Optics: measuring and testing – By dispersed light spectroscopy – For spectrographic investigation
Reexamination Certificate
2000-10-18
2003-09-30
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
For spectrographic investigation
C356S328000, C356S334000
Reexamination Certificate
active
06628383
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a simple, efficient and economic multiorder spectrograph based upon a modified Ebert-type mounting, using a standard low-blaze-angle plane reflection grating as the multiorder dispersing element. This invention also relates to the efficient usage of the available pixels in a modern two-dimensional detector array, such as a CCD, by filling the array area with a multiorder spectral display covering wavelengths ranging from the vacuum ultraviolet to the infrared, either in their entirety or in selected wavelength segments, at medium to high spectral resolutions.
Until recent years the photographic emulsion was typically used as the recording means in spectrographs. The advantage of the emulsion was that it provided durable data storage having an enormous number of detector elements (photographic grains) at low cost. But the low quantum efficiency (QE), and the numerous problems associated with processing and measuring photographic plates, gave rise to common usage of faster and more convenient scanning spectrometers, or monochromators, using a photomultiplier (PMT) for routine spectral measurements not requiring a large number of resolution elements.
More recently, electronic detector arrays, such as CCDs, having large numbers of pixels, and QEs significantly higher over wider wavelength ranges than even PMTs, have become the detector of choice in spectroscopy. But whereas the cost of those earlier detectors was normally a small fraction of the cost of the spectrograph, modern scientific-grade CCDs are often the most expensive part of the spectrograph system. This creates a significant need for an inexpensive high-performance spectrograph designed to make most efficient use of these powerful detector arrays, while keeping the overall cost of the spectrograph system within budget.
The vast majority of spectrographs and spectrometers continue to be used to measure spectra in one spectral order, usually the first order, with order-sorting filters used as needed to block other orders. In such applications, a square detector array having a million pixels will typically measure fewer than 500 spectral resolution elements at one time (due to the images typically overlapping 2-3 pixels). This is a serious under-utilization of the available pixels in these expensive detectors.
An obvious way to greatly improve utilization of detector arrays is to fill the pixel array with a multiorder spectrum using a grating and a cross-dispersing element. This approach is based upon the fundamental characteristic of blazed gratings, that the light diffracted in the blaze direction is comprised of radiation in a plurality of spectral orders, where the central wavelength of each order is given by &lgr;
o
/m, where &lgr;
o
is the first-order blaze wavelength and m is the spectral order. To clearly separate these orders in the image plane it is necessary to also introduce a cross-dispersing element (a prism or a second grating) to disperse this same radiation perpendicular to the first grating's dispersion. A thorough tutorial regarding the terminology, construction and theory of diffraction gratings is given in E. Loewen , et al., “Diffraction Grating Handbook, Bausch & Lomb, Inc., 1970, which is incorporated by reference as if fully set forth herein.
It is commonly assumed that multiorder spectrographs are in fact echelle spectrographs, inasmuch as essentially all multiorder spectrographs use echelle gratings. But the main purpose of echelle gratings is not that they should produce multiorder spectra, that being a necessary and often undesirable byproduct of their design. The reason and justification for using echelle gratings is the very high spectral resolution they afford as a direct result of their high blaze angles (typically 63°-76°), and the fact that angular dispersion of a reflection grating is proportional to Tan B, where B is the grating's blaze angle. Thus, the angular dispersion of an R
2
(Tan 63.4°=2) echelle is 10 times, and an R
4
(Tan 76°=4) echelle is 20 times that of a typical standard plane grating having a blaze angle of 11°. But very high spectral resolution is not a common requirement in spectroscopy, as verified by the fact that the relatively costly and complex echelle spectrographs comprise only a tiny fraction of the spectrographs that are in use.
A much simpler and less expensive way to perform multiorder spectroscopy for a majority of applications, where modest resolutions over a large wavelength range is the goal, is to use a cross-dispersed low-blaze-angle grating blazed at several times the longest wavelength to be studied. Such gratings, having a wide range of blaze and dispersion characteristics, are commercially available at reasonable cost A multiorder spectrograph using such a grating was reported by R. L. Hilliard, etal., “A Cross-Dispersed Echelette Spectrograph and a Study of the Spectrum of the QSO 1331+170”, Ap.J., 1975, 351-361, Vol. 196.
An important requirement for any multiorder spectrograph is that the image quality over the area of the detector be comparable to or smaller than the pixel size. The spectrograph must therefore have negligible astigmatism, coma, and spherical aberration over the required field; and to avoid chromatic aberration over such a large wavelength ranges effectively requires all mirror imaging optics.
An elegant optical system uniquely meeting these requirements has its roots in the Ebert-type mounting, originally described by W. G. Fastie, “A Small Plane Grating Monochromator”, J.O.S.A., 1952, 641-647, vol 42, no. 9. The popular Ebert has a single spherical mirror serving both as collimator and camera, and the plane grating is located near the sphere's focus. Although corrected for coma, the Ebert still has astigmatism and spherical aberration, which has restricted its use to that of a scanning spectrometer. But an essentially unnoticed article by W. T. Welford, “Stigmatic Ebert-Type Grating Mounting”, J.O.S.A., 1963, 766, vol. 53, revealed that the images would become free of aberration if the spherical mirror of the Ebert were simply replaced by a paraboloidal mirror of the same focal length. The only previous example of anyone actually using a paraboloidal mirror in an Ebert-type mounting appears to be I. Furenlid and O. Cardona, “A CCD Spectrograph with Optical Fiber Feed”, P.A.S.P., 1988, 1001-1007, vol. 100.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a simple, efficient, and economic multiorder spectrograph for use over a wavelength range from the vacuum ultraviolet to the infrared.
A further object of the invention is to provide such a spectrograph based upon the Ebert-type mounting, where the spherical collimator/camera mirror normally used therein is replaced by a paraboloidal mirror to eliminate the Ebert's astigmatism and spherical aberration, and to thereby create a spectrograph that has image quality comparable to the pixel resolution over the area of a two-dimensional electronic detector array.
A further object of the invention is to provide such a spectrograph that utilizes a low-blaze-angle reflection grating having first-order blaze so that spectra at shorter wavelengths of interest shall be most efficiently diffracted into higher spectral orders.
Another object of the invention is to provide such a spectrograph wherein a cross-dispersing element is located between the reflection grating and the paraboloidal mirror where it is used to cross disperse, and thereby separate, the grating orders perpendicular to the grating dispersion, to create a multiorder spectral, display.
It is also an object of the present invention to provide such a spectrograph that can create multiorder spectral displays for simultaneous recording of very large wavelength ranges at moderate to high resolution, using two-dimensional detector arrays having N
p
pixels to detect as many as N<N
p
/10 spectral elements in a single exposure.
Another object of the invention is to mount and permanently align the grating and
Evans F. L.
Geisel Kara
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