Concave grating spectrometer for use in near and vacuum ultraviolet region

Murty, M. V. R. K.

English

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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
WASIIINGTON. D C. 70546 
REPLY TO 
ATTN OF: October 1 2 ,  1970 
TO : USI/Scien t i f ic  & Technical 'Information Div is ion  
Attent$on: M i s s  Winnie M. Morgan 
, 
FROM: GP/Offkce of A s s i s t a n t  General 
Counsdl f o r  Pa ten t  Mat te rs  
SUBJECT: Announcement of NASA-Owned 
U.S.  Pa t en t s  i n  STAR 
I n  accordance wi th  t h e  procedures contained i n  t h e  Code GP 
t o  Code U S 1  memorandum on t h i s  s u b j e c t ,  dated June 8, 1970, 
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U , S .  Pa ten t  No. : 3,229,568 
Corporate Source : Univers i ty  of Rochester 
Supplementary 
Corporate Source 
NASA P a t e n t  C a s e  No, : XGS-01036 
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however', t h e  name of t h e  a c t u a l  i nven to r  (author)  appears  a t  
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words ' I .  e . wi th  respect t o  an  invent ion  of .  . . 
Gayle Parker 
Enclosure:  
Copy of Pa ten t  
https://ntrs.nasa.gov/search.jsp?R=19700030687 2020-03-17T02:09:47+00:00Z
Jan. 18, 1966 JAMES E. WEBB 3,229,568 
ADMINISTRATOR OF THE NATIONAL AERONAUTICS 
AND S P A C E  ADMINISTRATION 
CONCAVE GRATING SPECTROMETER 
Filed Sept. 28, 1962 2 Sheets-Sheet 1 
... 
Jan. 18, 1966 JAMES E. WEBB 3,229,568 
ADMINISTRATOR OF THE NATIONAL AERONAUTICS 
AND S P A C E  ADMlNlSTRATION 
CONCAVE GRATING SPECTROMETER 
Filed Sept. 28, 1962 2 Sheets-Sheet 2 
3 
INVENTOR. 
A TTORNE YS 
M. KR.  K: MURTY 
BY 
9 5 it Jan. 18, 1966 
~ - 
3,229,568 However, the mirror now i rcepts the diffracted beams 
from the concave grating, and the spectrum is formed on 
the circle near the grating. A viewing means, or exit slit, 
is located at this point on the circle where the spectrum 
The scanning of the spectmm formed on the Rowland 
circle is produced by causing the mirror to rotate about 
The invention described herein may be manufactured a Pivot Point at the center of the Rowland circle. The 
and used by and for the Government of the United States mirror is driven by a sine-bar (mechanism so that the plane 
of America for governmental purposes without the pay- 10 of the mirror in its rotational path always passes through 
ment of any royalties thereon or therefor. the center of the Rowland circle and the spectrum wave- 
This invention relates generally to a concave grating length scale is made linear. The rotation of the mirror 
spectrometer, and more particularly to  a concave grating in this manner moves the spectrum on the circle, and 
spectrometer capable of use in near and vacuum ultra- thus no defocusing occurs. An observer positioned at  the 
violet regions. 15 exit slit may view the spetcrum of the radiation which 
The extensive space exploration program of recent is formed upon diffraction of the ultraviolet beam by the 
years, and the laboratory experimentation associated concave grating. Also, in view of the fact that the spec- 
therewith, has caused a greatly increased interest in the trum is formed already on the Rowland circle, more than 
field of ultraviolet spectroscopy and the instrumentation one exit could be used so that different wavelengths could 
connected therewith. 
monochromators have been extensively used in the near In the embodiment of the invention above described, 
and vacuum ultraviolet regions. A great advantage of the the direction of the entrance beam is fixed, and the exit 
concave grating over the plane grating is that it provides beam changes its direction during the scan. However, 
a dispersing and foousing system free of collimating and the exit slit could be placed near the center of curvature' 
focusing lenses, thereby reducing radiation energy loss as 25 of the grating, and the entrance slit near the grating. In 
the beams of ultraviolet radiation pass through the optical this arrangement, the exit beam remains fixed in direc- 
system and, also, simplifying the structure of the spec- tion during the scan, but the entrance beam changes 
trometer. direction. 
