<figure><img src="image_1.png" /></figure>

# Ultra wide-field infrared astronomy in Antarctica

Tony Travouillon<fnref n="1" /> ID | Roger M. Smith<fnref n="2" /> | Jason Fucik<fnref n="2" /> | Don F. Figer<fnref n="3" /> |

Mansi Kasliwal<fnref n="4" /> | Anna M. Moore<fnref n="1" /> | Tristan Guillot<fnref n="5" />

**Correspondence**

Tony Travouillon, Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT, Australia.

Email: tony.travouillon@anu.edu.au

**Present address**

Tony Travouillon, RSAA, Mount Stromlo Observatory, Weston Creek, Australian Capital Territory Australia

**Funding information**

Ames Research Center, Grant/Award

Number: NNX13AH70G; National

Science Foundation, Grant/Award

Numbers: 1207827, 1509716, AST 2010041

The science enabled by the deep and high-cadence survey that will be performed by the Vera Rubin Observatory has led to an increase of survey and follow-up capabilities around the world. The infrared, has however, not match this growth due to the challenges caused by the atmospheric and the cost of large detector arrays. In this paper, we present solutions to resolve these challenges and a path toward an Antarctic observatory capable matching the volumetric speed of the Vera Rubin Observatory survey in the infrared k-band. We will detail the current state of infrared survey telescopes, demonstrate the benefits of Antarctic high-plateau for such observations, and show some of the development made in detector technologies to make large detector arrays a reality for such application.

**KEYWORDS**

instrumentation: detectors, telescopes, infrared: general

# 1 | INTRODUCTION

The approaching operation of the Vera Rubin Observatory (VRO, Ivezić et al. 2019) and the upcoming upgrade of

the LIGO gravitational wave detector (Abbott et al. 2018) are about to radically change how astronomers observe the dynamic universe. No longer considering the universe static is allowing us to understand physical phe-

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© 2023 The Authors. Astronomische Nachrichten published by Wiley-VCH GmbH.

<footnote n="1">¹Research School of Astronomy and Astrophysics, The Australian National University, Canberra, Australian Capital Territory Australia</footnote>

<footnote n="2">²Caltech Optical Observatory, California Institute of Technology, Pasadena, California</footnote>

<footnote n="3">³Center for Detectors, Rochester Institute of Technology, Rochester, New York</footnote>

<footnote n="4">⁴Cahill Center for Astrophysics, California Institute of Technology, Pasadena, California</footnote>

<footnote n="5">⁵Laboratoire Lagrange, Observatoire de la Cote D'azur, Nice, France</footnote>
nomena out of reach until now. The observations of cataclysmic events allow us to witness the mergers or deaths of stars and the resulting creation of heavy elements so far poorly understood. High-cadence observations have also accelerated our knowledge of planetary system and variable stars, notably from wide-field surveys, which capture large samples of such systems.

But while the amount of data produced by these surveys is immense, one piece of the puzzle is missing. Indeed, the vast majority of our galaxy is obscured by dust and out of reach of optical surveys. Piercing through this dust and increasing the number of sources a hundred-fold would allow the structure of the Milky Way to be mapped in much more details. It would also allow more accurate measurements of the distances to nearby galaxies and increase the number of habitable planets to be detected through transit microlensing methods. This need calls for an increase of our infrared surveying capabilities that can match the depth and cadence of the LSST survey. Such a capability would also be timely as LIGO will in 2023 be capable of detecting Neutron and Black hole mergers up to a distance of 400Mpc. Electromagnetic counterpart observations of such events are particularly well suited to the infrared as the blue luminosity decrease rapidly as the object becomes self-obscured.

The path to an infrared surveyor with the volumetric speed of VRO is not a trivial task and faces two major obstacles: the atmospheric absorption of infrared photons and the cost of large infrared detectors compared with their optical counterparts. In this paper, we will present how both challenges are being addressed and the path taken toward the fastest and deepest infrared survey.

## 2 | THE INFRARED SURVEY LANDSCAPE

Until fairly recently, the two best examples of infrared surveys were performed by the 2MASS (Skrutskie et al. 2006) and VISTA observatories (Sutherland et al. 2015). Both surveys covered the J (1.25 µm), H (1.65 µm), and Ks (2.16 µm) bandpasses with two 1.3-m telescopes in the case of 2MASS and a 4-m telescope in the case of VISTA. The goals of these surveys were not to produce high-cadence data but rather single reference surveys. 2MASS, which operated between 1997 and 2001, focused on producing an all-sky release of 471 million sources with high photometric accuracy. Vista extended this catalog on deeper, higher resolution observations focusing on specific areas of the sky. Now that 2MASS is retired and that VISTA is focusing on spectroscopy, new facilities have been constructed to focus on producing high-cadence surveys of the infrared sky.

