This guest editorial summarizes the Special Section on Detectors for Astronomy and Cosmology

Hamden, Erika

Hoenk, Michael

MacKenty, John

Nikzad, Shouleh

Nomerotski, Andrei

Shapiro, Chaz

Smith, Roger

English

Caltech Authors - Main

Special Section Guest Editorial:
Detectors for Astronomy and
Cosmology
Shouleh Nikzad
Erika Hamden
Michael Hoenk
John MacKenty
Andrei Nomerotski
Chaz Shapiro
Roger Smith
Shouleh Nikzad, Erika Hamden, Michael Hoenk, John MacKenty, Andrei Nomerotski,
Chaz Shapiro, Roger Smith, “Special Section Guest Editorial: Detectors for Astronomy and
Cosmology,” J. Astron. Telesc. Instrum. Syst. 6(1), 011001 (2020), doi: 10.1117/
1.JATIS.6.1.011001
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Special Section Guest Editorial:
Detectors for Astronomy and Cosmology
Shouleh Nikzad,a Erika Hamden,b Michael Hoenk,a John MacKenty,c
Andrei Nomerotski,d Chaz Shapiro,a and Roger Smithe
aJet Propulsion Laboratory, Pasadena, California, United States
bUniversity of Arizona, Tucson, Arizona, United States
cSpace Telescope Science Institute, Baltimore, Maryland, United States
dBrookhaven National Laboratory, Upton, New York, United States
eCalifornia Institute of Technology, Pasadena, California, United States
Astronomers are dreaming up fantastic measurements to understand our universe using plat-
forms varying from ground based to suborbital to CubeSats and SmallSats, all the way to giant
aperture telescopes orbiting Earth. While no one instrument component is clearly more essential
than the others (what is a telescope without the optics or the mechanical structures to point it?),
the guest editors of this detector-themed section of JATIS (split between the October-December
2019 and January-March 2020 issues) are eager to accept the often-repeated statement (by detec-
tor experts) that detectors are the heart of any astronomical instrument. It is noteworthy that
instrument designs meant to push current measurement capabilities to their limits are often based
on the limitations or advantages of existing detectors. This has less to do with any “essential”
quality and more to do with the relative effort needed to develop new detector technologies or
even achieve incremental improvements. In the short-term, while it is not always easy to improve
signal-to-noise by adjusting instrument specifications such as optics or observing strategy, these
are typically preferable to inventing a lower noise detector. In the long term, with each new
detector advancement, the instrument trade space expands, allowing more design flexibility
or enabling previously unattainable measurements.
Having obtained a detector with precisely the desired combination of sensitivity, pixel scale,
format, power consumption, readout options, and cost—that is to say, a unicorn—the work is not
yet done. In the era of precision astronomy, science reach is often limited by poorly understood
detector behavior. In cosmological surveys, where the number of sources can reach into the
billions, statistical errors are averaged down to levels below detector-induced systematic errors.
Exoplanet measurements using the radial velocity technique may require detecting spectral shifts
comparable to the silicon lattice spacing of the CCD! Successful campaigns thus require not only
state-of-the-art detectors but also advanced knowledge of how to operate and calibrate detectors
effectively for a particular science case. The Image Sensors for Precision Astronomy (ISPA)
workshop, held at Caltech in December of 2018, was dedicated to this theme. We are grateful
to the ISPA attendees who contributed to this section, especially Pierre Antilogus, who contrib-
uted the workshop introduction.
In the infrared, the paper by Cabrera et al. details the development of 15 μm cutoff infrared
arrays, which will enable wide-area searches for near-Earth objects with NEOCam. The arrays
are based on the popular HgCdTe detectors from Teledyne Imaging Systems and will thus benefit
from the work of various other groups who are taking deep dives into understanding the sys-
tematics of this family of devices. The paper by Ninan et al. demonstrates a model and correction
for the “crosshatch”—a high-frequency pattern of quantum efficiency variations on subpixel
scales that can degrade high-precision measurements such as radial velocities of exoplanet sys-
tems. Meanwhile, Donlon et al. provide a new correction scheme for inter-pixel capacitance,
a form of crosstalk between adjacent pixels that is crucial to understand for precision measure-
ments sensitive to the point spread function. Crosstalk also couples amplifier channels in these
infrared detectors, complicating the effect far beyond adjacent pixels. Thankfully, George et al.
provide a fast method to measure and correct the coupling between all detector channels. These
papers demonstrate the important role that detector users are playing in IR detector development.
© 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)
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Moving to the other end of the spectrum, Kyne et al. push the limits of detector sensitivity
toward ultraviolet by upgrading the detector of FIREBall—a balloon-borne instrument that mea-
sures UV emission around galaxies—to a delta-doped electron-multiplying CCD (EMCCD).
Optimized for a narrow atmospheric window for a balloon (190–215 nm), the UV-enhanced
photon counting detector contributed greatly to improvement in sensitivity relative to the pre-
