Astronomical spectrographs: Past, present, and future

400

Astronomical s p e c t r o g r a p h s : past, present, and future

preserved, though with diminished sharpness, while the primary one may have become completely blotted out. A time exposure of the fringe pattern can be used for evaluating the time average F of the visibility V, (equ. (1)), for the particular setting of the interferometer. If Vo is the visibility for the same setting in calm air, it is easily seen that ~ ---- ~V0 where the factor s < 1 depends on the degree of turbulence, but is independent of the interferometer setting. Thus, provided that s is not too small, a series of time photographs of the secondary fringes should, on the basis of WOLF'S theorem, still provide all information necessary for determining the light intensity distribution over the surface of the source under conditions of turbulence which make the observation by the original Michelson method impossible. It might be worth while to t r y out this procedure on some celestial objects with a suitably adapted stellar interferometer; if it should turn out to be successful it would open new and unexpected possibilities for the use of this instrument.

REFERENCES CARROLL, J. A . . . . . . . . . . . FURTH, R., SITTE, K. a n d APPEL, H . P . LACROUTE, P . . . . . . . . . . . . MICHELSON, A. A . . . . . . . . . . . WOLF, E . . . . . . . . . . . . .

1939 1939 1939 1918 1953 1954 1955

M . N . , 99, 735. M.N., 99, 141. M.N., 99, 733. Ap. J., 47, 283.

Nature (London), 172, 535. Proc. Roy. Soc., A, 225, 96. Vistas in Astronomy, Ed.

A. BEER, P e r g a m o n Press, L o n d o n ; p. 385.

Astronomical Spectrographs: Past, Present, and Future* I. S. BOWEN Mount Wilson and P a l o m a r Observatories Carnegie I n s t i t u t i o n of W a s h i n g t o n California I n s t i t u t e of Technology Pasadena, California, U.S.A.

AT the beginning of the present century photography was rapidly displacing visual observation for the study of stellar spectra. As a result of this and of the completion of several new observatories at about this time a substantial number of new spectrographs were built and described in the early years of this century (CAMPBELL,1898 ; VOGEL, 1900; FROST, 1902; SLIPHER, 1904; ADAMS, 1912; PLASKETT, 1919, 1924). The general pattern of these spectrographs, many of which are still in use, was a lens collimator of 1.5-2.5 in. aperture, a dispersing system of from one to three flint prisms and one or more cameras of focal length from 3 ft down to the shortest that the optical designs of the day would permit for the aperture used. These provided dispersions in the range from 10 to 100 =~ per mm. Somewhat later the demand for higher dispersion resulted in the construction, first at the Mount Wilson Observatory (ADAMS, 1911) and later at the McDonald * See also the article "Methods in Stellar Spectroscopy" by TH. DUNHAM,Jr., in Section 13 of Vohnne 2.

I. S. BOWEN

401

Observatory, of large coud~ instruments using either prisms or gratings. These had apertures of 3-6 in., focal lengths of up to 15 ft and yielded dispersions of as much as 2 A p e r m m . With the construction of telescopes of larger and larger aperture, however, it became increasingly evident that the efficiency of these spectrographs, particularly for the higher dispersions, was exceedingly low. Thus the telescope-spectrograph system has a focal length equivalent to that of a single lens having an aperture equal to that of the main telescope mirror and a focal ratio equal to t h a t of the spectrograph camera. I f the slit is removed and if the seeing image of a star has a diameter of F fi radians, the linear diameter of a monochromatic star image on the plate is A ~ ft. A is the aperture of the telescope and F and D are the focal length of the camera lens and the aperture of the beam coming from the collimator, respectively. This value for the image diameter is rigorous for a spectrograph with a symmetrical dispersing system in which a small deviation of the collimator beam produces an equal deviation of the camera beam. For non-symmetrical systems the above formula for the dimension of the image in the direction parallel to the dispersion should be multiplied by a factor, r, equal to the ratio of the deviations of the camera and collimator beams. In general r does not differ greatly from unity. I f this image size is greater than the limit of resolution of the plate, A, it is then necessary to introduce a slit to narrow the image to about this limit if optimum resolution is to be attained. The fraction of light that can pass this slit is, therefore, AD E

--

AFr--fl

....

