Common-mode Correction Research Articles

Low-resolution transit spectroscopy of three hot Jupiters using the 2 m Himalayan Chandra Telescope

ABSTRACT Here, we present the low-resolution transmission spectroscopy of three giant planets using the Himalayan Faint Object Spectrograph Camera (HFOSC) on the 2 m Himalayan Chandra Telescope (HCT) in Hanle, India. It is the first application of transmission spectroscopy with HCT. This study presents results from a single transit, each for three planets: HAT-P-1b, KELT-18b, and WASP-127b. The selection of suitable reference stars assisted in accurately tracking slit losses for the long cadence observations that are needed to achieve the required signal-to-noise ratio (SNR). We employ the common mode correction technique, utilizing a white light transit curve to minimize time-dependent systematic errors. The observed spectra for WASP-127b and HAT-P-1b agree with previous low-resolution transit spectroscopic observations using other observing facilities. We confirm the presence of Rayleigh scattering in the atmosphere of WASP-127b. In addition, we provide the first low-resolution transmission spectrum for KELT-18b. Modelling the exoplanet atmosphere with HFOSC and available IR observations from HST and Spitzer for WASP-127b and HAT-P-1b shows that HFOSC can be an alternative optical instrument to use in conjunction with IR observations to constrain the atmospheric parameters better.

ACCESS: Confirmation of a Clear Atmosphere for WASP-96b and a Comparison of Light Curve Detrending Techniques

One of the strongest Na i features was observed in WASP-96b. To confirm this novel detection, we provide a new 475–825 nm transmission spectrum obtained with Magellan/IMACS, which indeed confirms the presence of a broad sodium absorption feature. We find the same result when reanalyzing the 400–825 nm VLT/FORS2 data. We also utilize synthetic data to test the effectiveness of two common detrending techniques: (1) a Gaussian processes (GP) routine, and (2) common-mode correction followed by polynomial correction (CMC+Poly). We find that both methods poorly reproduce the absolute transit depths but maintain their true spectral shape. This emphasizes the importance of fitting for offsets when combining spectra from different sources or epochs. Additionally, we find that, for our data sets, both methods give consistent results, but CMC+Poly is more accurate and precise. We combine the Magellan/IMACS and VLT/FORS2 spectra with literature 800–1644 nm HST/WFC3 spectra, yielding a global spectrum from 400 to 1644 nm. We used the PLATON and Exoretrievals retrieval codes to interpret this spectrum, and find that both yield relatively deeper pressures where the atmosphere is optically thick at log-pressures between and bars, respectively. Exoretrievals finds solar to supersolar Na i and H2O log-mixing ratios of and , respectively, while PLATON finds an overall metallicity of dex. Therefore, our findings are in agreement with the literature and support the inference that the terminator of WASP-96b has few aerosols obscuring prominent features in the optical to near-infrared (near-IR) spectrum.

Solar-to-supersolar sodium and oxygen absolute abundances for a ‘hot Saturn’ orbiting a metal-rich star

ABSTRACT We present new analysis of infrared transmission spectroscopy of the cloud-free hot-Saturn WASP-96b performed with the Hubble and Spitzer Space Telescopes (HST and Spitzer). The WASP-96b spectrum exhibits the absorption feature from water in excellent agreement with synthetic spectra computed assuming a cloud-free atmosphere. The HST-Spitzer spectrum is coupled with Very Large Telescope (VLT) optical transmission spectroscopy which reveals the full pressure-broadened profile of the sodium absorption feature and enables the derivation of absolute abundances. We confirm and correct for a spectral offset of $\Delta R_{{\rm p}}/R_{\ast }=(-4.29^{+0.31}_{-0.37})\, \times 10^{-3}$ of the VLT data relative to the HST-Spitzer spectrum. This offset can be explained by the assumed radius for the common-mode correction of the VLT spectra, which is a well-known feature of ground-based transmission spectroscopy. We find evidence for a lack of chromospheric and photometric activity of the host star which therefore make a negligible contribution to the offset. We measure abundances for Na and O that are consistent with solar to supersolar, with abundances relative to solar values of $21^{+27}_{-14}$ and $7^{+11}_{-4}$, respectively. We complement the transmission spectrum with new thermal emission constraints from Spitzer observations at 3.6 and 4.5 $\mu$m, which are best explained by the spectrum of an atmosphere with a temperature decreasing with altitude. A fit to the spectrum assuming an isothermal blackbody atmosphere constrains the dayside temperature to be Tp = 1545 ± 90 K.

