JKAS Archive

Journal of the Korean Astronomical Society - Vol. 51 , No. 2

[ Article ]
Journal of the Korean Astronomical Society - Vol. 51, No. 2, pp.27-36
Abbreviation: JKAS
ISSN: 1225-4614 (Print) 2288-890X (Online)
Print publication date 30 Apr 2018
Received 20 Oct 2017 Accepted 20 Mar 2018
DOI: https://doi.org/10.5303/JKAS.2018.51.2.27

TOWARD A NEXT GENERATION SOLAR CORONAGRAPH: DIFFRACTED LIGHT SIMULATION AND TEST RESULTS FOR A CONE OCCULTER WITH TAPERED SURFACE
HEESU YANG1 ; SU-CHAN BONG1, 2 ; KYUNG-SUK CHO1, 2 ; SEONGHWAN CHOI1 ; JONGYEOB PARK1 ; JIHUN KIM1 ; JI-HYE BAEK1 ; JAKYOUNG NAH1 ; MINGZHE SUN3 ; QIAN GONG4
1Korea Astronomy and Space Science Institute, 776 Daedukdae-ro, Yuseong-gu, Daejeon 34055, Korea (hsyang@kasi.re.kr)
2University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Korea
3Sandong University, 180 Wenhua Xilu, Weihai, China
4NASA Goddard Space Flight Center, Greenbelt, Maryland, USA

Correspondence to : H. Yang


JKAS is published under Creative Commons license CC BY-SA 4.0.
Funding Information ▼

Abstract

In a solar coronagraph, the most important component is an occulter to block the direct light from the disk of the sun. Because the intensity of the solar outer corona is 10-6 to 10-10 times of that of the solar disk (I), it is necessary to minimize scattering at the optical elements and diffraction at the occulter. Using a Fourier optic simulation and a stray light test, we investigated the performance of a compact coronagraph that uses an external truncated-cone occulter without an internal occulter and Lyot stop. In the simulation, the diffracted light was minimized to the order of 7.6×10-10 I when the cone angle θc was about 0.39°. The performance of the cone occulter was then tested by experiment. The level of the diffracted light reached the order of 6×10-9 I at θc = 0.40°. This is sufficient to observe the outer corona without additional optical elements such as a Lyot stop or inner occulter. We also found the manufacturing tolerance of the cone angle to be 0.05°, the lateral alignment tolerance was 45 μm, and the angular alignment tolerance was 0.043°. Our results suggest that the physical size of coronagraphs can be shortened significantly by using a cone occulter.


Keywords: Sun: corona, Sun: solar wind, instrumentation: photometers

1. INTRODUCTION

Various dynamic properties of the solar corona, such as Solar Energetic Particles (SEPs), and Coronal Mass Ejections (CMEs), have a significant effect on the earth's magnetosphere and satellites. To create a warning system on the earth, it is necessary to track the initial phenomenon on the Sun. A coronagraph is an instrument that images the solar corona so that its dynamics can be understood. Since the corona is 10-6 to 10-10 times fainter than the intensity of the solar disk, an occulting disk must be used to create an artificial eclipse.

Classically, an internal occulter is used for inner corona observations, while an external occulter is used together with the internal occulter for outer corona observations. Such a combined internal and external occultation system is used in the Large Angle Spectroscopic Coronagraph (LASCO) C2 and C3 (Brueckner et al. 1995). In the system, the diffracted light generated in the external occulter is rejected by the internal occulter. The scattered light occurring at the primary lens is also blocked by the Lyot spot and the Lyot stop. Thanks to an advanced scattering/diffraction rejection mechanism, the C2 and C3 achieve sufficient signal to noise ratio that allows the observation of coronal dynamics. However, the overall length of the C2 and C3 needed to be extended to make multiple images and pupil planes. If, on the other hand, the diffracted light from the external occulter and the scattered light due to the primary lens are suppressed, the coronagraph can be shortened without a need for the formation of multiple images and pupil planes.

