A Fabry-Perot spectrometer for auroral observations
Nilson, John A
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Spectrometers utilizing prisms or gratings have been in use for a number of years. Interferometers such as the Fabry-Perot and Michelson are often considered as being extremely delicate and specialized instruments, not the kind of instrument one would choose for routine observation of spectra. Recently, however, special properties of the Fabry-Perot have been recognized and a very useful and versatile spectrometer designed, which puts the Fabry-Perot ahead of other spectrometers in many respects. The Fabry-Perot was invented in the nineteenth century, and used almost immediatly in wavelength measurements and standardization of the meter. The Fabry-Perot interferometer has also been used a great deal for hyperfine structure studies. It has always been recognized as being a useful instrument because of its properties of being able to convert directly from wavelength of light to a standard length and the fact that its theoretical resolution has no limit. The Fabry-Perot is a multiple beam apparatus that splits the beam up by successive reflections between two plates, at each reflection allowing some of the beams to pass through, thus making many beams interfere. The result is that the interference fringes are very sharp. The method by which a Fabry-Perot has been and still is often used to measure wavelengths and study fine structure is what will be called here the photographic method. In this method an objective lens focuses the Fabry-Perot fringes, which are at infinity, onto a photographic plate. When looking at a monochromatic source the image consists of a number of concentric circles which become closer together as one proceeds from the center. The Fabry-Perot is often crossed with a prism or grating spectrometer, because the free spectral range is small, when something other than a nearly monochromatic source is being used. These fringe patterns are quite difficult to analyse but for many years it was the best method to get such high resolution, and it is still used. The photoelectric method of recording Fabry-Perot fringes was developed by Jaequinot and Dufour (1949). The method used is to replace the photographic emulsion at the focus of the objective lens with a diaphragm that allows a portion of the pattern to go through, be collected by a lens and be focused on a photomultiplier tube. If operated in this manner it is possible, by changing the optical path length between the plates, to make the instrument scan its pass bands linearly with time over a wavelength or wavenumber interval; thus it is comparable to other spectrometers. The recorded output is the spectrum, unlike the photographic method where considerable analysis is required to get a spectrum. Other advantages of a Fabry-Perot spectrometer compared to a photographic Fabry-Perot are discussed below. A spectrum. can be obtained in a shorter period due to higher sensitivity of the photomultiplier which is especially" important in aurora because of the short life of some forms that have interesting spectral characteristics. The developing of plates and subsequent reduction to intensities by microphotometer tracings is eliminated, which is very important. This means the non linearities in emulsions are overcome, the time required to get data is reduced, and there is an increase in accuracy due to direct photoelectric methods. The objective lens does not have to be of high quality since only near-axial rays are used, and a high f/number to get high speed is not required (in fact at high resolution a lens of long focal length is advantageous). An important advantage that any spectrometer has over a spectrograph is that it is possible to see the results as the observations are being taken, and thus observing time can be used more efficiently. If rapidly moving sources are being studied perhaps a photographic method has some advantages, but if the spectrum is changing rapidly this advantage is lost. The great advantage in light gathering power for a given resolving power for a Fabry-Perot spectrometer, over prism and grating spectrometers, is clearly pointed out by Jacquinot (1954). He comparess instruments of the same effective area and resolving power and finds the grating spectrometer always will have greater light gathering power than a prism spectrometer, and a Fabry-Perot spectrometer will have as much as 30 to 400 times the light gathering power as a grating spectrometer. This shows why the Fabry-Perot can be superior even for low resolution where good light gathering power may be important. A Fabry-Perot has further advantages over a grating spectrograph in that one set of plates can be used for any wavelength region in which they will transmit, and the resolution can be adjusted to any value. This is not true for a grating spectrograph since a particular grating is designed to be most efficient in one spectral region and at one resolution. This is why the Fabry-Perot spectrometer is a very versatile instrument. In upper atmospheric observations until very recently, the Fabry-Perot has been limited to measuring wavelengths. The classic examples using the photographic method are the exact measurement of wavelength of the auroral green line by Babcock (1923), and the confirmation of the sodium D lines in the twilight by Bernard (1938). Others were the study of both 5577A and 6300A by Vegard (1937) and a study of all of these by Dufay, Cabannes and Gauzit (1942). Babcock (1923) realized that the Fabry-Perot could be used for temperature