One of the problems of the concave grating spectrom- Another feature of the instant invention is that the 
eter is that of scanning the spectrum. One approach to 30 inherent astigmatism of the concave grating spectrometer 
this problem has been to  rotate the concave grating about may be reduced. This is accomplished by replacing the 
an axis passing through its center. The undesirable fea- plane mirror by a cylindrical mirror of proper radius of 
ture of rotating the grating is that a certain amount of curvature with its axis in the plane of the Rowland circle. 
defocusing of the spectrum occurs. By a proper choice Other objects and many of the attendant advantages of 
of the position of the radiation entrance and exit slits, 35 this invention will be readily appreciated as the same be- 
the defocusing may be minimized, although still existent. comes better understood by reference to the following 
Another approach to the scanning problem is to  rotate detailed description when considered in conjunction with 
the grating about the center of the Rowland circle. Since the accompanying drawings, wherein: 
the Rowland circle now moves on itself, there is no de- FIG. 1 is a plan view of the grating spectrometer of 
focusing. However, the disadvantage of this approach is 40 the instant invention; and 
that the directions of both the entrance and exit beams FIG. 2 is a schematic view of the spectrometer, similar 
must also change as the grating is rotated. to FIG. 1, illustrating the relative positions of the plane 
Accordingly, one object of the present invention is to mirror for imaging the spectrum line of different wave- 
provide a simple scanning arrangement for a concave 45 lengths on the exit slit located on the Rowland circle. 
grating spectrometer. Referring now to the drawings, wherein like reference 
Another object of the instant invention is to  provide numerals designate identical or corresponding parts 
a scanning arrangement for the concave grating spectrom- throughout the several views, the spectrometer 11 is shown 
eter wherein movement of the concave grating is elimi- in FIG. 1 as comprising a concave grating 12, a mount 
nated and the Rowland circle associated therewith is 14 for an en t rane  slit 13, shown in FIG. 2, for passage 
fixed. 6o of ultraviolet radiation, and a mount 16 for an exit slit 
A further object of the instant invention is to provide 15, also shown in FIG. 2, for viewing the spectrum of 
a scanning spectrometer wherein there is no defocusing the ultraviolet radiation. The elements 12, 14, 16 are 
of the spectrum. placed so as to position the spherical surface of the con- 
The foregoing and other objects are attained in the in- 55 cave grating 12, the entrance slit 13, and the exit slit 15 
stant invention by providing a mirror normal to  the plane on the Rowland circle associated with the concave grat- 
of the Rowland circle associated with the concave grating ing spectrometer. The Rowland circle is a circle drawn 
spectrometer, i.e., the circle drawn tangent to the face of tangent to the face of the grating at its midpoint, having 
the grating a t  its midpoint, having a diameter equal to a diameter equal to the radius of curvature of the grating 
the radius of curvature of the grating surface. The ad- 60 surface. If a radiation source or slit is placed at any 
&tion of a mirror obviously adds an additional source of Point on the circumference of this circle, the spectrum 
reflection loss into the optical system. However, in recent produced by the grating will be imaged at other points 
times there has been progress in the direction of improv- dong the circumference of this same circle. 
ing the far ultraviolet reflection of surfaces and the small Concave gratings rather than plane gratings are used in 
reflection loss introduced is more than compensated for 65 the vacuum ultraviolet region 30 as to  achieve dispersion 
by the novel and desirable optical and mechanical fea- and focusing with one element, and thus reduce energy 
tures imparted to the spectrometer. losses as the radiation passes through the optical system. 
The ultraviolet radiation source or entrance slit is A concave grating is a pattern of rulings on a polished 
placed near the center of curvature of the concave grating concave spherical surface, the rulings being the inter- 
on the circumference of the circle. Without the mirror 70 sections with the surface of equidistant planes parallel to 
the radiation spectrum, upon dispersion from the grating, the principal axis of the surface. The surface acts a t  the 
is formed on the Rowland circle near the entranec slit. same time as a grating and as a concave mirror. 