To minimize diffracted light in the external occulter, several rejection concepts, such as toothed or multiple discs, have been suggested. Theoretical and experimental studies have proposed that the diffracted light level of a triplet disk is on the order of 10-5 (Bout et al. 2000; Gong & Socker 2004), and that of the multi-thread disk is on the order of 10-6 (Koutchmy 1988; Bout et al. 2000; Thernisien et al. 2005). This is much smaller than the order of 10-3 of the singlet disk (Fort et al. 1978; Lenskii 1981; Koutchmy 1988). A toothed external occulter can also suppresses diffracted light efficiently (Fort et al. 1978; Koutchmy 1988; Verroi et al. 2008; Sun et al. 2013), but the performance is likely limited due to vignetting of the teeth.

The multi-thread, or truncated-cone-shaped occulter, is expected to have performance similar to an infinite stack of occulting disks. As described above, a level of 10-6 diffracted light was achieved using it and it was adopted in the LASCO C2 (Bout et al. 2000). But by optimizing the angle of the side of the cone occulter (hereafter cone angle θc), the diffracted light can likely be significantly reduced compared to the previous result. Gong & Socker (2004) performed a simulation of the multiple disk external occulter and investigated the rule of the optimization of the cone angle. Thernisien et al. (2005) also obtained a similar result in their simulation. Additionally, he suggested that if the side of the cone had an arc shape, it may be more efficient.

In this paper, we simulated an occulter designed in the shape of the truncated-cone with a tapered surface (hereafter the cone occulter) and estimated its performance considering the conceptual design of an externally occulted coronagraph (hereafter compact coronagraph) using a Fourier optics based simulation. We also demonstrated its performance in a laboratory experiment. The diffracted light from the cone occulter can be suppressed by the order of 10-10 I, whereas we achieved 10-9 I in the experiment. Our simulations and the experiments are intended for the next generation space-borne coronagraph described in Cho et al. (2017).

In Section 2, we describe the simulation result. We present the experimental results in Section 3. Finally, we summarize our result in Section 4.


2. FOURIER OPTICS SIMULATION USING GLAD
2.1. Specification of the Simulation

We investigate the performance of a compact coronagraph that consists of an external truncated-cone occulter and a simple lens group. Table 1 describes the specifications of the compact coronagraph. To miniaturize, we adjusted the optical path from the external occulter to an image plane to be shorter than 50 cm. The aperture size was enlarged to 40 mm to increase the signal. The wavelength of light was fixed at 0.4 μm because we intended to employ the temperature measuring method in Cho et al. (2016). We determined the height of the occulter to be 60 mm. The diameter of the occulter was about 47.58 mm at the bottom of the cone, and the diameter at the front was a bit larger than that.

Table 1 
Parameters of the simulation
Parameter Value Unit
Total length 496 mm
Optical system size 142 mm
Aperture size 40 mm
External occulter front diameter 47.58 mm
Height of the cone occulter 60 mm
Central wavelength 0.4 μm
Simulation array size 214 × 214 pixels×pixels
FOV of the Simulation 80 × 80 mm × mm

The diffracted light simulation was performed with the aid of Fourier optics-based software, General Laser Analysis & Design (GLAD) of Applied Optics Research (Landini et al. 2005). The GLAD works in both of the Fraunhofer and the Fresnel diffraction regime. Our simulation for the cylindrical occulter satisfied the condition for a Fourier optics simulation: the direction of the beam entering the coronagraph was well defined and the light was coherent.

The sampling in our simulation was 4.88 μm at the first occulting disk plane. The sampling was reasonable considering the diffracted pattern at the edge of the occulting disk. Figure 1 shows the simulated intensity 7.5 mm behind the occulting disk. The diffraction generated at the edge of the occulter creates a ring-shaped (or oscillatory) pattern as shown in the figure. Compared with the sharp edge in the analytical solution, it describes oscillatory patterns near the edge of the occulting disk well. The first four to five oscillatory patterns (m < 4 - 5) are drawn with more than four pixels. The outer parts (m > 4 - 5), however, are drawn with only a few pixels. But this is not likely to cause significant error in our result because the amplitude variations are much smaller than the oscillatory patterns near the edge.


Figure 1. 
Simulated intensity curve 7.5 mm behind the single occulting disk from the center to the edge of the plane. It shows a typical oscillatory pattern.