(1)

Formula (1) is exact for a square star image. The much more complicated formula for the actual distribution of light in a star image differs from this by an amount much smaller than the uncertainties introduced by large fluctuations in ft. For most fast plates A is usually assumed to be about 0.02 mm while under average seeing conditions at the most favourably located observatories the seeing image has a diameter of 1-1.5 sec of arc, or fl = 6 × 10-6 radians. Under these conditions 3300D E

--

AF~

....

(2)

in which all dimensions are in millimetres. The fraction passed is therefore equal to 3300 mm or 132 in. divided by the effective focal length of the telescope-spectrograph system. This means that to attain 100 per cent efficiency cameras should operate at a focal ratio of two-thirds or less for a 200-in. telescope, of four-thirds for a 100-in., and of 2.2 for a 60-in. Formula (2) also shows that only 2-20 per cent of the light passes the slit of the older high-dispersion spectrographs operating at focal ratios of 10-30 when attached to these large telescopes. In view, however, of the enormous cost of collecting light by means of modern telescopes it is obviously of the greatest importance t h a t the light once collected be used efficiently. Fortunately optical advances in recent years have made possible the use of apertures and focal ratios of a different order than those hitherto available. These advances are : ~7

402

Astronomical spectrographs: past, present, and future

(1) The development of the "blazed" grating by WOOD (1944) and by the BABCOCKS (1944; 1951). Gratings of this type throw 60-70 per cent of the incident light into one order. This represents a considerably higher efficiency than even a smallaperture prism train giving the same angular dispersion. Furthermore the grating maintains the same efficiency at all apertures while the efficiency of a prism train falls off rapidly as larger apertures are attempted. Because of this advance all new spectrographs at the Mount Wilson and Palomar Observatories are being built with gratings and all of the older prism instruments are being replaced with gratings as rapidly as shop facilities will permit. (2) The development of cameras giving critical definition over moderate fields at very low focal ratios. Thus a standard air-type Schmidt camera gives critical definition over a field having a diameter of roughly one-fifth the focal length at focal ratios down to ~ =

~ in which the aperture D is expressed in inches. In eases

where the focal length is not so great that absorption in the glass path becomes F _ 1/~ prohibitive an even smaller ratio of D n ~ may be obtained with a solid block or thick-mirror-type Sehmidt (I-IENDRIX, 1939; MI~KOWSK~, 1944), n being the index of the glass used. I f a somewhat smaller field of one-tenth to one-fifteenth of the focal length is permissible still lower focal ratios may be attained with the Sehmidt-aplanatie sphere camera (BowEN, 1952). This means that at the present time one can design spectrographs around cameras that approach the theoretical limit of the focal ratio of one-half for an air system or 1/2n for an immersion system in which n = 1.52, the index of the gelatine of the photographic plate ; this index sets the ultimate limit on the focal ratio of any photographic camera. Obviously a reassessment of spectrograph designs should therefore be made in the light of these new optical possibilities. In equation (1) the quantities A, ~ and A are fixed by the telescope used, by seeing conditions, and by the resolution required. Furthermore, F is related to the linear dispersion, K, expressed in Angstroms per millimetre, and the angular dispersion, e, expressed as Angstroms per radian, through the relationship F

-

....(3)

K

From grating theory we may obtain the following relationship : sin O + sin Op --

n~

....(4)

in which ® and (I) are the angles of incidence and diffraction, d is the grating space, and n the order of the spectrum. For a monochromatic image we may differentiate holding 2 constant and obtain cos O dO ~ cos (I)d(I) = 0 or

r=

d~ cos ® --d~=cos¢ .

.

.

.

.

(5)

I. S. B o w n N

403

To obtain the angular dispersion ~ we m a y differentiate holding @ constant, which gives cos (P d e --

or

~ --

nd,~

d2

~ cos ¢

d(I)

n

....