CONCAVE G 
James E. Webb, Administrator of the National Aeronau- 
tics and Space AdmhisPration, with respect to an h e n -  
tion of Ra. V. W. K. Murty 
Filed Sept. 28,1962, Ser. NO. 227,692 
5 is formed. 
3 Claims. (CL 88-14) 
Concave grating spectrometers or 20 be scanned simultaneously. 
3,229,568 
4 
The entrance slit 13 is placed on the Rowland circle 
near the center of curvature of the concave grating. The ly given by the following expression 
viewing means, exit slit 15, is located on the Rowland 
circle adjacent to the concave grating 12. L>R sin p 
The grating, and the exit and entrance slits, are struc- 
turally connected together through a ring member 18 
secured to a base member 20. The grating 12, the en- Where L=length of the Plane mirror, and @=angle of 
trance slit mount 14, and the exit slit mount 16 are at- diffraction for the longest wavelength 
tached to the ring member through tubular arms 22, 24 The height of the plane mirror is about one half of the 
and 26 extending from corresponding openings in the ring rupng length neglecting astigmatism and slit length. The 
member 20. The grating 12 may be connected height should be estimated taking these factors into ac- 
to its tubular arm through any conventional clamping or count* The size Of the Plane mirror required could as 
securing means. well be established by a suitable drawing to scale. 
A plane mirror 28 secured to a pivotable member 30 In the physical embodiment upon which the schematic 
is positioned to extend normal to the plane of the Rowland 15 illustration of FIG. 2 was based, the concave grating 
circle. me member 30 is attached to a pivot point 32 radius=lOOO mm.; ruled area=llfOx100 mm.; the grat- 
located at the center of the Rowland circle at its one ing constant=600 L/mm.; and the mirror size approxi- 
extremity and at its other extremity is adapted to be mate1y=9"X4". 
driven through a rotational path by sine-bar mechanism 34 In an alternate embodiment of the instant invention, not 
secured to base 20. A sine-bar mechanism carriage 36 20 shown, the exit slit could be placed near the center of 
has formed therein a groove 38 and is reciprocated by a cur\rature Of the grating and the entrance near the 
drive shaft 40. The drive shaft 40 may be connected to grating* In this a r ~ w w m n t ,  the exit beaim lWnains 
any well known actuation such as an electric motor, fixed in direction during the scan but the entrance beam 
or may be hand operated. A roller 42 positioned on an changes direction and is reflected by the plane mirror onto 
arm 43 extends from member 310 and coacts with groove 25 the grating surface. 
38 to rotate the plane mirror 213 about an axis passing In a further modification of the instant invention, 
through the center of the Rowland circle. The Rowland the astigmatism inherent in the concave grating spec- 
circle is imaged virtually on itself in this plane mirror 28. trometer may be controlled to a limited extent by sub- 
the absence of plane mirror 28, the spectrum of the stituting a cylindrical mirror in place of the plane mir- 
radiation passing through the entrance slit 13 would be 30 ror. The cylindrical mirror must be of proper radius 
defracted from grating 12 and would be fonmed the of curvature with its axis located in the plane of the Row- 
Rowland circle near the entrance slit, as shown by the land circle. When astigmatism is corrected in this way 
dashed lines in PIG. 2. However, the plane mirror 28 for a wavelength is midway between the two extreme 
rotating about the center of the circle intercepts the beams wavelengths the astigmatism for the Bong wavelength end 
dispersed from grating 12 and images the spectrum on the 35 is reduced and for the short wavelength end is increased. 
