CN109323763B

CN109323763B - Large-view-field far-ultraviolet spectral imager

Info

Publication number: CN109323763B
Application number: CN201811120666.5A
Authority: CN
Inventors: 娄铮; 纪丽
Original assignee: Purple Mountain Observatory of CAS
Current assignee: Purple Mountain Observatory of CAS
Priority date: 2018-09-26
Filing date: 2018-09-26
Publication date: 2020-12-29
Anticipated expiration: 2038-09-26
Also published as: CN109323763A

Abstract

The invention discloses a large-view-field far-ultraviolet spectrum imager which comprises a Gregorian telescope, a monochromator, imaging optics and a detector. The monochromator separates an incident beam collected by the telescope into a plurality of wavelength channels along the dispersion direction, each separated channel is collected by an independent imaging lens to form a full-field image, and the images of different channels are finally formed in different areas of the same detector. The invention can realize a large field of view with a diameter of more than 20 degrees, a spatial resolution superior to 30 arc seconds and a medium and adjustable spectral resolution of 500-2000, and is suitable for scientific application with requirements on the large field of view and the medium spectral resolution.

Description

Large-view-field far-ultraviolet spectral imager

Technical Field

The invention relates to a spectral imager, in particular to a large-field far-ultraviolet spectral imager.

Background

The imaging spectrometer is an astronomical observation instrument with optical imaging and spectrum measurement functions, and can simultaneously acquire two-dimensional spatial distribution and spectral characteristics of an astronomical target, so that abundant three-dimensional information of the observed target is constructed. In the Far Ultraviolet (FUV) band, an imaging spectrometer is one of important observation means for researching the fields of intersatellite warm gas, intersatellite media, extraterrestrial planets and the like, so that important international science frontier problems such as 'cosmic gravity electron deficiency' and 'astrological absorption and feedback' are answered.

In the FUV band, the main difficulty in designing the imaging spectrometer is that the imaging spectrometer lacks a lens material with good light transmittance, so that only reflective optical elements can be considered in the optical design. For example, in the optical and near ultraviolet bands, two-dimensional spectral imaging can be achieved using integrated field of view spectroscopy (IFS) techniques, but not in the FUV band due to the lack of corresponding fiber or microlens materials. On the other hand, even if the coating process of the reflector surface is optimized, the typical reflectivity of the reflector in the FUV waveband can only reach 40-70%, which is far lower than that of visible light and near ultraviolet waveband. In order to obtain the greatest possible viewing efficiency, it is desirable to minimize the number of reflecting surfaces and to use as simple an optical layout as possible in the design.

There have been several precedents internationally for spectral imaging projects in the FUV band, such as SPEAR, ALICE, and WSO-UV. Most of these projects use Long Slit Spectroscopy (LSS) for spectral imaging. The LSS field of view is a strip-shaped one-dimensional field of view, and two-dimensional imaging is realized through the push-broom motion of the instrument. The method can obtain full-spectrum wavelength coverage and higher spectral resolution, but has the defect of longer imaging time of a diffuse spread source, and cannot give consideration to both high spectral resolution and large field of view. In addition, there is a Tomographic Imaging algorithm (Tomographic Imaging Technique) used in the SPIDR project, for example, which uses a Tomographic reconstruction algorithm to acquire a two-dimensional monochromatic image of an observation target, but this method requires multiple rotations of an instrument and has some controversy in a signal-to-noise ratio calculation method. The imaging spectrometer adopted in the ISIS project changes a secondary mirror in the Gregorian telescope into a dispersion element, so that different emission lines of an observation target are imaged separately. However, when there are spectral lines close to each other in the emission spectrum of the target, images of different spectral lines overlap each other and cannot be distinguished. In addition, for such a seamless system, the suppression of stray light would be very difficult.

Disclosure of Invention

The invention aims to provide a large-field far-ultraviolet spectrum imager.

