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Register a new userRemoval of systematics in photometric measurements: static and rotating illumination corrections in FORS2@VLT data
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Lodovico Coccato
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Images taken with modern detectors require calibration via flat fielding to obtain the same flux scale across the whole image. One method for obtaining the best possible flat fielding accuracy is to derive a photometric model from dithered stellar observations. A large variety of effects have been taken into account in such modelling. Recently, Moehler et al. (2010) discovered systematic variations in available flat frames for the European Southern Observatorys FORS instrument that change with the orientation of the projected image on the sky. The effect on photometry is large compared to other systematic effects that have already been taken into account. In this paper, we present a correction method for this effect: a generalization of the fitting procedure of Bramich & Freudling (2012) to include a polynomial representation of rotating flat fields. We then applied the method to the specific case of FORS2 photometric observations of a series of standard star fields, and provide parametrised solutions that can be applied by the users. We found polynomial coefficients to describe the static and rotating large-scale systematic flat-field variations across the FORS2 field of view. Applying these coefficients to FORS2 data, the systematic changes in the flux scale across FORS2 images can be improved by ~1% to ~2% of the total flux. This represents a significant improvement in the era of large-scale surveys, which require homogeneous photometry at the 1% level or better.
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We present ARC2 (Astrophysically Robust Correction 2), an open-source Python-based systematics-correction pipeline to correct for the Kepler prime mission long cadence light curves. The ARC2 pipeline identifies and corrects any isolated discontinuities in the light curves, then removes trends common to many light curves. These trends are modelled using the publicly available co-trending basis vectors, within an (approximate) Bayesian framework with `shrinkage priors to minimise the risk of over-fitting and the injection of any additional noise into the corrected light curves, while keeping any astrophysical signals intact. We show that the ARC2 pipelines performance matches that of the standard Kepler PDC-MAP data products using standard noise metrics, and demonstrate its ability to preserve astrophysical signals using injection tests with simulated stellar rotation and planetary transit signals. Although it is not identical, the ARC2 pipeline can thus be used as an open source alternative to PDC-MAP, whenever the ability to model the impact of the systematics removal process on other kinds of signal is important.
We reduced ESOs archival linear spectropolarimetry data (4000-9000AA) of 6 highly polarized and 8 unpolarized standard stars observed between 2010 and 2016, for a total of 70 epochs, with the FOcal Reducer and low dispersion Spectrograph (FORS2) mounted at the Very Large Telescope. We provide very accurate standard stars polarization measurements as a function of wavelength, and test the performance of the spectropolarimetric mode (PMOS) of FORS2. We used the unpolarized stars to test the time stability of the PMOS mode, and found a small ($leq$0.1%), but statistically significant, on-axis instrumental polarization wavelength dependency, possibly caused by the tilted surfaces of the dispersive element. The polarization degree and angle are found to be stable at the level of $leq$0.1% and $leq$0.2 degrees, respectively. We derived the polarization wavelength dependence of the polarized standard stars and found that, in general, the results are consistent with those reported in the literature, e.g. Fossati et al. (2007) who performed a similar analysis using FORS1 data. The re-calibrated data provide a very accurate set of standards that can be very reliably used for technical and scientific purposes. The analysis of the Serkowski parameters revealed a systematic deviation from the width parameter $K$ reported by Whittet et al. (1992). This is most likely explained by incorrect effective wavelengths adopted in that study for the R and I bands.
Context: Polarimetry is a very powerful tool to uncover various properties of astronomical objects that remain otherwise hidden in standard imaging or spectroscopic observations. However, the reliable measurement of the low polarization signal from astronomical sources requires a good control of spurious instrumental polarization induced by the various components of the optical system and the detector. Aims: We perform a detailed multi-wavelength calibration study of the FORS2 instrument at the VLT operating in imaging polarimetric mode (IPOL) to characterize the spatial instrumental polarization that may affect the study of extended sources. Methods: We use imaging polarimetry of a) high signal-to-noise blank fields BVRI observations during full-moon, when the polarization is expected to be constant across the field-of-view and deviations originate from the instrument and b) a crowded star cluster in broad-band RI and narrow-band H{alpha} filters, where individual polarization values of each star across the field can be measured. Results: We find an instrumental polarization pattern that increases radially outwards from the optical axis of the instrument reaching up to 1.4% at the edges, depending on the filter. Our results are well approximated by an elliptical paraboloid down to less than {sim0.05%} accuracy,and {sim0.02%} when using non-analytic fits. We present 2D maps to correct for this spurious instrumental polarization. We also give several tips and tricks to analyze polarimetric measurements of extended sources. Conclusions: FORS2 is a powerful instrument allowing to map the linear polarimetry of extended sources. We present and discuss a methodology to measure the polarization of such sources, and to correct for the spatial polarization induced in the optical system. This methodology could be applied to polarimetric measurements using other dual-beam polarimeters.
Space-based transit search missions such as Kepler are collecting large numbers of stellar light curves of unprecedented photometric precision and time coverage. However, before this scientific goldmine can be exploited fully, the data must be cleaned of instrumental artefacts. We present a new method to correct common-mode systematics in large ensembles of very high precision light curves. It is based on a Bayesian linear basis model and uses shrinkage priors for robustness, variational inference for speed, and a de-noising step based on empirical mode decomposition to prevent the introduction of spurious noise into the corrected light curves. After demonstrating the performance of our method on a synthetic dataset, we apply it to the first month of Kepler data. We compare the results, which are publicly available, to the output of the Kepler pipelines pre-search data conditioning, and show that the two generally give similar results, but the light curves corrected using our approach have lower scatter, on average, on both long and short timescales. We finish by discussing some limitations of our method and outlining some avenues for further development. The trend-corrected data produced by our approach are publicly available.
ESOs two FOcal Reducer and low dispersion Spectrographs (FORS) are the primary optical imaging instruments for the VLT. They are not direct-imaging instruments, as there are several optical elements in the light path. In particular, both instruments are attached to a field rotator. Obtaining truly photometric data with such instruments present a significant challenge. In this paper, we investigate in detail twilight flats taken with the FORS instruments. We find that a large fraction of the structure seen in these flatfields rotates with the field rotator. We discuss in detail the methods we use to determine the cause of this effect. The effect was tracked down to be caused by the Linear Atmospheric Dispersion Corrector (LADC). The results are thus of special interest for designers of instruments with LADCs and developers of calibration plans and pipelines for such instruments. The methods described here to find and correct it, however, are of interest also for other instruments using a field rotator. If not properly corrected, this structure in the flatfield may degrade the photometric accuracy of imaging observations taken with the FORS instruments by adding a systematic error of up to 4% for broad band filters. We discuss several strategies to obtain photometric images in the presence of rotating flatfield pattern.
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