Light from the solar limb enters with an angle of about 0.25° as shown in Figure 2(a). But GLAD works only with a well-defined collimated coherent on-axis beam. We solved the off-axis beam passing through the aperture by tilting the overall optical system, as shown in Figure 2(b). Then the incident ray can be considered horizontal. This assumption might cause a small margin of error since the structure of the simulated multi-disk occulter differs from the actual cylindrical structure.


Figure 2. 
Schematic diagram of (a) the overall optical system and the ray incident with an angle δ and (b) the overall optical system tilted by δ instead of the horizontal incidence ray.

Figure 3 shows the simulated intensity derived for different incident angles δ. We simulated the off-axis beam coming from the solar disk by dividing the region into off-axis angles of 0.025°, 0.075°, 0.125°, 0.175°, and 0.225°.


Figure 3. 
Intensity maps at the image plane derived from the various incident angles δ. The circle at the right bottom represents the solar disk and the position of the simulated ray (black dots). The area including the dots is multiplied by the simulated result to weight the contribution of each ray.

The intensity images derived from the different incident angles were multiplied by the area of the solar disk for 10° to weight the contribution. We also considered limb darkening for the off-axis incident ray. Then we summed each intensity image after rotating the intensity image every 10° over 360°. Note that the light coming from the solar limb (δ = 0.225°) causes a large diffraction compared to the symmetric condition (δ = 0°).

Assuming the cone occulter was the same as an infinite stack of occulting disk, we simulated the doublet, triplet, quintet, and nonet occulter. We extrapolated them to derive the diffracted light of the cone occulter. We chose to simulate the doublet, triplet, quintet, and nonet occulter because of the equal gap between the occulting disks, as shown in Figure 4. For example, the nonet occulter has a gap of 7.5mm between each occulting disk, the quintet has 15 mm, triplet 30 mm, and doublet 60 mm. We fitted the intensity of the series of the occulters to the equation Y = P0/(X - P1) + P2, where X is the number of the occulter, Y is the intensity, and P0, P1, and P2 are the fitting parameters. Then we inferred the result of the cone occulter from the fitted value of P2. We omitted the intensity of the singlet occulter in our extrapolation because it does not have a mechanism to block diffracted light.


Figure 4. 
Schematic diagram of the simulated doublet, triplet, quintet, and nonet occulter.

2.2. Simulation Results

The front part of the occulter blocks the disk light, and the diffracted light at the edge of the occulter is captured using the cone structure. Therefore the cone angle θc is important to minimize the diffracted light. Figure 5(a) represents the intensity curves of the various cone angle occulters at the image plane. Note that the curves are the results for a triplet of occulters so that the level of the diffracted light is only on the order of 10-8 I.


Figure 5. 
(a) Intensity curves at the image plane for various angles θc of the triplet occulter. From black to red, the occulter angle changes from 0.3° to 0.5°. (b) Peak intensity around 3 R versus cone angle. The red solid and dotted lines represent the lowest point and its 10% intensity variation point of the peak intensity.

The peak intensity around 3 R for the different cone angles is shown in Figure 5(b). The peak intensity is minimized at a cone angle of θc = 0.39° in the simulation. We determined the mechanical tolerance of the cone angle to be 0.05° by requiring that the additional irradiance caused by the mechanical error was one-tenth of its original value. In other words, a cone angle change of 0.05° causes a 10% increase in peak intensity. For the conditions of the given optical system, 0.05° corresponds to about a 52 μm difference in radius from the bottom to the top of the cone occulter. 52 μm is commercially available precision.

Figure 6 shows the simulated intensity map of a nonet (a nine disk) occulter at the image plane. The inner circle diameter of 2 R filled with bright dots represents Arago spots. The Arago spots are caused by the in-phase intensity at the center of the geometrical shadow of a circular obscuration (Harvey & Forgham 1984). These are virtual because we simulated a parallel beam that creates a bright Arago spot. If one considers an extended source like the sun, the intensity of this is negligible. The intensity at R < 3 is blocked by the occulter, and the intensity increases and shows ring-shaped pattern at around 3 - 4 R. This ring-shaped, or oscillatory pattern is the diffracted light caused by the cone occulter and the aperture stop.


