Table of contents

Hard X-ray and low-energy gamma-ray coded-aperture imaging (CAI) instruments have been highly successful as high-energy surveyors and transient-source discoverers and trackers over the past decades. Albeit having relatively low sensitivity as compared with focusing instruments, coded-aperture telescopes still represent a very good choice for simultaneous, high cadence spectral measurements of individual point sources in large source fields. Here, I present a review of the fundamentals of CAI instruments in high-energy astrophysics, with an emphasis on the fundamental aspects of the technique, coded-mask instrument characteristics, and properties of the reconstructed images.

The citizen Continental-America Telescopic Eclipse (CATE) Experiment was a new type of citizen science experiment designed to capture a time sequence of white-light coronal observations during totality from 17:16 to 18:48 UT on 2017 August 21. Using identical instruments the CATE group imaged the inner corona from 1 to 2.1 RSun with 143 pixels at a cadence of 2.1 s. A slow coronal mass ejection (CME) started on the SW limb of the Sun before the total eclipse began. An analysis of CATE data from 17:22 to 17:39 UT maps the spatial distribution of coronal flow velocities from about 1.2 to 2.1 RSun, and shows the CME material accelerates from about 0 to 200 km s⁻¹ across this part of the corona. This CME is observed by LASCO C2 at 3.1–13 RSun with a constant speed of 254 km s⁻¹. The CATE and LASCO observations are not fit by either constant acceleration nor spatially uniform velocity change, and so the CME acceleration mechanism must produce variable acceleration in this region of the corona.

By analyzing the short-cadence K2 photometry from the observing Campaign 12 we refine the system parameters of hot Jupiter WASP-28b and hot Saturn WASP-151b. We report the non-detection and corresponding upper limits for transit-timing and transit-duration variations, starspots, rotational and phase-curve modulations and additional transiting planets. We discuss the cause of several background brightening events detected simultaneously in both planetary systems and conclude that they are likely associated with the passage of Mars across the field of view.

Weak gravitational lensing studies aim to measure small distortions in the shapes of distant galaxies, thus placing very tight demands on the understanding of detector-induced systematic effects in astronomical images. The Wide-field Infrared Survey Telescope will carry out weak lensing measurements in the near-infrared using the new Teledyne H4RG-10 detector arrays, which makes the range of possible detector systematics very different from traditional weak lensing measurements using optical CCDs. One of the nonlinear detector effects observed in CCDs is the brighter-fatter effect (BFE), in which charge already accumulated in a pixel alters the electric field geometry and causes new charge to be deflected away from brighter pixels. Here we describe the formalism for measuring the BFE using flat-field correlation functions in infrared detector arrays. The autocorrelation of CCD flat fields is often used to measure the BFE, but because the infrared detector arrays are read out with the charge "in place," the flat-field correlations are dominated by capacitive cross-talk between neighboring pixels (the interpixel capacitance, or IPC). Conversely, if the BFE is present and one does not account for it, it can bias correlation measurements of the IPC and photon transfer curve measurements of the gain. We show that one can compute numerous cross-correlation functions between different time slices of the same flat exposures, and that correlations due to IPC and BFE leave distinct imprints. We generate a suite of simulated flat fields and show that the underlying IPC and BFE parameters can be extracted, even when both are present in the simulation. There are some biases in the BFE coefficients up to 12%, which are likely caused by higher-order terms that are dropped from this analysis. The method is applied to laboratory data in the companion Paper II.

The Wide Field Infrared Survey Telescope (WFIRST) will investigate the origins of cosmic acceleration using weak gravitational lensing at near-infrared wavelengths. Lensing analyses place strict constraints on the precision of size and ellipticity measurements of the point-spread function. WFIRST will use infrared detector arrays, which must be fully characterized to inform data reduction and calibration procedures such that unbiased cosmological results can be achieved. Hirata & Choi introduces formalism to connect the cross-correlation signal of different flat field time samples to nonlinear detector behaviors such as the brighter fatter effect (BFE) and nonlinear inter-pixel capacitance (NL-IPC), and this paper applies that framework to a WFIRST development detector, SCA 18237. We find a residual correlation signal after accounting for classical nonlinearity. This residual correlation contains a combination of the BFE and NL-IPC; however, further tests suggest that the BFE is the dominant mechanism. If interpreted as a pure BFE, it suggests that the effective area of a pixel is increased by (2.87 ± 0.03) × 10⁻⁷ (stat.) for every electron in the 4 nearest neighbors, with a rapid ∼r^−5.6±0.2 fall-off of the effect for more distant neighbors. We show that the IPC inferred from hot pixels contains the same large-scale spatial variations as the IPC inferred from auto-correlations, albeit with an overall offset of ∼0.06%. The NL-IPC inferred from hot pixels is too small to explain the cross-correlation measurement, further supporting the BFE hypothesis. This work presents the first evidence for the BFE in an H4RG-10 detector, demonstrates some of the useful insights that can be gleaned from flat field statistics, and represents a significant step toward calibration of WFIRST data.

High fidelity iodine spectra provide the wavelength and instrument calibration needed to extract precise radial velocities (RVs) from stellar spectral observations taken through iodine cells. Such iodine spectra are usually taken by a Fourier Transform Spectrometer (FTS). In this work, we investigated the reason behind the discrepancy between two FTS spectra of the iodine cell used for precise RV work with the High Resolution Spectrograph (HRS) at the Hobby–Eberly Telescope (HET). We concluded that the discrepancy between the two HRS FTS spectra was due to temperature changes of the iodine cell. Our work demonstrated that the ultra-high resolution spectra taken by the TS12 arm of the Tull Spectrograph One at McDonald Observatory are of similar quality to the FTS spectra and thus can be used to validate the FTS spectra. Using the software IodineSpec5, which computes the iodine absorption lines at different temperatures, we concluded that the HET/HRS cell was most likely not at its nominal operating temperature of 70°C during its FTS scan at National Institute of Standards and Technology or at the TS12 measurement. We found that extremely high resolution echelle spectra (R > 200,000) can validate and diagnose deficiencies in FTS spectra. We also recommend best practices for temperature control and nightly calibration of iodine cells.