In order to solve the technical problems, the technical scheme adopted by the invention is as follows:

a big visual field far ultraviolet spectrum imager which is characterized in that: the device comprises a Gray Gaokui telescope, a monochromator, imaging optics and a detector, wherein the monochromator separates an incident beam collected by the telescope into a plurality of wavelength channels along a dispersion direction, each separated channel is collected by an independent imaging lens to form a full-field image, and the images of different channels are finally formed in different areas of the same detector.

Further, the grignard telescope adopts an off-axis grignard telescope, the grignard telescope comprises a primary mirror and a secondary mirror, the primary mirror adopts a rectangular off-axis parabolic reflector, the length-width ratio of the rectangle is 10:1, the secondary mirror adopts an off-axis ellipsoidal mirror, the mouth surface is a long-strip rectangle, and the size of the mouth surface is far smaller than that of the primary mirror.

Furthermore, the primary mirror is an off-axis parabolic mirror with the aperture of 500 x 50mm, the focal length is 613.4mm, the secondary mirror is an off-axis ellipsoidal mirror with the aperture of 28 x 6mm, the magnification is 16.9, and the included angle between the symmetric axes of the primary mirror and the secondary mirror is 10.5 degrees.

Furthermore, the monochromator comprises an incident slit, a concave spherical grating and a plurality of emergent slits, wherein each emergent slit corresponds to one wavelength channel, the incident slit is positioned on an image of a main mirror formed by a secondary mirror, the concave spherical grating adopts a non-equidistant holographic grating, a substrate is spherical fused quartz glass, three-order diffraction is adopted, the equivalent linear density is 3000-4000 g/mm, and the emergent slits are positioned on an optical pupil plane formed at a certain distance behind the grating in parallel.

Further, the concave spherical grating adopts a spherical third-order holographic grating, the equivalent linear density is 3350g/mm, the aperture is 102 × 114mm, the radius of curvature of the substrate is 681.03mm, and under the condition that the spectral resolution ratio R =500 is achieved, the aperture of the entrance slit and the aperture of the exit slit are 23 × 2.3mm and 25 × 2.5mm respectively.

Furthermore, the imaging optics comprises a plurality of imaging mirrors, each imaging mirror corresponds to an exit slit of one wavelength channel, the imaging mirrors are off-axis ellipsoidal mirrors, the mouth surface is a rectangular with a long strip shape, and the off-axis angle is selected as the minimum off-axis angle for avoiding the light blocking of the detector.

Furthermore, the calibers of the imaging mirrors are 34 multiplied by 10.8mm, the off-axis angles are 40 degrees, and the three imaging mirrors are processed on the same mirror blank.

Furthermore, the detector is a planar MCP detector, a full-field image formed by each channel is separately imaged in different areas on the same planar MCP detector, and in a spectral scanning mode, the MCP detector, the imaging optical lens corresponding to each channel and the exit slit keep fixed relative positions and integrally move along a circle with an equal distance to the grating, so that different imaging wavelengths are selected.

Compared with the prior art, the invention has the following advantages and effects:

1) an off-axis Gregorian telescope is used for collecting light, and the outline of the mouth surfaces of a primary mirror and a secondary mirror of the telescope are both rectangular in a long strip shape;

2) separating the full-field light beam into a plurality of wavelength channels along the dispersion direction by using a large dispersion grating;

3) adjusting the spectral resolution by adjusting the widths of the entrance slit and the exit slit;

4) the entrance slit and the exit slit are both arranged on the pupil surface, and the grating is positioned at a position close to the Gregorian image surface;

5) the integral image quality is improved by optimizing the included angle of the symmetric axes of the primary mirror and the secondary mirror of the Gregorian telescope;

6) obtaining a complete spectral profile of an observation target through a wavelength scanning mechanism;

7) the spectral resolution is independent of the field of view characteristics, and large field of view and medium adjustable spectral resolution can be realized simultaneously.