(6)

Furthermore if L is the length of the ruled surface measured perpendicular to the lines, then D, the largest aperture of collimator beam that can be handled by the grating without loss, is L cos ®. Substituting these values of F, r, and ~, in (1) and simplifying, one obtains for the efficiency of a grating instrument AKLn

E = - -

....(7)

n

I f we substitute for ~ from (4) in order to show more clearly the effect of tbe limitation on the value of n, we obtain E = AKL(sin (9 -7 sin (I)) Afl,~

. . . . (s)

It is evident that any increase in efficiency depends on the possibility of making L(sin ® -7 sin (I)) as large as possible. In the design of the Coud~ spectrograph for the 200-in. Hale telescope L was pushed to 14 in. by resort to the use of a composite made up of four matched gratings each having a ruled area 5½ × 7 in. Since the optical resolving power of an individual grating was greater than t h a t of the photographic plate for all focal lengths used, it was not necessary to bring the four gratings into phase with each other. The much easier condition of so adjusting the gratings that the spectra formed by them were in coincidence within the limit of plate resolution could be satisfied with the aid of proper mechanical devices. Unfortunately it has thus far not been possible to obtain the 60-70 per cent concentration of light in an order higher than the third for a 400 line per mm grating and consequently gratings of this type were used. For the centre of the camera field at ~ about 4200 A this gives a value of (sin 0 -7 sin ¢) = ½. For this spectrograph cameras of focal length 144 in., 72 in., 36 in., 18 in., and 8.4 in., operating at focal ratios of 12, 6, 3, 1.5, and 0.7, are provided. These yield dispersions in the third order violet of 2.3, 4.5, 9, 18, and 38 A per mm (BOWEN, 1952), and efficiencies of 6 per cent, 13 per cent, 25 per cent, 50 per cent, and 100 per cent, respectively, under the conditions of seeing and slit width mentioned above. For dispersions in the 110-440 A per mm range a small grating spectrograph has been constructed for use at the prime focus of the Hale telescope. This has a 3-in. aperture collimator beam and is provided with cameras of the thick-mirror Schmidt F type operating at focal ratios of ~ = 0.47 and 0.95. Looking to the future for possible ways of still further increasing efficiency, two general possible directions of attack need consideration. The first of these is some

404

Astronomical spectrographs: past, present, and future

scheme such as the image slicer (BowE~, 1938), which changes the shape of the star image to a long narrow one. Such a device by itself increases the fraction of light passing the slit by widening the spectrum rather than by increasing its surface brightness. The brightness m a y than be increased by narrowing this spectrum with a cylindrical lens placed in front of the photographic plate. A simple cylindrical lens can, without introducing objectionable aberrations, reduce the effective focal ratio in the direction perpendicular to the dispersion to about F/D = 2, while the original value of the focal ratio of the camera lens is retained in the direction parallel to the dispersion. With the older cameras operating at focal ratios of F/D = 10-30 this made possible a substantial gain in speed. With the much smaller focal ratios now possible for spectrograph cameras the gain in speed is much less, although such an image slicer m a y often prove useful to widen spectra that otherwise would be unduly narrowed by the small ratios of camera to collimator focal lengths now coming into use. The second and more direct procedure for increasing efficiency through taking advantage of the factors in equation (7) would appear to have its greatest possibility of success by improving grating ruling techniques in such a way as to increase (sin 0 -~ sin (b) and at the same time retain an efficient blaze. Thus a gain of a factor of over 3, compared to the present 200-in. Coud6 spectrograph, is theoretically possible and is probably easier of attainment than by ruling gratings three times as large as the present maximum size. Since the field of view of the present very fast cameras is limited to between one-fifteenth and one-fifth of the focal length, it is desirable that the angular length of the blaze in each order should not exceed this. By combining the grating with a small prismatic cross dispersion all wavelengths may then be recorded on one plate. This would indicate a fairly coarse grating of the order of 100 line per ram. Gratings of this general type have been suggested by SHANE and WOOD (1947) and by HARRISON (1949). The one serious difficulty encountered in the use of a reflection grating, at large values of 0 and (I), is the interference of the camera with the collimator beam. Either 0 must differ largely from (I), in which case a highly elliptical camera beam results, or the camera lens or correcter plate of a Schmidt system must be placed at a large distance from the grating with resultant vignetting difficulties unless an aperture much larger than the beam is used. Either choice results in a very inefficient use of lens aperture and seriously limits the effective focal ratios that may be used. An ideal solution would be a grating made up of a series of equally-spaced reflecting surfaces arranged like the slats of an open Venetian blind and embedded in a transparent medium, the planes of the individual surfaces being nearly perpendicular to the plane of the grating. By placing the camera and collimator on opposite sides of this grating, O and q) could be large and equal without causing any interference. Such a grating would have the additional advantage that the position of the blaze in the spectrum could be shifted by a change of 0 and d). One procedure for making such a "Venetian-blind" grating would be to rule on a thin layer of plastic bonded to glass with a triangular groove one side of which was held nearly perpendicular to the plastic surface. An aluminium coat would next be evaporated from such a direction that the perpendicular side only of the groove would receive a coat. The grooves would then be filled up with a second application of plastic and possibly covered with a glass optical flat.