Rowland circle adjacent to the grating at exit slit 15. The It will therefore be seen, upon consideration of the 
rotation of the plane mirror 28 moves the spectrum along above disclosure, that a relatively simple and effective 
the Rowland circle across the viewing slit and thus no scanning arrangement has been provided for a concave 
defocusing occurs. B~ using the sine-bar mechanism 34 grating spectrometer. It will be further noted that the 
to rotate the plane mirror, the wavelength scale is made 40 described scanning arrangement has been provided with 
linear. a minimum of additional parts, and that the scan spec- 
In FIG. 2, the relative position of the entrance and exit tJXm remains in perfect focus O n  the Rowland circle. 
slits 13, 15, the concave grating 12 and the rotatable mir- Obviously, numerous modifications and variations of 
ror 213 are shown when the mirror is in position to image a the PI-eSent invention are possible in the light of the 
900 Angstrom line on the viewing means and again when 45 above teachings. It is therefore to be understood that 
the mirror is in position to h a g e  a 6000 Angstrom line within the scope of the appended claims, the invention 
on the viewing means. I t  will be Seen that the plane may be practiced otherwise than as described herein. 
mirror during its rotational path will scan the radiation What is claimed as new and desired to be secured by 
spectrum, successively imaging the various spectrum wave- Letters Patents of the United States is: 
lengths onto the viewing means 15. 1. A spectrometer capable of use in the ultraviolet 
To again briefly summarize the operation of spectrom- 
eter 11, ultraviolet radiation passes through entrance slit a fixedly mounted concave grating; 
13, is incident upon and diffracted by concave grating 12, an entrance slit positioned on the circumference of the 
is reflected by rotating mirror 28, and is imaged upon the Rowland circle associated with said grating and lo- 
viewing means 15. It may be readily seen, therefore, 55 cated adjacent the center of C~~rvature of the grating 
that the spectrometer provides an effective arrangement for passage of radiation directly incident thereon; 
for spectrum scanning in a concave grating spectrometer. a viewing means for radiation dispersed from said 
Also, it will be noted that in view of the fact that the grating positioned on the circumference of said circle 
spectrum is forsmed already on the Rowland circle, more and located adjacent the grating; 
than one exit could be used so that different wavelengths 60 a mirror located within the circumference of said 
could be scanned simultaneously. circle and positioned normal to the plane thereof 
In the above described embodiment, it is essential that for intercepting only the dispersed radiation from 
the plane mirror must not intercept the incident beam the grating and reflecting this radiation onto said 
from the entrance slit to the concave grating. This means viewing means, the plane of the mirror reflecting 
that the shortest wavelength beam required must be 65 surface passing through the center of the circle; and 
separated from the incident beam so that there is no means for rotating said mirror about an axis passing 
overlapping between them beyond the center of the Row- through the center of said circle, thereby moving 
land circle. To  insure against overlapping, it may be the spectrum of said dispersed radiation across said 
shown that the following relationship is necessary viewing means. 
a > [ ( W / 2 R ) - ( h 1 / 2 a ) ]  2. A spectrometer as defined in claim 1, wherein said 
where ol=angle of incidence on the grating, W=width 3. A spectrometer as defined in claim wherein said of the grating ruled area, R=radius of curvature of the 
grating, u=grating constant, and hl=shortest wavelength 
required. 75 (References on fdswing page) 
The length of the plane mirror required is approximate- 
5 2 
50 
comprising: 
70 
rotating means comprises a sine-bar mechanism. 
viewing comprises an exit slit. 
References Cited by the Examiner 
UNITED STATES PATENTS 
3,229,568 
2,805,343 9/1957 Lang _ _ _ _ _ _ _ _ _ _ _ _ _  250-51.5 
3,090,863 5/1963 McPherson _ _ _ _ _ _ _ _  2 5 0 4 3 . 5  
2,540,821 2/1951 Harker _ _ _ _ _ _ _ _ _ _ _  250-51.5 JEWELL H. pEDERsENy Primary Examiner- 
2,548,548 4/1951 Menzies et al. _ _ _ _ _ _ _ _  88-14 5 RALPH G. NILSON, FREDERICK M. STRADER, 
2,723,589 3/1955 Bullock _ _ _ _ _ _ _ _ _  2 5 0 4 3 . 5  X Examiners. 
2,805,342 9/1957 Lang _ _ _ _ _ _ _ _ _ _ _ _ _  250-51.5 Assistant Examiners. 
2,764,909 10/1956 Caldecourt -___-_-_-__ 88-14 H. S. MILLER, G. E. MAPHE\Vs,  R. L. WIBERT, 


Concave grating spectrometer Patent

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