Figure 6. 
Simulated intensity map of the nonet occulter on a logarithmic scale to enhance the contrast of the ring-shaped pattern around 3 − 4 R. The cone angle of the occulter is 0.39°.

Figure 7 represents the result of the extrapolation for a cone angle of 0.39°. The intensity decreases with the number of occulting disks from the order of 10-6 I for a singlet occulter to the order of to 10-10 I for the cone occulter. The vignetting effect is reflected in the corona brightness curve. For the cone occulter curve, the diffracted light overwhelms the corona brightness in the range of 2.5 - 4 R. But it is not necessary to concern because the photon noise of the diffracted light that we only have to consider is much lower than the corona brightness.


Figure 7. 
Diffracted light for singlet, doublet, triplet, quintet, nonet and infinite occulter (cone occulter) system. The black dotted line represents the corona brightness by November & Koutchmy (1996) considering the vignetting effect.


3. DIFFRACTED LIGHT TEST IN DARK TUNNEL

We tested our coronagraph to determine the performance of the cone occulter. The test was conducted at the class 1000 clean dark tunnel installed at Sandong University, Weihai, China. Figure 8 shows the basic geometric mechanism of the coronagraph including the solar simulator. The solar simulator included a 1 kW powered Xenon lamp and a collimating lens of 150 mm diameter. It was installed at the end of the tunnel. The coronagraph was installed at 10 meter distance from the collimating lens considering the beam height. The intensity of the Xenon lamp varied with time so that we could calibrate the intensity using the photodiode power meter beside the coronagraph.


Figure 8. 
Test setup of the coronagraph and the solar simulator. Not to scale.

The light beam encounters successive parts in the order occulter, the aperture stop, the neutral density (ND) filter, the lens, the short bandpass filter, and the camera. The external cone occulter blocks the sun's light from directly entering the lens. The cone occulters in our experiment have a threaded surface to reduce the diffracted light (Bout et al. 2000). Its pitch was 0.25 mm and the occulter had about 80 threads. The occulter was micro-positioned using a 6-axis table (Hexapod, Hexapod, PI Physik Instrumente, Germany), HX-811K. Using the 6-axis table, we positioned the occulter with a resolution of 0.1 μm in x-, y-direction, 0.05 μm in z-direction, and 0.02arcsec in rotation. From the external occulter to the aperture stop, a threaded baffle encases the light path to reduce scattering. We also created a threaded surface at the aperture stop rather than the reflection mirror, to minimize development effort.

To calibrate the level of the diffracted light intensity, we measured the intensity of the artificial solar disk with an ND filter installed in front of the lens. After removing the occulter, we installed a reflective 10-3 ND filter overlapped with the reflective 10-4 ND filter so that the light was attenuated by the order of 10-7 magnitudes. The ND filters were manufactured by Andover. Because the level of diffraction was expected to be smaller than 10-9 I, we adjusted the exposure time to record the appropriate intensity.

We used a commercial camera lens Hasselblad HC 4/210 to reduce the development effort. It has 10 elements in 6 groups and an F/4 configuration; vignetting is smaller than 5% in our FOV of interest of 6 R so that we did not consider vignetting in data processing. The camera lens was focused at infinity and the focus was tested by observing the pointing source.

After that, the light passes the short bandpass filter with 10nm bandwidth centered at a wavelength of 423.3nm manufactured by Andover. Then the light is focused on the image plane of the CCD PL9000 manufactured by Finger Lakes Institute. Several specifications associated with our experiment are listed in Table 2.

Table 2 
Experiment specification
Parameter Value Unit
Aperture diameter π * (20 * 10−3)2 m2
Filter wavelength 423.3 nm
Filter width 10 nm
Transmission of the filter > 0.5
Transmission of the lens > 0.9
CCD Quantum efficiency > 0.3

Note that we installed a 10nm bandpass filter centered at the wavelength of 423.3nm in the light path because it will be used in the coronagraph targeted in this paper. The electron temperature of the corona can be measured using the intensity ratio of two bandpass filters (Cho et al. 2017).