The first, which opened its doors in 2018, is the Palomar-Gattini-IR telescope (De et al. 2020). The 30-cm telescope boasts a 25 sq. deg. field of view and is designed to survey the entire accessible sky (20,000 sq deg) to 16.4 $M_{AB}$ (J-band) every night. The telescope uses a cooled Teledyne H2RG camera with a cutoff at 1.7 µm to avoid the thermal background at 2 µm. While the Palomar-Gattini-IR telescope is in operation, a complementary surveyor is being built in the Southern observatory. This observatory, the Dynamic REd All-sky Monitoring Survey (DREAMS), is due to be commissioned by Q2 2023 and will provide similarly high-cadence data of the southern sky (Soon et al. 2020). The 0.5-m telescope will feed 6 InGasAs Princeton 1280 SciCams for a total 7.8 Mpix and a field of view of 4 sq. deg. The initial survey covering initially covering the J-band at a depth of 17.9 $M_{AB}$ will eventually be extended to cover the H and Y bands. Later in 2023, the 1m WINTER telescope (wide-field infrared transient explorer, Lourie et al. 2020) will also be installed at Palomar to offer similar bands and achieve even deeper observation using similar InGasAs technology (FLIR AP1020). Together, these three telescopes offer a full sky coverage of both hemispheres with a combination of depth and cadence that will cover many science cases in transient astronomy. They, however, do not cover the prized k-band due to their small apertures and strong sky background.

This wavelength is ideally observed from space where the thermal background is completely eliminated. It will in fact be exploited by the upcoming Euclid and the Nancy Grace Roman Space telescopes. While these telescopes will reach unmatched resolutions and depths at this wavelength, their field of view and science mission do not make them suitable for transient astronomy. We do, however, have an alternative from the ground, where cost is far more attractive, data rate more manageable and flexibility for upgrades and repair: the Polar regions. In this paper, we will describe the atmospheric conditions of the Antarctic plateau in Section 3 and why they lend themselves to an infrared observatory. We will then describe our work toward retiring two of the major challenges of such an observatory: A prototype telescope designed for the Antarctic conditions (Section 4) and the large detector array that will be required to optimize the volumetric speed of the telescope (Section 5).

## 3 | OBSERVING CONDITIONS IN ANTARCTICA

A popular region to carry out astronomy in the low energy bands is the High Antarctic Plateau. The region offers elevations above 2,000 m over an area the size of Australia. The South Pole station (2,835 m) currently hosts an
American research station specializing in submillimeter observations as well as high-energy particles detections. Higher locations such as Dome A (4,093 m) and Dome C (3,233 m), respectively ran by China and France/Italy, have observatories observing at optical wavelength. The Dome C station is particularly attractive to an infrared survey telescope because it is operated year round. The scientific advantages of operating such facilities at this location are as follow

**long nights:** Achieving high-cadence observations is key for certain science case and being able to follow-up a LIGO detection without being interrupted by the day/night cycle is crucial. This is only possible on the ground near the poles.

**Low sky background:** As shown in Ashley et al. (1996). The k-band sky background is about 40 times lower than the best temperate sites thanks to a combination of low temperature and water vapor content. With the minima at 2.4 μm caused by the disappearance of air-glow lines, the *K*<sub>Dark</sub> window (2.0–2.4 microns) is the one to be optimally exploited for astronomy (Li et al. 2016)

**Low seeing:** The seeing that quantifies the degradation of the resolution of astronomical images is caused by turbulence when air of different temperature mixes together. The seeing on the Antarctic plateau is rather unique as it is mostly located near the ground. Indeed the ice causes a strong thermal inversion that spans a few tens of meters. Above this height, the atmosphere is very stable and shielded from mixing with warm air by the circumpolar vortex. At Dome C, this boundary layer is between 20 and 30 m, above which the seeing averages a quarter of an arc second on average (Lawrence et al. 2004). A telescope placed at this height would therefore experience the world's best seeing conditions and optimize its sensitivity (Figure 1).