Ground-based transmission spectroscopy with FORS2: A featureless optical transmission spectrum and detection of H2O for the ultra-hot Jupiter WASP-103b

ABSTRACT We report ground-based transmission spectroscopy of the highly irradiated and ultra-short period hot-Jupiter WASP-103b covering the wavelength range ≈400–600 nm using the FORS2 instrument on the Very Large Telescope. The light curves show significant time-correlated noise which is mainly invariant in wavelength and which we model using a Gaussian process. The precision of our transmission spectrum is improved by applying a common-mode correction derived from the white light curve, reaching typical uncertainties in transit depth of ≈2 × 10−4 in wavelength bins of 15 nm. After correction for flux contamination from a blended companion star, our observations reveal a featureless spectrum across the full range of the FORS2 observations and we are unable to confirm the Na absorption previously inferred using Gemini/GMOS or the strong Rayleigh scattering observed using broad-band light curves. We performed a Bayesian atmospheric retrieval on the full optical-infrared transmission spectrum using the additional data from Gemini/GMOS, HST/WFC3, and Spitzer observations and recover evidence for H2O absorption at the 4.0 σ level. However, our observations are not able to completely rule out the presence of Na, which is found at 2.0 σ in our retrievals. This may in part be explained by patchy/inhomogeneous clouds or hazes damping any absorption features in our FORS2 spectrum, but an inherently small scale height also makes this feature challenging to probe from the ground. Our results none the less demonstrate the continuing potential of ground-based observations for investigating exoplanet atmospheres and emphasize the need for the application of consistent and robust statistical techniques to low-resolution spectra in the presence of instrumental systematics.

Characterization and correction of OMPS nadir mapper measurements for ozone profile retrievals

Abstract. This paper verifies and corrects the Ozone Mapping and Profiler Suite (OMPS) nadir mapper (NM) level 1B v2.0 measurements with the aim of producing accurate ozone profile retrievals using an optimal-estimation-based inversion method to fit measurements in the spectral range 302.5–340 nm. The evaluation of available slit functions demonstrates that preflight-measured slit functions represent OMPS measurements well compared to derived Gaussian slit functions. Our initial OMPS fitting residuals contain significant wavelength and cross-track-dependent biases, resulting in serious cross-track striping errors in the tropospheric ozone retrievals. To eliminate the systematic component of the fitting residuals, we apply soft calibration to OMPS radiances. With the soft calibration the amplitude of fitting residuals decreases from ∼ 1 to 0.2 % over low and middle latitudes, and thereby the consistency of tropospheric ozone retrievals between OMPS and the Ozone Monitoring Instrument (OMI) is substantially improved. A common mode correction is also implemented for additional radiometric calibration; it improves retrievals especially at high latitudes where the amplitude of fitting residuals decreases by a factor of ∼ 2. We estimate the noise floor error of OMPS measurements from standard deviations of the fitting residuals. The derived error in the Huggins band ( ∼ 0.1 %) is twice the OMPS L1B measurement error. OMPS noise floor errors constrain our retrievals better, leading to improving information content of ozone and reducing fitting residuals. The final precision of the fitting residuals is less than 0.1 % in the low and middle latitudes, with ∼ 1 degrees of freedom for signal for the tropospheric ozone, meeting the general requirements for successful tropospheric ozone retrievals.

VLT/FORS2 comparative transmission spectroscopy II: Confirmation of a cloud deck and Rayleigh scattering in WASP-31b, but no potassium?

We present transmission spectroscopy of the hot-Jupiter WASP-31b using FORS2 on the VLT during two primary transits. The observations cover a wavelength range of $\approx$400-840nm. The light curves are corrupted by significant systematics, but these were to first order invariant with wavelength and could be removed using a common-mode correction derived from the white light curves. We reach a precision in the transit depth of $\approx$140 ppm in 15 nm bins, although the precision varies significantly over the wavelength range. Our FORS2 observations confirm the cloud-deck previously inferred using HST/STIS. We also re-analyse the HST/STIS data using a Gaussian process model, finding excellent agreement with earlier measurements. We reproduce the Rayleigh scattering signature at short wavelengths ($\lesssim$5300 $\AA$) and the cloud-deck at longer wavelengths. However, our FORS2 observations appear to rule out the large potassium feature previously detected using STIS, yet it is recovered from the HST/STIS data, although with reduced amplitude and significance ($\approx$2.5$\sigma$). The discrepancy between our results and the earlier STIS detection of potassium ($\approx$4.3$\sigma$) is either a result of telluric contamination of the ground-based observations, or an underestimate of the uncertainties for narrow-band features in HST/STIS when using linear basis models to account for the systematics. Our results further demonstrate the use of ground-based multi-object spectrographs for the study of exoplanet atmospheres, and highlight the need for caution in our interpretation of narrow-band features in low-resolution spectra of hot-Jupiters.

FPGA-based signal processing for the LHCb silicon strip detectors

We have developed an electronic board (TELL1) to interface the DAQ system of the LHCb experiment at CERN. Two hundred and eighty-nine TELL1 boards are needed to read out the different subdetectors including the silicon VEertex LOcator (VELO) (172 k strips), the Trigger Tracker (TT) (147 k strips) and the Inner Tracker (129 k strips). Each board can handle either 64 analog or 24 digital optical links. The TELL1 mother board provides common mode correction, zero suppression, data formatting, and a large network interface buffer. To satisfy the different requirements we have adopted a flexible FPGA design and made use of mezzanine cards. Mezzanines are used for data input from digital optical and analog copper links as well as for the Gigabit Ethernet interface to DAQ.