Drawings

Fig. 1 is a light path diagram of a large field of view extreme ultraviolet spectral imager of the present invention.

FIG. 2 is an imaging profile of a detector of an embodiment of the invention.

FIG. 3 is a table of 80% energy concentration diameters for each channel of an embodiment of the present invention.

Detailed Description

The present invention is further illustrated by the following examples, which are illustrative of the present invention and are not to be construed as being limited thereto.

As shown in fig. 1, the large-field-of-view extreme ultraviolet spectral imager of the present invention comprises a griigly telescope, a monochromator, imaging optics and a detector. The monochromator separates the incident beam collected by the telescope into a plurality of wavelength channels (3 are shown in the figure) along the dispersion direction, and the bandwidth of the channels can be adjusted by the width of the incident and emergent slits of the monochromator so as to adapt to different spectral resolution requirements. And each separated channel is converged by an independent imaging lens to form a full-field image, and images of different channels are finally formed in different areas of the same MCP detector.

1) Gregorian telescope

The telescope is an off-axis griigy system in optical layout. The griigy system is used in order to form an optical pupil plane (image of the primary mirror) shortly behind the secondary mirror, and to place the entrance slit 3 of the monochromator at this position. The primary mirror 1 is a rectangular off-axis parabolic mirror, with the rectangular aspect ratio being designed to be 10: 1. The rectangular aperture of the elongated shape is used because the size of the pupil in the dispersion direction is required to be limited in order to be able to spatially separate the pupils of different wavelengths. The secondary mirror 2 is an off-axis ellipsoidal mirror, and the mouth surface is also a long-strip rectangle, but the size of the mouth surface is far smaller than that of the primary mirror. The field range of the telescope is matched with the focal ratio of the monochromator by designing the magnification of the Gregorian telescope. In addition, the design method improves the image quality by optimizing the included angle between the symmetric axes of the primary mirror and the secondary mirror, namely the off-axis astigmatism of the off-axis griigy system can just compensate the astigmatism generated by the off-axis imaging ellipsoid mirror by optimizing the included angle, so that the image quality in the full field of view is optimal. The griiglioy focus 8 is located between the entrance slit 3 and the concave spherical grating 4.

2) Monochromatic instrument

The monochromator system consists of an entrance slit 3, a concave spherical grating 4 and a plurality of exit slits 5, and each exit slit corresponds to a wavelength channel. The width of the entrance slit can be adjusted to change the bandwidth of the channel or the spectral resolution of the system. As described above, the entrance slit is located on the image of the primary mirror formed by the secondary mirror. This also means that when the slit width is adjusted to change the spectral resolution, the actually usable primary mirror area is also changing. When a narrower slit is used to achieve high spectral resolution, the effective collection area of the primary mirror is also reduced.

The grating adopts a non-equidistant holographic grating, the substrate is spherical fused quartz glass, and in order to obtain the largest possible dispersion ratio, three-order diffraction is adopted, and the equivalent linear density is 3000-4000 g/mm. The grating holographic parameters can also participate in the overall image quality optimization, but the effect of compensating the off-axis astigmatism of the imaging mirror by utilizing the grating holographic parameters is not obvious, mainly because the effective light spot size on the grating is small, and the generated astigmatism is far smaller than the contribution of the imaging ellipsoidal mirror. Because the grating is closer to the grignard focus, the line scale covered by the light spot on the grating is smaller, and certain influence is generated on the spectral resolution. Thus, in design, the grating is offset back from the grignard focus by a distance along the optical axis to increase the effective spot size on the grating. A finite spot size will result in a practical spectral resolution that is slightly lower than ideal, but with little impact on the signal-to-noise ratio of the system (little energy loss).

