Astronomical Telescope System Research: Test a Coronagraphic Instrument Designed for Direct Imaging of Exoplanets with a Supercontinuum Laser

Since Mayor and Queloz’s groundbreaking discovery in 1995 of a planet orbiting a star like 51 Pegasi, followed by Butler and Marcy’s detection of planets around 47 Ursae Majoris and 70 Virginis in 1996, the hunt for exoplanets has grown into a well-established field within astrophysics [1]. The discovery of hundreds of exoplanets over the past two decades has provided fresh insights into planetary structure. The planets in our solar system are no longer considered the definitive models for planetary formation and evolution; they represent just one of many possible outcomes. Exoplanets, or planets that orbit stars beyond our solar system, are now recognized as common in our galaxy, exhibiting a vast array of physical characteristics. These characteristics range from extremely puffy gas giants to dense rocky planets with densities comparable to iron [2].?

Astronomers are currently characterizing exoplanetary properties, identifying correlations between a star’s classification and the types of planets orbiting it, and are beginning to analyze the atmospheres of exoplanets. These studies suggest that our solar system’s planets may be atypical in terms of their diversity, composition, or orbits. This has introduced an era of comparative planetology, signaling a significant shift in our understanding of planetary formation and evolution that is still in its early stages [3].?

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From Distances Spanning Many Light Years, How Do We Gather Insights Into the Internal Composition and Structure of Exoplanets?

Instead of developing entirely new facilities, there is a proposal to utilize existing instruments that excel in high-contrast imaging or high-dispersion spectroscopy, integrating them via optical fibers.??

Achieving this objective poses a significant challenge due to the narrow angular distance and brightness contrast compared to their parent stars, in fact researchers use coronagraphic instruments which are designed and used to block out the bright light from stars so that faint nearby objects like exoplanets can be observed. The effectiveness of a coronagraph depends on its ability to block specific wavelengths of light while allowing others to pass through? [4].?

But before reaching exoplanets and stars, researchers proceed with the Space Coronagraph Optical Bench (SCoOB) which is a specialized testbed designed to showcase techniques for suppressing starlight in visible wavelengths, simulating conditions like those found in space within a vacuum environment [5].?

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Experimental Set Up: High-Precision Optical Experiments.

The researchers from the University of Arizona used the Iceblink Supercontinuum light source, (Figure 1), which is fed into a Thermal Vacuum Chamber (TVAC), which simulates the vacuum and thermal conditions of space. The light first passes through a linear polarizer to ensure it is linearly polarized, it then goes through a Quarter Wave Plate (QWP) that converts the linearly polarized light into circularly polarized light. This circularly polarized light is directed to a spatial filter using a pair of achromatic doublets, which are lenses designed to reduce chromatic aberration by focusing different wavelengths of light to the same point. The light then enters a tip/tilt mirror, which controls the angle and alignment of the beam, and the entrance pupil is defined at this point.?

Next, the light is re-imaged onto the Kilo-C Deformable Mirror (DM), a mirror with numerous small actuators that adjust its shape to correct optical aberrations. The light beam, now at an f/48 focal ratio, passes through a focal plane mask (FPM) that selectively blocks parts of the beam to enhance image quality and contrast.?

A 95% Lyot stop, which blocks unwanted light, is placed in a conjugate pupil plane, allowing most of the light to pass through while reducing glare. Part of the light is reflected to a Low-order Wavefront Sensor (LLOWFS) to measure and correct for lower-order aberrations in the wavefront. The remaining transmitted light passes through a field stop in a focal plane, limiting the field of view. It then passes through a circular analyzer, which consists of another QWP and LP, ensuring the light is correctly polarized for detection. Finally, the light is focused onto a science camera, where the image or data is captured [5].?

Iceblink Supercontinuum Fiber Laser for Test Space Coronagraph Optical Bench.

Using a supercontinuum light source like the Iceblink with the Boreal accessory in the SCoOB setup enhanced the experiments of the researchers from the University of Arizona in several ways:?

• Power stability (<0.5%) is essential for maintaining consistent experimental conditions. It minimizes fluctuations in light intensity, leading to more reliable and reproducible data. This consistency is crucial for calibrating optical components like the Kilo-C Deformable Mirror and Low-order Wavefront Sensor, ensuring accurate aberration correction and precise measurements which is crucial for detailed analysis and interpretation in the SCoOB setup;?

• Broad Spectral Coverage from 450 nm to 2300nm: this broad spectral range allows for the investigation of optical properties across visible, near-infrared, and part of the mid-infrared spectrum. It enables comprehensive studies of phenomena that have different responses at different wavelengths, providing a more complete understanding of the subject under study;?

• Adequate VIS Power of 150 mW: Having this significant power in the visible range is important for imaging and spectroscopic experiments. It ensures that there is enough illumination for capturing high-quality images and detailed spectral data, which are critical for identifying and analyzing specific features or components within the experiment;?

• Precise Wavelength Selection: thanks to the Boreal accessory there is the possibility to select specific wavelengths within the visible spectrum allows for targeted experiments on wavelength-dependent phenomena. It enables high-contrast imaging and detailed spectroscopic analysis by isolating specific wavelengths, reducing background noise, and enhancing the clarity of the detected signal. This is particularly useful for studying details and subtle effects in the SCoOB setup.?

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Resources.?

[1] S. Udry and N. C. Santos, “Statistical properties of exoplanets,” in Annual Review of Astronomy and Astrophysics, Annual Reviews Inc., 2007, pp. 397–439. doi: 10.1146/annurev.astro.45.051806.110529.?

[2] D. S. Spiegel, J. J. Fortney, and C. Sotin, “Structure of exoplanets,” Proc Natl Acad Sci U S A, vol. 111, no. 35, pp. 12622–12627, Sep. 2014, doi: 10.1073/pnas.1304206111.?

[3] A. S. Burrows and G. W. Marcy, “Exoplanets,” Proceedings of the National Academy of Sciences of the United States of America, vol. 111, no. 35. National Academy of Sciences, pp. 12599–12600, Sep. 02, 2014. doi: 10.1073/pnas.1409934111.?

[4] A. Vigan et al., “First light of VLT/HiRISE: High-resolution spectroscopy of young giant exoplanets,” Astron Astrophys, vol. 682, Feb. 2024, doi: 10.1051/0004-6361/202348019.?

[5] K. Van Gorkom et al., “The space coronagraph optical bench (SCoOB): 4. vacuum performance of a high contrast imaging testbed,” Jun. 2024, [Online]. Available: https://arxiv.org/abs/2406.18885 ?