Figure 9 shows the coronagraph installed on the optical bench. To align the coronagraph parallel to the light beam, we used lab jacks and micro-positioning stages. The occulter was installed on the 6-axis table. Each occulter was aligned by checking the diffracted light recorded in the camera. The ND filter was deployed between the aperture stop and the commercial lens with the filter wheel. The position of the ND filter was chosen because a large amount of the reflected light in the ND filter is likely to produce scattering when the ND filter is installed behind the lens.


Figure 9. 
Coronagraph on the optical bench.

Six cone occulters were manufactured with cone angles of 0.35°, 0.37°, 0.39°, 0.41°, 0.43°, and 0.45°, respectively. Their surfaces were anodized with T4. Table 3 shows the measured cone angles of the cone occulters. The diameters were precisely measured with a coordinate measuring machine that has a resolution of 0.1 μm with an accuracy of 2 μm manufactured by Dukin, South Korea.

Table 3 
Nominal and actual cone angles of the manufactured cone occulters
Nominal
cone angle
(θc)
Front
diameter
Rear
diameter
Measured
diameter
0.35 47.5785 46.8487 0.3494
0.37 47.5788 46.8041 0.3695
0.39 47.5786 46.7559 0.3905
0.41 47.5790 46.7225 0.4092
0.43 47.5782 46.6795 0.4297
0.45 47.5809 46.6370 0.4507

3.1. Experiment Result
3.1.1. Cone Angle Test

First, we tested the performance of each cone occulter. Figure 10 shows the intensity maps of the cone occulters at the image plane. The bright ring represents the light diffracted from the individual occulters. The light from the corona at the inner part of the circle is blocked completely, whereas the coronal light reaches the outer part with the vignetting effect of the occulter. In our experiment, the diffracted light has a peak at around 2.5 R and it rapidly decreases with radial distance to the background intensity. The level of the background is about 10-8 I and the level of the diffracted light is about a few of 10-9 I.


Figure 10. 
Measured intensity maps of the six cone occulters at the image plane.

The bright arc structures that are tangent to the circle of the diffracted light represent the light scattered by dust at the surface of the occulter and the aperture stop. With the 0.45° occulter, we failed to clean the surface, so that it was seriously contaminated by dust. Sometimes this scattered light overwhelmed the diffracted light. The faint ellipse structure at the right of the diffraction circle represents the light scattered by the pylon, and the rectangular shape represents the light scattered by the frame of the dark tunnel.

Figure 11(a) represents the measured intensity curve from the image center to the limb. The average of each radius was used to derive the graph. The peak intensity is located at around 2.5 R, and the diffraction oscillatory pattern is identified at > 3 R in the simulation result in Figure 7. The height of the peak reaches a few 10-9 I as mentioned in the previous paragraph. Notably, the diffraction peak of the 0.39° occulter is about 6×10-9 I, which is about an order of magnitude larger than the simulation. The peak height of the order of 10-9 is comparable with the performance of the nonet occulter. We note that, even though we calibrated the results using the intensity variation recorded by the power meter, the background intensities of the occulters varied on the order of 10-10 I.


Figure 11. 
(a) Intensity curves from the image center to the limb of the six occulters and (b) peak intensity versus the cone angle of the occulter. The intensity of diffracted light is minimized at θc = 0.40°. Error bars are 1σ errors.

Figure 11(b) represents the peak intensity at around R = 2.5 R versus the cone angle of the occulter. The minimized cone angle is 0.4° which in the simulation was expected to be around 0.39°. This discrepancy might come from the surface roughness of each occulter. The diffracted light on the order of 10-9 also indicates that the surface is not the same as the ideal case.

A variation in the cone angle by 0.05° increased the diffracted light of 1×10-9 I, which is 1/6 of the total diffracted light of 6×10-9 I. This is in good agreement with the simulation result in subsection 2.2. We set 10-9 I for the tolerance intensity. This might be bigger than 10% of the overall intensity in the simulation, however, a diffracted light of 10-9 I allows a suitable signal to noise ratio (Cho et al. 2016). In this regard, we conclude that the cone occulter is tolerated to the manufacturing error of the cone angle of 0.05°.

We note that the level of diffracted light in the test was not minimized. The occulters were shifted and tilted from the best-aligned position. This is because the occulter was aligned to the level of the diffracted light recorded in the image with a short exposure time. The next two subsections show that the 0.39° occulter in this test was misaligned by 100 μm in the x-direction and 0.043° in the xy-plane.