Detection of muons at 150 GeV/ c with a CMS Preshower prototype

The analysis of 150 GeV/ c muon data collected during a test of a CMS Preshower prototype is presented. The test took place in 2004 in the H4 beam at CERN. The muon signal extraction is possible after pedestal subtraction and common mode correction. The results of a Geant-4-based simulation, developed for the Preshower prototype test, are also presented. The results of the simulation are found to be in excellent agreement with the data. It is also demonstrated that by combining the results of the data analysis and simulation an absolute calibration of the CMS Preshower detector system can be performed.

The LHCb DAQ interface board TELL1

We have developed an electronic board (TELL1) to interface the DAQ system of the LHCb experiment at CERN. 289 TELL1 boards are needed to read out the different subdetectors. Each board can handle either 64 analog or 24 digital optical links. The TELL1 mother board provides common mode correction, zero suppression, data formatting, and a large network interface buffer. To satisfy the different requirements we have adopted a flexible FPGA design and made use of mezzanine cards. Mezzanines are used for data input from digital optical and analog copper links as well as for the Gigabit Ethernet interface to DAQ. The LHCb timing and trigger control signals are transported by a dedicated optical link, while the board slow-control is provided by an embedded PC running a Linux kernel.

A micropower CMOS continuous-time filter with on-chip automatic tuning

This paper presents a CMOS implementation of a low-voltage micropower G/sub m/-C biquad with on-chip automatic tuning. The filter is suitable for any kind of application involving low-frequency ranges, and very low-power consumption, such as biomedical devices. The operational transconductance amplifier (OTA) is implemented with the transistors working in the weak inversion saturation region, thus allowing the use of very small currents that minimize the power consumption. The aspect ratios are small enough not to degrade the frequency response. The tuning algorithm is based on amplitude tracking. The filter output amplitude is quantized using a low-power amplifier and an asymmetric comparator. A digital controller varies the tuning parameters until the maximum quantized amplitude is found. The system works down to a voltage supply of 1.75 V. The center frequency is tunable over one and a half decades, from 300 Hz to 10 kHz for bias currents changing from 6 to 200 nA and a 20-pF integrating capacitance, giving an overall filter accuracy of up to 99.55%. The power consumption of the second-order filter including the common-mode correction circuitry is in the order of 200 nW for the 10-nA bias current. It exhibits a dynamic range of 54 dB and occupies an area of 0.06 mm/sup 2/ excluding the area of the integrating capacitances.

A low-jitter PLL clock generator for microprocessors with lock range of 340-612 MHz

A fully integrated, phase-locked loop (PLL) clock generator/phase aligner for the POWER3 microprocessor has been designed using a 2.5-V, 0.40-/spl mu/m digital CMOS6S process. The PLL design supports multiple integer and noninteger frequency multiplication factors for both the processor clock and an L2 cache clock. The fully differential delay-interpolating voltage-controlled oscillator (VCO) is tunable over a frequency range determined by programmable frequency limit settings, enhancing yield and application flexibility. PLL lock range for the maximum VCO frequency range settings is 340-612 MHz. The charge-pump current is programmable for additional control of the PLL loop dynamics. A differential on-chip loop filter with common-mode correction improves noise rejection. Cycle-cycle jitter measurements with the microprocessor actively executing instructions were 10.0 ps rms, 80 ps peak to peak (P-P) measured from the clock tree. Cycle-cycle jitter measured for the processor in a reset state with the clock tree active was 8.4 ps rms, 62 ps P-P. PLL area is 1040/spl times/640 /spl mu/m/sup 2/. Power dissipation is <100 mW.

Differential synchronous demodulation for small signal amplitude estimation

A circuit for estimating the amplitude of small signals (in the range of microvolts to a few millivolts) developed initially for an electrical impedance tomography (EIT) data acquisition system is presented. The paper is focused on two points, the accurate demodulation of the difference of two sinewave signals and the common-mode correction of the measurement. The method is oriented for applications in measuring instruments, where a signal source (sinewave oscillator) is assumed to be incorporated. It uses a sample-and-hold circuit, suitably driven, as an amplitude demodulator. One of the advantages of this method is that the realization of the synchronous demodulator (or phase-sensitive detector) does not require a low-pass fitter at the output, resulting in a smaller settling time, that is, in higher speed of measurement. Finally, to improve the accuracy, a self-calibration technique has been developed. >

Performance of a differential synchronous demodulator for electrical impedance tomography.

The performance of a differential synchronous demodulator is presented. This demodulator is part of an electrical impedance tomography system developed by our group. In addition a technique for common-mode correction instead of common-mode feedback is proposed. A series of measurements as a function of the common-mode signal is given.