Device and process for extinguishing a source

Details Device and process for extinguishing a source Device and process for extinguishing a source

1999-09-27

2001-02-20

Kim, Robert (Department: 2877)

Optics: measuring and testing

By light interference

Reexamination Certificate

active

06191863

ABSTRACT:

The present invention relates to a device and a process for extinguishing a source and an achromatic interferential coronograph.
Different types of coronographs are known. They enable observations in the vicinity of a principal source and constitute in a certain way antiglare systems. The coronographs are used in astronomy and have applications in the research or the examination of entities in the vicinity of a central star, such as close stars.
The occultation coronographs, used conventionally, block the central portion of an incident beam from a central source while letting through the peripheral portion of the beam. The shortcoming of these coronographs is that they do not extinguish totally the contribution of the central source, since they let through a fraction of light forming rings far away from the diffraction spot. Increasing the dimension of the occultation enables to reduce this fraction of light, but the cost is the total loss of efficiency in areas close to the central source. The occultation coronographs are therefore not suited to the observation of objects in the close vicinity of the central source, whereas these objects are of relative small dimensions and thus hardly detectable because of the glare caused by the central source.
Such an interference extinguishing system has been suggested, involving two telescopes. The shortcoming of such a system is to call for cophasing of both telescopes and to generate a chromatic source extinguishing.
Jean GAY and Yves RABBIA have suggested in an article ‘principle of an interferential coronography’ published in C. R. Acad. Sci. Paris. Vol. 322, series 2b, pp. 265-271, 1996, a device for extinguishing a source, cancelling the contribution of a punctual source in an achromatic fashion. The coronograph suggested comprises an interferometric means, consisting of a beam splitter that splits an incident beam in two secondary beams having the same optical power and two arms having optical paths of the same length. The arms are provided with afocal systems having opposite enlargements that produce pupil turn-up in one of both arms.
The article of Wei shen et al. ‘Application of a shack cube as a beam splitter in interferometers with a local reference beam’, published in the magazine Applied Optics, vol. 35, no. 34, pp. 6648-6654, December 1996, relates to the test of a convergent wave front using a Shack cube used as a beam splitter in a Mach-Zehnder interferometer. The technique described enables to control an optical surface made up of such a convergent wave front.
The object of the invention is a coronograph that enables to observe objects in the close vicinity of a central star, such as an exoplanet, gas envelopes and circumstellar dusts and protoplanetary disks.
The invention thus relates to a coronograph enabling to extinguish the effects of a central source in its immediate vicinity, i.e. up to an angular distance projected onto the sky equal to one third of the first radius of the spot of the Airy spot, with rigorous achromatism.
The invention aims at such a coronograph that improves mastering chromatic and/or spherical chromatic aberrations differential with respect to the article mentioned above of Jean GAY and Yves RABBIA, and/or allowing to meet particular requirements, such as for example forming an image at a preset distance in an on-board assembly or obtaining an enlargement fixed at the output.
The invention relates more generally to a device for extinguishing a source, enabling to extinguish the effects of a central source up to its immediate vicinity and in an achromatic fashion, which is simple to realise, easy to implement and accurate.
The object of the invention is also a process for extinguishing a source, thereby cancelling the effects of a central source closest to this source with rigorous achromatism.
To this end, the invention relates to a device for extinguishing a source comprising:
means for receiving an incident light beam emanating from a punctual central source,
a beam splitter receiving the incident beam from the receiving means and separating this beam into two secondary beams having the same optical power,
two arms each provided with an optical system and traversed respectively by the secondary beams, whereas the arms have optical paths of the same length and whereas the optical systems have opposite enlargements causing pupil turn-up in one of both arms with respect to the pupil of the other arm, and
means to combine secondary beams into an output beam,
so that the contribution of the punctual source is cancelled in the output beam.
According to the invention, the optical systems having focuses, principal planes and antiprincipal planes, each of the focuses, principal planes and antiprincipal planes of one of the two optical systems is respectively superimposed to a focus, an antiprincipal plane and a principal plane of the other optical system after formation of the output beam. Moreover, the arms each having a number of passages through at least one of the focuses of the optical system corresponding to this arm, the number of passages differs by an odd number.
While stating that the incident beam ‘originates from a punctual central source’, we mean that the beam is transmitted from an area comprising a light source on which the beam is centred. The source is said to be punctual with reference to the weak angular radii that it occupies in the incident beam. More precisely, if we want an extinction better than 1/m, both following conditions must be respected:
the alignment of the punctual source must not deviate from the ideal position by more than one angle &thgr; carried over infinitely (on the sky in the case of a coronograph), the angle &thgr; being given in relation to the radiation of wavelength &lgr; and to the diameter D of a lens of the receiving means (diameter of the telescope for a coronograph), by the relation:
&thgr;=(&lgr;/
D
)·(∞{square root over (2)}/&pgr;
∞{square root over (m)}
)
the dimension of the source must not overflow infinitely (on the sky in the case of a coronograph) of a circle of angular radius &ohgr;, whereas the radius &ohgr; is given by:
&ohgr;=(&lgr;/
D
)·(2
/&pgr;~{square root over (m)}
).
The device for extinguishing a source can also serve to eliminate the influence of the central portion of a light source associated with an angular radius greater than that defined above, notably if the luminosity of the source is concentrated essentially in this central portion. In such a case, we designate by ‘punctual central source’ the central portion of the source to be extinguished.
An additional condition that the incident beam must check, which carries a wave originating from the punctual central source, is that this wave is centrosymmetrical. This condition can be explained in the following way. The incident beam propagating along a propagation axis on which is centred the punctual central source, we define the symmetry of the wave originating from the source, with respect to the axis of propagation. To achieve a global extinction better than 1/m at the output for the radiation of wavelength &lgr;, the mean quadratic deviation &egr;′ between the wave and its symmetry must then check the equation:
&egr;′≦&lgr;/2
&pgr;∞{square root over (m)}
An additional condition to achieve an extinction better than 1/m concerns the arms traversed by the secondary beams. The arms must generate to do so a differential velocity &dgr; along the radii on the principal axis which bears out the equation:
&dgr;≦&lgr;/2
&pgr;∞{square root over (m)}
The device for extinguishing a source is based on an interferometric assembly comprising the beam splitter, the two arms provided with optical systems and the means of combination. The interferometer can be for instance of the Michelson or Mac-Zehnder type.
The effects of the central source are extinguished by a destructive interference between any point of the output field for all the radiations, whatever the frequency. Thus, the extinction is achromatic.
Extinction by destructive

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