3.1.2. Lateral Alignment Tolerance

We measured the lateral alignment tolerance by shifting the 0.39° occulter vertical to the beam. Figure 12(a) shows the diffracted light curves when the occulter was shifted from x = -0.7 mm to 0.8 mm. x = 0 represents the position used in previous cone angle test. The light curve was obtained by averaging over all radii within a sector of 22.5° around the parallel axis as shown in Figure 10(b). The left wing of the diffracted light decreased exponentially while the right wing increased following the motion of the occulter.


Figure 12. 
(a) Diffracted light curves after shifting the 0.39° occulter vertical to the beam and (b) peak intensity of the diffracted light curves of the left (blue) and right (red) wings.

Figure 12(b) represents the peak intensity of the diffracted light curves of the left (blue) and right (red) wings. The diffracted light of both wings decreased and increased exponentially when the occulter was shifted. The diffracted light of both wings was minimized at x = -0.1 mm. Around x = -0.1 mm, the lateral alignment tolerance was 45 μm in both directions by restricting the diffracted light variation of 10-9 I.

3.1.3. Angular Alignment Tolerance

We measured the angular alignment tolerance by tilting the occulter vertical to the beam for the lateral tolerance measurement. Figure 13(a) represents the light curves when the occulter was tilted from θc = -0.3° to 0.4°. The light curve was obtained by averaging over all radius within 22.5° sectors around the parallel axis. The diffracted light of both wings was increased by tilting the occulter. The peaks of each wing are plotted in Figure 13(b). The logarithmic intensities of the diffracted light exhibit parabolic shapes. The angular alignment tolerance was 0.043° near the aligned position (θc = 0.05°) by restricting the diffracted light variation of 10-9 I.


Figure 13. 
(a) Diffracted light curves observed after tilting the 0.39° occulter vertical to the beam and (b) peak intensity of the diffracted light curves of both wings.


4. SUMMARY AND CONCLUSION

We investigated the diffracted light generated by a cone occulter with a tapered surface. We performed simulations to determine the amount of diffracted light, which was estimated by extrapolating results for doublet, triplet, quintet, and nonet occulter systems. In the compact coronagraph configuration, 0.39° was the optimum cone angle θc for the diffracted light. Ideally the diffracted light can be suppressed down to the order of 10-10 I at θc = 0.39°.

The cone angle was tested in a dark tunnel experiment. The diffracted light was minimized when the cone angle was 0.40°. The tolerance of the cone angle was determined to be about 0.05° considering the level of the diffracted light of 10-9 I.

Considering a diffracted light level of 10-9 I, the tolerances of the lateral and angular alignments were measured to be about 45 μm and 0.043°, respectively. The logarithmic intensities of the diffracted light produced by shifting and tilting the cone occulter exhibited a parabolic shape. This result indicates that the cone occulter is more insensitive to its alignment offset than reported by Bout et al. (2000) near the aligned position. Bout et al. (2000) suggested that the variations in the diffracted light could be fitted with a parabola.

Our result shows that the truncated cone occulter with a tapered surface reduces the diffracted light of an externally occulted coronagraph to permit the observation of the corona of > 2.5 R. Because the coronal brightness is vignetted by the external occulter, the level of coronal brightness reaches about 10-10 I (see Figure 7). Even though the diffracted light, on the order of 10-9 I is higher than the coronal brightness, it is sufficient to obtain a reasonable signal to noise ratio, in view of the fact that it is necessary to consider the photon noise of the diffracted light, not the overall brightness.

In our experiment, the level of scattered light overwhelmed the diffracted light. The diffracted light was reduced as small as the order of 10-9 I, whereas the scattered light reached about 10-8 I. Because the intensity level at the shade of the occulter also reached that amount, we conjecture that most of the scattered light comes from inside the optics. This can be improved by using a rejection mirror.


Acknowledgments

This work was supported by the Korea Astronomy and Space Science Institute under the R&D program 2018185003 supervised by the Ministry of Science and ICT. This work was also supported by the National Natural Science Foundation of China (Grant No. 41627806).


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