Image smear in a CCD digital image arises when light from the scene being imaged impinges on the detector during the initial flush before the exposure begins, during readout, or both. We have found a rigorous method for removing "two-sided" smear that is linear with the number of pixels and does not involve matrix inversion.

Optimizing on-sky single-mode fiber (SMF) injection is an essential part of developing precise Doppler spectrometers and new astrophotonics technologies. We installed and tested a prototype SMF-injection system at the Large Binocular Telescope in 2016 April. The fiber injection unit was built as part of the derisking process for a new instrument named iLocater that will use adaptive optics (AO) to feed a high resolution, near-infrared spectrograph. In this paper we report Y-band SMF coupling measurements for bright, M-type stars. We compare theoretical expectations for delivered Strehl ratio and SMF coupling to experimental results, and evaluate fundamental effects that limit injection efficiency. We find the pupil geometry of the telescope itself limits fiber coupling to a maximum efficiency of ρ_tel ≈ 0.78. Further analysis shows the individual impact of AO correction, tip-tilt residuals, and static (noncommon-path) aberrations contribute coupling coefficients of ρ_Strehl ≈ 0.33, ${\rho }_{\mathrm{tip}/\mathrm{tilt}}\approx 0.84$, and ρ_ncpa ≈ 0.8 respectively. Combined, these effects resulted in an average Y-band SMF efficiency of 0.18 for all observations. Finally, we investigate the impact of fiber coupling on radial velocity precision as a function of stellar apparent magnitude.

α Centauri A is the closest solar-type star to the Sun and offers an excellent opportunity to detect the thermal emission of a mature planet heated by its host star. The MIRI coronagraph on the James Webb Space Telescope can search the 1–3 au (1''–2'') region around α Cen A which is predicted to be stable within the α Cen AB system. We demonstrate that with reasonable performance of the telescope and instrument, a 20 hr program combining on-target and reference star observations at 15.5 μm could detect thermal emission from planets as small as ∼5 R_⊕. Multiple visits every 3–6 months would increase the geometrical completeness, provide astrometric confirmation of detected sources, and push the radius limit down to ∼3 R_⊕. An exozodiacal cloud only a few times brighter than our own should also be detectable, although a sufficiently bright cloud might obscure any planet present in the system. While current precision radial velocity (PRV) observations set a limit of 50–100 M_⊕ at 1–3 au for planets orbiting α Cen A, there is a broad range of exoplanet radii up to 10 R_⊕ consistent with these mass limits. A carefully planned observing sequence along with state-of-the-art post-processing analysis could reject the light from α Cen A at the level of ∼10⁻⁵ at 1''–2'' and minimize the influence of α Cen B located 7''–8'' away in the 2022–2023 timeframe. These space-based observations would complement on-going imaging experiments at shorter wavelengths as well as PRV and astrometric experiments to detect planets dynamically. Planetary demographics suggest that the likelihood of directly imaging a planet whose mass and orbit are consistent with present PRV limits is small, ∼5%, and possibly lower if the presence of a binary companion further reduces occurrence rates. However, at a distance of just 1.34 pc, α Cen A is our closest sibling star and certainly merits close scrutiny.

A coherent-dispersion spectrometer (CODES) system using the radial velocity (RV) method for exoplanet searches is established in this paper. This spectrometer utilizes a new Sagnac interferometer with common-path and asymmetric designs. Compared with the traditional Michelson interferometer-based spectrometer for exoplanet detection, these designs markedly improve the stability of the optical path difference (OPD) to positional changes in optical elements or air density changes, reducing the need for active cavity stabilization. Furthermore, the asymmetric Sagnac design allows convenient separation of the two complementary outputs. In order to verify the feasibility of the CODES, an optical Doppler shift experiment is constructed in the laboratory. The RV signal was extracted by the phase shifts of the interference fringes produced by the spectrometer. The experimental results show that the obtained retrieved RV is 76.7 m s⁻¹, with an absolute error of 0.3 m s⁻¹ compared to simulated values. And the root mean square error (RMSE) and the standard deviation (STD) are 21.3 m s⁻¹ and 21.4 m s⁻¹, respectively. Error analyses show that the OPD change caused by the temperature variation is the main factor for the RMSE and STD.

PolCam, a wide-angle polarimetric camera, will be onboard the Korea Pathfinder Lunar Orbiter to investigate the lunar surface based on polarimetry. PolCam will construct a global lunar map of polarimetric parameters, such as maximal polarization (P_max) and titanium distribution using three color bands centered at 320, 430, and 750 nm. PolCam's twin cameras are mounted at 45° tilt angles from the nadir across the orbital track in opposite directions. PolCam will obtain polarimetric measurements of sunlight scattered by the lunar surface at various phase angles up to ∼140° to achieve the scientific goals during the one-year mission. Since degree of linear polarization is a function of phase angle, it is essential to perform several measurements at various phase angles to properly retrieve, e.g., P_max, P_min, and the inversion angle, α_inv. In this work, we show PolCam's phase-angle coverage as a function of selenographic longitudes from the equator to latitudes up to 70°N. Decreasing tilt angle to 25° and 0° limits the high end of phase-angle coverage, as well as the observational chances at high latitudes.