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New, Ultra-High-Sensitivity, Deeply Cooled InGaAs Cameras
for Ground-Based Astronomy in the NIR-II / SWIRIntroduction“Ground-based observationsin the J and H astronomicalbands can be improved byutilizing scientific InGaAs
cameras from Princeton
Instruments.”

Infrared astronomy started about three decades after infrared (IR) radiation was discoveredby Sir William Herschel in 1800. Work in the IR range has been an important facet of
astronomical investigation since the 1950s and 1960s, when technological advances in IR
detectors began being leveraged to augment the success of contemporary radio astronomy.
By the mid-1980s, array detectors engineered to provide photonic sensitivity in thenear-infrared (NIR) range — at wavelengths between 700 and 3000 nm — were letting
astronomers explore the universe in new and exciting ways.
Since then, the development of InGaAs, InSb, and HgCdTe detectors has facilitated groundbased observations in the J and H astronomical bands of the NIR-II / short-wavelength
infrared (SWIR) range extending from 1050 nm to beyond 1700 nm. The J and H bands, which
are depicted in Figure 1, correspond to 1100–1400 nm and 1500–1800 nm, respectively.
These two astronomical bands are among several “infrared windows” (wavelengths) at which
the Earth’s atmosphere is essentially transparent to infrared light. Although most infrared light
is absorbed by water vapor in the Earth’s atmosphere, atmospheric transmission of light within
the so-called infrared windows is much better. To optimize ground-based observations, mostIR-capable telescopes on the Earth’s surface are located in dry geographical regions and at
rather extreme elevations that are higher than most of the water vapor in the atmosphere.
J bandFigure 1.

Most infrared light is absorbed bywater vapor in the Earth’s atmosphere;
however, there are a number ofastronomical bands that effectively
serve as “infrared windows” throughwhich light at certain wavelengths
reaches the surface of the Earth.

atmospheric transmission (%)

100H band

80

60

40

200400

600

800

1000

1200

1400

1600

1800

2000

2200

2400

wavelength (nm)Today, infrared astronomy remains a critical subfield of astronomy, a fact underscored by a2015 NRC report that recommends improvements in the US ground-based optical and infrared
astronomy system so as to meet specific long-term scientific goals pertaining to astronomy,
planetary science, and astrophysics.1 The use of infrared radiation for astronomy is crucial

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because unlike light at visible wavelengths, infrared light is not blocked by interstellar dust.
Ground-based observations via the “infrared windows” described above can therefore aid in
the ongoing search for planets (such as M dwarfs) that may possess the potential to support
life. Furthermore, for astrophysics investigations, the red-shifted light originating from objects
in the early universe can only be detected within the IR spectral region.

J & H Band Applications
Over the past three decades, myriad investigations have relied upon ground-based
observations in the J and H astronomical bands. In the 1990s, for instance, such observations
helped astronomers produce a new grid of faint NIR standard stars as well as a secondary listof red stars suitable for determining color transformations between photometric systems.2At the dawn of the twenty-first century, the advent of silicon immersion grating technology
coupled with adaptive optics at large ground-based telescopes started to revolutionize highresolution infrared spectroscopy.3 And earlier this decade, studies of integrated J- and H-band
spectra of globular clusters in the LMC4 as well as spectroscopic investigations of M dwarfs5,6
using data collected from the J and H bands have been published.
Applications utilizing the J and H astronomical bands may also include ground-based
observations of phenomena associated with planets in the Solar System, such as volcanic
activity on Jupiter’s moon Io and storms on Saturn.

New Data: Sunspot Images
An example of the utility of the latest generation of deeply cooled, scientific InGaAs detector
technology from Princeton Instruments for ground-based observations in the J and H
astronomical bands is presented in Figure 2. These two images of sunspots were acquired in2014 using a Princeton Instruments NIRvana:640 InGaAs camera at Yunnan Observatories’
Fuxian Lake Solar Observing Station (China).

Figure 2.

Sunspot images taken at 1565.3 nm(left) and 1083.0 nm (right) using aPrinceton Instruments NIRvana:640
InGaAs camera with the New Vacuum
Solar Telescope at the Fuxian Lake
Solar Observing Station. Courtesyof Yunnan Observatories, Chinese
Academy of Sciences.