measurement even though it was not known at the time what was producing the oxygen 5577A auroral and night airglow green line. The method of obtaining temperature is called the Doppler method. The profile of the spectral line is a pure Gaussian and results from the Doppler shift arising from the the atoms random motion in thermal equilibrium. The half-width s, and absolute temperature T are related by s = 7.16 X 10-7 ÏƒâˆšT/M where Ïƒ is the wavenumber or the line in cm-1, and M is the molecular weight. To give good results when using the Doppler method a line should have narrow natural half-width, due to internal structure and transition probability. To fill the second need a forbidden transition is best because of its long time in the excited state. Another important consequence of the forbidden transition is that the atoms spend a sufficiently long time in the excited state to ensure that they will reach thermal equilibrium.. The oxygen 5577A [OI] fills these requirements since it is forbidden, having life time of about 0.7 seconds in the upper state, and is not known to have any internal structure that would broaden the line compared to the Doppler width. Another important requirement is that it be bright enough so that enough luminosity can be obtained to use high resolution. The oxygen green line is one of the most prominent features in the aurora. Babcock assigned an upper limit to the half-width of the green line of 0.035 A. Vegard (1937) tried to get a temperature for the oxygen red line (6300A), but could not obtain enough resolution with the luminosity required. Because of this there was a time lapse of almost twenty years where the idea of getting temperatures by this method was given up. With the advent of dielectric multilayers, the absorption in the reflecting layers was greatly reduced and interest was again aroused. Wark and Stone (1955), Wark (1956), (1960), and Cabannes and Duray (1955), (1956a), (1956b), resumed photographic work on the aurora and night airglow using a Fabry-Perot, and found it possible to obtain temperatures in this way. When the Fabry-Perot interferometer was investigated by Jacquinot (1954) interest was aroused in this method and some Doppler temperatures were tried again. The reason for the difficulty experienced by these many observers is the high resolution required. Using the half-width obtained by Babcock (1923), and assuming the instrument would need a passband at least as narrow, the minimum resolution required becomes 5577A/0.023A = 2.4 X 105. However, with the increase in sensitivity of photomultipliers and of course dielectric layers, it was thought worth while to try to get Doppler temperatures. Armstrong (1956), (1959) used a Fabry- Perot spectrograph to try and measure the Doppler temperature of the 5577A line in night airglow and a bit of aurora and he obtained some preliminary results. Karandikar (1956), (1959) has an instrument capable of measuring Doppler temperatures, but at the time of writing his results are not known since he had not the opportunity to observe aurora until recently. It is considered that Doppler half-width measurements of the auroral green line, 5577A, would. give the most reliable spectroscopic temperatures of those available, since the interpretation is beyond question. The other methods of obtaining temperatUres are vibration and rotation. Vibrational temperatures are not very reliable because, for temperatures below 1000Â°K the excitation process dominates and largely determines the population of the upper states, rather than temperature. Rotational temperatures give reasonable values but the interpretation depends upon the excitation process. Thus Doppler teaperatures should be the best and the oxygen green line should give very reliable results. The reason that more temperatures have not been obtained by the Doppler method is that high resolution is required that cuts down the intensity since the product ot luminosity and resolution is a constant for spectrometers. Because of the obvious suitability for auroral studies, it seemed desirable to build a Fabry-Perot spectrometer at the University of Saskatchewan. Of the possible studies, the most worth while appeared to be a high resolution instrument that would be capable of observing aurora for the oxygen green line with Doppler temperatures in mind and Which would be easily convertable to other studies. Other observers have had little chance to observe aurora Where the gain in intensity over night airglow is great. A pair of 4 inch diameter quartz plates were ordered from Hilger and Watts before the author undertook this thesis. They arrived in the winter of 1959 and this study was begun in the spring. A Fabry-Perot interferometer was to be built with the first purpose in mind to study the auroral green line for temperatures. The next chapter consists of same theory required for subsequent chapters but it is not intended to be complete. Next in Chapter III a full description of the apparatus is given. This is described in detail in most parts because the design is thought to be original and appears very satisfactory. In Chapter IV the contours of the plates are determined. This is important because of the great effect the defects of the plates have on the way in which the plates are to be used and on the end result. The next chapter contains some results on auroral observations which are of a preliminary nature, but thought to be worth while. The final chapter gives an estimate of the value of the results and a few ideas on what may be done next with the four-inch Fabry-Perot interferometer.