) has recently built a stepping drive for his far-infrared interferometer and has used it with very good success, for reasons which are comparable to our own. The main differences are that he does not use true integration nor interferometric control. L. Delbouille at the Institut d'Astrophysique Université de Liège, has also been independently developing a stepping drive.

J. Pinard, Echantillonnage des interferogrammes en spectroscopie par transformation de Fourier—These de 3eme cycle, Université de Paris, 1963—Preliminary results were also briefly described by J. Connes at the Liège Colloquium on Infrared Astronomical Spectra, June 1963 [Mem. Soc. Roy. Sci. de Liège 9, 81 (1964)].

While the interferometer itself remained basically unchanged throughout the experiments, the fore optics suffered considerable modifications; only the latest version need be fully described here.

This is by far the greatest part of the time, if we can believe most astronomers.

The automatic guider was not available during the early part of the experiments (Mount Wilson and Kitt Peak).

In fact, this system was used only for Mars at Observatoire de Saint Michel and the dimensions were then Φ = 3 mm, l = 100 mm. For Venus and Jupiter at Mount Wilson and Kitt Peak a different device was tried, because it could be improvised more quickly: a rapidly rotating dove prism was placed right after the coudé focus and made the whole beam rotate at 300 revolutions/sec. The results were about as good as those with the light pipe, but the system was very noisy and cumbersome; moreover, since the speed of revolution was not very stable it was difficult to avoid beats with harmonics of the chopper frequency.

L. Mertz, J. Phys. (France) 1, 233 (1958).

M. Cuisenier, Interferomètre de Michelson pour spectroscopic par transformation de Fourier, These de 3eme cycle, Université de Paris, 1964.

We have also made a CaF2 beam splitter with Ge coating for use at longer wavelengths. Since the absorption of Ge would be too large in the visible, the quadrants corresponding to C and D are covered with a TiO layer.

These were cut from a single paraboloid of 20-mm focal length and 120-mm aperture by the Atelier de 1'Institut d'Optique Paris under the direction of J. Demarcq and were found, even when used at full aperture, to give an image definition better than 0.1 mm.

On the other hand, balancing does not depend on the sensitivity of the receivers and it is not necessary to match them.

198Hg with high-frequency excitation is used for D>5 cm.

Sanborn Company, Waltham, Massachusetts.

Prior to the speed servo, an oil damper was used to stabilize the velocity. It gave reasonably good results, but the speed was still subject to large, slow variations and had to be readjusted by hand control; also, transportation of the instrument was made difficult. On the other hand, it gave better damping of high-frequency external vibrations.

Bell Telephone Company.

Even airplane transmission is not error proof. The tapes sometimes went to London.

Several reasons for preferring pure cosine transforms have been given earlier (Ref. 11, p. 133): Optimum signal-to-noise ratio and perfect linearity in the computed spectrum; reduction in the scanning length and the computing time. Another important reason is given by L. Delbouille, G. Roland, H. A. Gebbie [Mem. Soc. Roy. Sci. de Liège 9, 125 (1964)]; it is the only method which does not rectify negative portions of the spectrum. It has, however, the disadvantage that an extremely accurate value of e is needed.

The unapodized profile from the same interferogram shows a resolving power R0 = 105.

For this reason apodization is never used with gratings. However, it has been shown [P. Jacquinot and B. Roizen Dossier, in Progress in Optics III, edited by E. Wolf (North-Holland Publishing Co., Amsterdam, 1964), p. 31.], that it is actually possible to improve the diffraction pattern of an extremely good grating. But the device used for the demonstration, a diamond-shaped diaphragm, would be inconvenient in any practical spectrometer or spectrograph because it needs a point source. Also, the making of an apodizing screen of the size of a large modern grating would no doubt be a delicate operation.

To a physicist turned astronomer some of the environmental difficulties involved appeared rather more severe than anticipated. For instance, the usually reliable servo once refused to operate. A check of the electrical, then of the optical system, was made, and revealed the unexpected combination of a "cat's eye" with a mouse's nest inside. On a different occasion, the paper punch appeared to be making a slightly irregular noise; upon investigation a rattlesnake was found to be responsible.

We thank W. E. Mitchell for the loan of this record which shows increased resolving power when compared with the Mc-Math-Hulbert—University of Michigan atlas. The latest improvements in the instrument are described by W. E. Mitchell and O. C. Mohler, Appl. Opt. 3, 467 (1964).

For instance, the PbS cells were then much too large, because the system had originally been designed with the hope of using it with a bigger telescope.

Defined by taking the center of the half-width on special plots, using 50 spectral points per resolution width. Since the resolving power is not large enough to separate these lines completely, their profiles are slightly asymmetrical and there would be no point in giving the absolute positions.

G. P. Kuiper, Mem. Soc. Roy. Sci. Liège 9, 365 (1964).

Furthermore, the 1965 opposition was not a near one; the energy available in 1971 should be larger by a factor of 3.

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