GEORGE R. HARRISOl~

405

REFERENCES ADAMS, WALTER S . . BABCOCK, HAROLD D . . . . BABCOCK, HAROLD D. a n d HORACE W.

BOWEN, I. S . . CAMPBELL, W. W. FROST, EDWIN B. HARRISON, g . n .

HENDRIX, D. 0. MINKOWSKI, R. PLASKETT, J. S. SLIPHER, V. M. VOGEL, H. C . . ~l~7OOD, R. W.

1911 1912 1944 1951 1938 1952 1898 1902 1949 1939 1944 1919 1924 1904 1900 1944 1947

Ap. J., 33, 64. Ap. J., 35, 163. J. Opt. Soc. Amer., 84, 1. J. Opt. Soc. Amer., 41, 776. Ap. J., 88, 113. Ap. J., 116, 1. Ap. J., 8, 123. Ap. J., 15, 1. J. Opt. Soc. Amer., 39, 522. Publ. Astr. Soc. Pacific, 51, 158. J. Opt. Soc. Amer., 34, 89. Ap. J., 49, 209. Ap. J., 59, 65. Ap. J., 20, I. Ap. J., 11, 393. J. Opt. Soc. Amcr., 34, 509. J. Opt. Soc. Amer., 37, 733.

Spectroscopy with the Echelle GEORGE R . HARRISON Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A. SUMMARY

The echelle is a type of dispersing element intermediate in properties between the blazed plane diffraction grating and the reflection etalon of MICHELSON and WILLIAMS. Echelle spectrographs have now been produced in six forms which for many spectroscopic problems give advantages in speed, compactness, and stigmatic properties, and compete effectively with much larger instruments in resolving power and dispersion. An echelle is crossed with a small high-speed stigmatic prism or grating spectrograph for the production of cyclic spectra, giving the equivalent of a large concave grating instrument. A special advantage of the echelle spectrograph lies in its free spectral range, which is sufficient to reveal hyperfine structures and Zeeman patterns effectively without overlapping. Echelles are now produced by the Bausch and Lomb Optical Company by three methods, involving respectively ruling in deep alumininm layers on glass as with ordinary gratings, cutting laps in metal with a fairly precise tool room dividing engine and using these to grind into an optical flat the desired set of flat-faced grooves with appropriate procedures to average out periodic errors, and replicating in plastic of either ruled or lapped echelles. I t has been found that wavelength precision of about one part in five million, intermediate between that given by the usual large concave gratings and that given by the reflection echelon or etalon, is obtainable with echelles.

I. T~E ECHELLE AT the London Conference on Optical Instruments, held 19th to 26th July, 1950, HARRISONand BAUSCH(1950) reported on the production of a new type of dispersing element, intermediate in properties between the blazed plane diffraction grating (WooD, ]910) or echellette, and the reflection echelon (MICHELSON, 1898, 1899). This so-called echelle has since been found useful in many fields of spectroscopy, especially where compactness combining high resolution with broad spectral coverage is i m p o r t a n t . Echelle spectrographs have now been produced in at least six forms, several of which give advantages over concave grating instruments in speed, compactness, and stigmatic properties, and compete effectively with much larger instruments in resolving power and dispersion. The application of the echelle to astronomical s p e c t r o s c o p y h o l d s m u c h p r o m i s e ( H A R R I S O n , 1949).