Due to the converging action of the concave grating, another optical pupil plane is formed at a certain distance behind the grating. A plurality of exit slits are positioned in parallel on the pupil surface, and each slit corresponds to a channel with a specific wavelength. Like the entrance slit, the width of the exit slit is adjustable to correspond to different spectral resolutions. The distance from each exit slit to the grating is equal, and scanning motion can be carried out along the circumference with equal distance from the grating, so that channels with different wavelengths can be selected.

3) Imaging optics and MCP detector

An imaging mirror 6 is correspondingly arranged behind the exit slit of each channel, and a full-field image formed by each channel is divided into different areas on the same plane MCP detector 7. The imaging mirror 6 is an off-axis ellipsoidal mirror, the mouth surface is a rectangular strip, and the off-axis angle is selected as the minimum off-axis angle for avoiding the light blocking of the detector. The final focal ratio of the system is determined by the effective detection area of the detector. To ensure that the images of the channels are in the same plane, the focal ratio of the channels will be slightly different. In the spectral scanning mode, the MCP detector 7, the imaging optical mirror 6 corresponding to each channel, and the exit slit 5 maintain fixed relative positions and move as a whole along a circle equidistant from the grating, thereby selecting different imaging wavelengths. Through the mechanism, wavelength scanning of an observation target can be realized, so that a complete spectrum profile of a detection target is obtained.

The invention provides a novel design method of an FUV imaging spectrometer. The spectral imager is essentially a multi-channel narrow-band imager, namely, a grating with large dispersion is used for separating a full-field light beam in the dispersion direction, a plurality of narrow-band spectral channels are formed by using an emergent slit, and then imaging optics are used for imaging each separated spectral channel respectively. And finally, the spectral characteristics in the full-wave band range can be obtained by combining wavelength scanning. On the technical index, the method can realize a large field of view with the diameter of 20 angular degrees or more, a spatial resolution superior to 30 angular seconds, and a medium and adjustable spectral resolution of 500-2000. In conclusion, the spectrometer is suitable for scientific application with large field of view and medium spectral resolution requirements.

The invention is further illustrated by the following specific examples:

the design method provided by the invention designs a set of optical scheme for the HI Ly-alpha narrow-band imager in the ultraviolet space detection project CAFE (center of WHIM, Acction, feed Explorer). The imager covers a wave band of 124.9-127.7 nm and is divided into three wavelength channels, the central wavelength interval of each channel is 1nm, the channel bandwidth is adjustable within 0.25-0.06 nm, and the spectral resolution corresponds to 500-2000. The imaging field of view of each channel is a circular field of view with 20 angular diameters, and the spatial resolution is better than 21 arc seconds.

The designed light path is shown in fig. 1. The primary mirror is an off-axis parabolic mirror with the aperture of 500 multiplied by 50mm, and the focal length is 613.4 mm. The secondary mirror is an off-axis ellipsoidal mirror with the aperture of 28 multiplied by 6mm and the magnification of 16.9. The included angle between the symmetric axes of the primary mirror and the secondary mirror is 10.5 degrees after optimization. The grating adopts a spherical third-order holographic grating, the equivalent linear density is 3350g/mm, the caliber is 102 multiplied by 114mm, and the curvature radius of the substrate is 681.03 mm. Under the spectral resolution condition of R =500, the aperture of the entrance slit and the aperture of the exit slit are 23 × 2.3mm and 25 × 2.5mm, respectively. When higher spectral resolution is required, the narrower slits can be replaced with slit switching devices. Three ellipsoidal imaging mirrors are arranged behind the three exit slits. The size of the imaging lens is 34 multiplied by 10.8mm, the off-axis angle is 40 degrees, and the three imaging lenses can be processed on the same lens blank. The resulting three channels are distributed as shown in fig. 2. The resulting focal ratios for the three channels were F/3.58, F/3.4 and F/3.22, respectively, each channel covering an imaging area of approximately 10X 10 mm. The MCP detector, the imaging optical lens corresponding to three channels and the exit slit keep fixed relative positions and can integrally move along a circle with the center of the grating as an origin and the radius of 731mm, so that the wavelength scanning function is completed.