The main instrument hosted by the station is the New Vacuum Solar Telescope (NVST),
a 1 meter infrared solar observation telescope. The primary scientific mission of NVSTAPPLICATION NOTE

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astronomers consists of obtaining high-resolution images and spectra of the sun atwavelengths ranging from 0.3 to 2.5 microns for the purposes of studying the fine structuresof solar magnetic fields, as well as evolutionary processes in high temporal and spatialresolution.

Enabling Technology
Ground-based observations in the J and H astronomical bands of the NIR-II / SWIR can be
improved by utilizing newly developed, deeply cooled, scientific InGaAs cameras (see Figure3) from Princeton Instruments.
The NIRvana® family of InGaAs cameras from Princeton Instruments differentiates itself fromother InGaAs cameras via a number of scientific performance features, including deep cooling,
low dark noise, high linearity, low read noise, high frame rates, intelligent software, andprecision control over integration times.

Figure 3.

Princeton Instruments’ new NIRvana
scientific cameras feature athermoelectrically or LN-cooled 640
x 512 InGaAs detector and offer
excellent sensitivity in the J and H
astronomical bands.

First and foremost, either maintenance-free thermoelectric cooling or liquid nitrogen can be
employed to chill the NIRvana’s two-dimensional 640 x 512 InGaAs focal plane array down
as low as –85°C (188 K) or –190°C (83 K), respectively. A proprietary cold shield design andsealed vacuum technology facilitates the lowest possible dark noise, which helps increase
sensitivity as well as preserve signal-to-noise ratio (SNR) for long exposure times. It should be
noted that all infrared detectors used by modern telescopes (i.e., detectors designed for NIR
sensitivity as well as detectors designed for mid-IR or far-IR sensitivity) require deep cooling tominimize thermally generated dark noise that would otherwise swamp the desired
photonic signal.
Thermoelectrically cooled NIRvana cameras have the ability to expose up to many minutes,
whereas LN-cooled NIRvana camera exposure times can reach 1 hour. Ultra-low-noise
readout electronics help ensure good SNR even when the cameras are operated at theirmaximum full-frame readout rates (i.e., 110 full frames per second for thermoelectricallycooled cameras; 2.77 full frames per second for LN-cooled cameras). Furthermore, excellent
system linearity means that NIRvana cameras collect highly reliable astronomical data.
Princeton Instruments’ 64-bit LightField® software, available as an option, provides a powerful
yet simple-to-use interface that puts real-time online processing capabilities at astronomers’
fingertips. NIRvana cameras are easy to integrate with telescopes. Full triggering support isprovided for precise synchronization with external equipment.
Deeply cooled, ultra-high-sensitivity NIRvana InGaAs cameras are ideal for making groundbased observations in the J and H astronomical bands.

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Resources
For additional information about Princeton Instruments NIRvana cameras, please refer to:www.princetoninstruments.com/products/imcam/nirvana

References
1. National Research Council. Optimizing the U.S. Ground-Based Optical and Infrared
Astronomy System. Washington, DC: The National Academies Press, 2015.
2. Persson S.E., Murphy D.C., Krzeminski W., Roth M., and Rieke M.J. A new system offaint near-infrared standard stars. Astron. J. 116, 2475–2488 (November 1998). DOI:
10.1086/300607
3. Ge J., McDavitt D., Chakraborty A., Bernecker J., and Miller S. Adaptive optics highresolution IR spectroscopy with silicon grisms and immersion gratings. Proceedings ofSPIE. (February 2003). DOI: 10.1117/12.459156
4. Lyubenova M., Kuntschner H., Rejkuba M., Silva D.R., Kissler-Patig M., and TacconiGarman L.E. Integrated J- and H-band spectra of globular clusters in the LMC:implications for stellar population models and galaxy age dating. Astron. Astrophys. 543(May 2012). DOI: 10.1051/0004-6361/201218847
5. Goulding N.T., Barnes J.R., Pinfield D.J., Kovacs G., Birkby J., Hodgkin S., Catalan S.,
Sipocz B., Jones H.R.A., del Burgo C., Jeffers S.V., Nefs S., Galvez-Ortiz M.-C., andMartin E.L. J band variability of M dwarfs in the WFCAM transit survey. Mon. Not. R.
Astron. Soc. 427(4). (November 2012). DOI: 10.1111/j.1365-2966.2012.21932.x
6. Terrien R.C., Mahadevan S., Bender C.F., Deshpande R., Ramsey L.W., and Bochanski J.J.
An H-band spectroscopic metallicity calibration for M dwarfs. Astrophys. J. Lett. 747(2)(February 2012). DOI: 10.1088/2041-8205/747/2/L38

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