These conditions are ideally for wide-field observations in the *K*<sub>Dark</sub> window: The combination of high atmospheric transmission and low seeing translates directly to unprecedented sensitivity, which allows a small aperture telescope to compete with a much larger one located at temperate latitudes. Table 1 shows the performance of a 1-m telescope with a 50 sq. deg. field of view can achieve compared with the upcoming state of the art VRO. This table also includes Cryoscope, the quarter scale prototype of such telescope currently under construction in the laboratories of the Caltech Optical Observatory.

Building an observatory also comes with some challenges, especially since the telescope would need to be

<figure><img src="image_2.png" /><figcaption>**FIGURE 1** Statistical distribution of the seeing at Dome C (DC) from a height of 30m compared with ground seeing at Mauna Kea (MK) and Cerro Paranal (CP). This plot is extracted from Lawrence et al. (2004)</figcaption></figure>

elevated to a height of 20 m to properly take advantage of the observing conditions. This requires a logistical effort that can be a stretch to the polar program running the stations. Like with most Antarctic bases, getting equipment to Dome C first requires a boat trip to the Antarctic coast followed by a 1,100-km drive to the station. The logistical challenge does not unfortunately end with transportation. Once at Dome C, the erection of the tower and construction of the observatory will have to be carried out by a limited crew without the help of large crane and heavy machinery. It is for these reasons that we are working on designing an observatory and tower that can meet these limitations.

## 4 | CRYOSCOPE: TELESCOPE DESIGN AND SPECIFICATIONS

To optimize its volumetric speed, an Antarctic telescope must comply with a few key requirements. First, an extremely wide field of view that compensates for its relatively small aperture size, and second, a low thermal emission and diffraction-limited design to take advantage of the atmospheric conditions. These requirements led to a design based off the DUET telescope re-optimized to image over the k-band consisting of a double-meniscus corrector window. A cryogenically cooled telescope (140 K) feeds the light to a detector located at prime focus after passing through two field flattening lenses. In order to reach high sensitivity, the thermal emission of the telescope needs to be less than the sky and also restrict the amount of sky seen by the detector to the field of view of the telescope. The classic solution to use an ambient telescope and feed the
<table><caption>TABLE 1 Performance comparison of an infrared surveyor located in Antarctica compared with the upcoming VRO's LSST survey.</caption><thead><tr><th>Survey</th><th>Filter</th><th>Depth (AB)</th><th>FoV</th><th>Exposure/Visit (sec)</th><th>Volumetric speed (Mpc<sup>3</sup>s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>LSST</td><td>Y</td><td>22</td><td>9.6</td><td>30/44</td><td>8.5 × 10<sup>4</sup></td></tr><tr><td>1m class at Dome C</td><td>K</td><td>20.75</td><td>50</td><td>30/40</td><td>8.6 × 10<sup>4</sup></td></tr><tr><td>0.25m prototype at Dome C</td><td>K</td><td>17.5</td><td>16.5</td><td>60/70</td><td>185</td></tr></tbody></table>

light to a cryogenic re-imager is not scalable to wide-field observations as they do not address emissions by optical surfaces that are both too fast and too large. This has led to using a design where the entire telescope is cooled from the primary mirror up to the inner of the two meniscus lenses, which also serves as a vacuum window. These features are illustrated in the telescope solid model shown in Figure 2.

Cryoscope, a quarter-sized pathfinder of the final telescope is currently under construction and is to be deployed in Antarctica in 2025. While the optical design has been shown to work at F1.2 over a field of view of 78.5 sqdeg., Cryoscope will be scaled down to a F/2 and a 260 mm aperture in order to match the pixel scale of the available H2RG detector that will be used or this demonstration. The goals of the pathfinder are to demonstrate:

**The as-built performance** of the new design on sky and under Antarctic conditions: The wide range of temperatures that the telescope will experience during the Antarctic night will be significantly larger than what telescopes experience at temperate latitudes. Before moving to the construction of the full-scale telescope, Cryoscope's design will be tested in real conditions, assuring not only the survivability of the system but also its general optical performance. The effectiveness of the vacuum window seal and the stability of the PSF are areas that are difficult to test in lab conditions and will benefit from operation during a full Antarctic winter.

**The low thermal background** of the site and the baffling of the telescope: Since this has the largest impact on the telescope performance, it is crucial to verify that the site infrared background is as low as expected and that the telescope's own thermal emission does not dominate. This will test that the baffling system of the telescope is functioning as predicted by the nonsequential ray tracing simulations.