vious FIREBall design while removing high voltage requirement. Gleisinger et al. use the same
model of EMCCDs to measure UV quantum yield—a challenging but essential calibration now
that CCDs are being used in the far UV. Characterizing quantum yield with a photon counting
CCD cleverly decouples the measurement from the device’s quantum efficiency and provides
a window into the physics of photon transfer. As silicon UV detectors become more widely used
for astronomy, particularly in space-based instruments, the need for better characterization
will grow.
Several authors remind us there is still much to learn even from mature devices like CCDs.
Snyder and Roodman take an in-depth look at charge-transfer inefficiency (CTI) in CCDs devel-
oped for the Large Synoptic Survey Telescope (LSST). Beyond the CTI effect incurred with each
pixel transfer, they find additional contributions possibly due to charge trapping and an elec-
tronic offset drift. Park et al. characterize the “tree ring” pattern of astrometric distortions in
the same CCDs, quantifying the length scale and amplitude of the pattern as a function of wave-
length and back bias voltage. Also for LSST, Juramy et al. investigate “tearing patterns” in CCD
flat fields. In addition to mitigating the effect by changing how they operate the CCDs, they
provide insight into the physics of the effect, theorizing that parallel clocking is moving pos-
itively charged holes into regions around channel stops. Creative CCD clocking can also be used
to solve issues originating outside the detector, such as thermal instability, as shown by Blake et al.
By shifting charge within a pixel during exposures, they are able to mitigate temperature var-
iations in the NEID spectrograph, which aims to detect line shifts of a few nanometers on the
focal plane. Finally, Weatherill et al. take a closer look at the origins of correlated double sam-
pling (CDS) noise and, by analyzing an oversampled video signal, derive an optimal CDS timing
scheme. In these instances, valuable insights come from researchers with high precision needs
pushing beyond the typical use cases for their devices.
As astronomical measurements come to require more precision and sophistication, detector
characterization must evolve to match. Often, subtle detector behavior goes unnoticed in conven-
tional testing, only becoming apparent later in the astronomical data analysis, at which point
mitigation schemes are limited. The paper by Vorobiev et al. describes a precision spot scanner
designed for characterization of intra-pixel response variations, which contribute to photometric
uncertainties in e.g., exoplanet detection. Using a spot less than 3.5 microns wide, they generate
detailed, multiwavelength response maps of a Kepler CCD. In addition to investigating known
detector issues, it is important to also uncover the “unknown unknowns” that may undermine
scientific measurements. The paper by Shapiro et al. argues for the importance of characterizing
detectors by projecting realistic images onto sensors and performing analyses that allow one to
validate both the detector performance and the calibration strategy for challenging science cases.
They provide an overview of the Precision Projector Laboratory, a flexible testbed designed for
this purpose, which has investigated detector issues for JWST, WFIRST, Euclid, and other proj-
ects. Similarly, Christov et al. describe the development of a flexible new characterization setup
that will allow sensors to be tested with a range of stimuli, including various light sources, image
patterns, and an x-ray source.
Detectors are particularly easy to admire when showcased as part of new instruments. For
instance, the 3 gigapixel focal plane of LSST consists of 21 autonomous “Raft Tower Modules,”
each housing 9 CCDs. O’Connor presents characterization results for these modules, which
themselves have strict mechanical and electro-optical requirements to meet the speed and sen-
sitivity needs of LSST. Another ambitious instrument, the Palomar Radial Velocity Instrument
(PARVI), aims to use a near infrared spectrograph (resolution ∼100;000) to detect exoplanets
from radial velocity signals as low as 30 cm∕s (i.e., the star moves at the speed of crawling).
As described in the paper by Gibson et al., PARVI is enabled in part by a guider based on the
InGaAs C-RED 2 camera from First Light Advanced Imagery. The camera, operated at −40C
and clocking 400 FPS, is used in a tip-tilt loop to keep starlight centered on the spectrograph
fiber, making low noise a crucial feature.
Special Section Guest Editorial
J. Astron. Telesc. Instrum. Syst. 011001-2 Jan–Mar 2020 • Vol. 6(1)
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The papers in this special section give us a glimpse of the powerful impact of detectors on
astronomy. They illustrate how state-of-the-art detectors are enabling exciting new science, and
they highlight work that must be done to bring them to their full potential. Advancements that
lead to the next great discoveries will surely include new detectors as well as improved under-
standing of existing ones. We thank all of our contributors and wish them, and the readers, best of
luck in their endeavors in the exciting days ahead in astronomy and astrophysics.
Special Section Guest Editorial
J. Astron. Telesc. Instrum. Syst. 011001-3 Jan–Mar 2020 • Vol. 6(1)
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Special Section Guest Editorial: Detectors for Astronomy and Cosmology

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