Ray tracing simulation was performed on the HI Ly- α narrowband imager optical system using ZEMAX software, and 80% energy concentration diameter over each field of view was calculated for two spectral resolution conditions (R =500 and R = 2000) in the 125.3nm, 126.3nm and 127.3nm channels, as shown in fig. 3. It can be seen from the table that if the 80% energy concentration diameter is used as the standard for measuring image quality, the image quality in the full field of view of 20 angles is within 21 arc seconds.

The above description of the present invention is intended to be illustrative. Various modifications, additions and substitutions for the specific embodiments described may be made by those skilled in the art without departing from the scope of the invention as defined in the accompanying claims.

Claims

1. A big visual field far ultraviolet spectrum imager which is characterized in that: the system comprises a Gray Gaokui telescope, a monochromator, imaging optics and a detector, wherein the monochromator separates an incident beam collected by the telescope into a plurality of wavelength channels along a dispersion direction, each separated channel is collected by an independent imaging lens to form a full-field image, and the images of different channels are finally formed in different areas of the same detector;

the griigly telescope adopts an off-axis griigy telescope, the griigy telescope comprises a primary mirror and a secondary mirror, the primary mirror adopts a rectangular off-axis parabolic reflector, the length-width ratio of the rectangle is 10:1, the secondary mirror adopts an off-axis ellipsoidal mirror, the mouth surface is a long-strip rectangle, and the size of the mouth surface is far smaller than that of the primary mirror;

the monochromator comprises an incident slit, a concave spherical grating and a plurality of emergent slits, wherein each emergent slit corresponds to a wavelength channel, the incident slit is positioned on an image of a primary mirror formed by a secondary mirror, the concave spherical grating adopts a non-equidistant holographic grating, a substrate is spherical fused quartz glass, three-order diffraction is adopted, the equivalent linear density is 3000-4000 g/mm, and the emergent slits are positioned on an optical pupil surface formed at a certain distance behind the grating in parallel;

the imaging optics comprises a plurality of imaging mirrors, each imaging mirror corresponds to an emergent slit of one wavelength channel, the imaging mirrors are off-axis ellipsoidal mirrors, the mouth surfaces of the imaging mirrors are rectangular, and off-axis angles are selected as the minimum off-axis angles for avoiding light blocking of the detector;

the detector is a planar MCP detector, full-field images formed by each channel are separately imaged in different areas on the same planar MCP detector, and in a spectral scanning mode, the MCP detector, the imaging optical lenses corresponding to the channels and the emergent slits keep fixed relative positions and integrally move along a circle with equal distance to the grating, so that different imaging wavelengths are selected.

2. A large field of view extreme ultraviolet spectral imager as claimed in claim 1, wherein: the primary mirror is an off-axis parabolic mirror with the aperture of 500 multiplied by 50mm, the focal length is 613.4mm, the secondary mirror is an off-axis ellipsoidal mirror with the aperture of 28 multiplied by 6mm, the magnification is 16.9, and the included angle between the symmetric axes of the primary mirror and the secondary mirror is 10.5 degrees.

3. A large field of view extreme ultraviolet spectral imager as claimed in claim 1, wherein: the concave spherical grating is a spherical third-order holographic grating, the equivalent linear density is 3350g/mm, the aperture is 102 x 114mm, the curvature radius of the substrate is 681.03mm, and under the condition that the spectral resolution ratio R =500 is high, the aperture of an incident slit and the aperture of an emergent slit are 23 x 2.3mm and 25 x 2.5mm respectively.

4. A large field of view extreme ultraviolet spectral imager as claimed in claim 1, wherein: the calibers of the imaging mirrors are 34 multiplied by 10.8mm, the off-axis angles are 40 degrees, and the three imaging mirrors are processed on the same mirror blank.