**A scalable strategy** to suppress condensation on its first optical surface: A regular problem for Antarctic telescopes caused by the radiative cooling of optical windows below ambient. Here again the baffling system plays a role, absorbing approximately 25% of

the radiation that would otherwise be radiated back through the window. Forward baffles, which reduce the solid angle subtended by the sky, will also be investigated and tested. For the full-scale version of the observatory, which will be sitting on a 20-m tower to experience the low seeing conditions, we expect to also leverage the temperature difference between the ground and the 20-m elevation by blowing the ground air through a ducting system to the top of the tower and reduce the dew point below the window temperature in the process.

Cryoscope will not however solve all the challenges that are needed to operate a full-scale infrared observatory in Antarctica. Challenges dealing with a 20-m tower that has to respect the logistical limitation of the station and its transport capabilities remain to be studied. Work on the tower is also intrinsically linked to the performance expected from the mount, which beside being required to operate in low temperatures will need to actively cancel the movements and vibrations caused by the tower.

## 5 | THE DETECTOR ARRAY

For infrared survey telescopes to become wide spread, availability and cost of large arrays of detectors will need to be improved in the near future. With a field of view of 50 sqdeg., our proposed diffraction-limited observatory will require the largest array of infrared sensor ever built in astronomy. To properly sample the PSF over this field of view, an array of 36 k × 36 k pixels will be necessary. To make such an array more affordable, Raytheon has partnered with the Center for Detectors at the Rochester Institute of Technology to develop large-format infrared detectors using HgCdTe grown on silicon substrate. The current program is to produce and test 4 k × 6 k sensors with 10 µm pixels. The detectors are built using Molecular Beam Epitaxy, a cheaper process than the Liquid Phase Epitaxy more commonly used for HgCdTe manufacturing. This new process has been demonstrated with the SATIN program run by the same institutions on smaller wafers and has achieved dark currents of 0.02 e/sec/pixel and read noise of 10 e.
<figure><img src="image_3.png" /><figcaption>**FIGURE 2** Solid model of the pathfinder Cryoscope. The telescope is an F/2 with an aperture stop of 260 mm, a field of view of 16sqdeg., and a PSF FWHM of 1.9". The top pictures show the meniscus window fabricated for the telescope (left), its interferogram showing compliance to the required wavefront error (middle), and the primary mirror design (right).</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**FIGURE 3** Quantum Efficiency of the current MBE arrays manufactured by Raytheon and characterized by the Center for Detectors.</figcaption></figure>

For the current phase, one important goal is to extend the high QE to the 2.5μm window, and early tests that have already achieved 70% QE at this wavelength (See Figure 3) with more optimizations planned for the next foundry run.

The Read Out Integrated Circuit (ROIC) for the 4 k × 6 k detectors is being developed by Sensor Creations Inc. It consists of 162 pads, all placed along the top edge with three sides "buttable" to create larger gap-less arrays. The development program is on schedule to be aligned with the planned timeline of the full-scale version of Cryoscope and is expected to be its first use case. Such development is crucial to the future of wide-field astronomy operating in the infrared bands as well as large spectroscopic telescopes such as the Maunakea Spectroscopic Explorer (MSE) and the Widefield Spectroscopic Telescope (WST), which will match the volumetric speed of VRO and provide the spectroscopic information to the images it gathers. We can therefore expect the need for large infrared arrays to significantly increase in the next decade.

## ACKNOWLEDGMENTS

The telescope work was performed under the auspices of the National Science Foundation under grant AST 2010041. The detector development is based upon work supported by the National Aeronautics and Space
Administration under Grant No. NNX13AH70G issued through the APRA Program and by the National Science Foundation under Awards 1207827 and 1509716 through the ATI Program. Open access publishing facilitated by Australian National University, as part of the Wiley - Australian National University agreement via the Council of Australian University Librarians.

CONFLICT OF INTEREST STATEMENT

The authors declare no potential conflict of interests.

ORCID

Tony Travouillon https://orcid.org/0000-0001-9304-6718

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AUTHOR BIOGRAPHY

**Tony Travouillon** is an Assistant Professor at the Advanced Instrumentation and Technology Centre at the Australian National University. He is the lead investigator of the DREAMS telescope and leads the development of telescope instrumentation for the centre.

How to cite this article: Travouillon, T., Smith, R. M., Fucik, J., Figer, D. F., Kasliwal, M., Moore, A. M., & Guillot, T. 2023, *Astron.Nachr./AN*, **344**, e230063. https://doi.org/10.1002/asna.20230063