Is Flat Fielding Safe for Precision CCD Astronomy? [IMA]

The ambitious goals of precision cosmology with wide-field optical surveys such as the Dark Energy Survey (DES) and the Large Synoptic Survey Telescope (LSST) demand, as their foundation, precision CCD astronomy. This in turn requires an understanding of previously uncharacterized sources of systematic error in CCD sensors, many of which manifest themselves as static effective variations in pixel area. Such variation renders a critical assumption behind the traditional procedure of flat fielding–that a sensor’s pixels comprise a uniform grid–invalid. In this work, we present a method to infer a curl-free model of a sensor’s underlying pixel grid from flat field images, incorporating the superposition of all electrostatic sensor effects–both known and unknown–present in flat field data. We use these pixel grid models to estimate the overall impact of sensor systematics on photometry, astrometry, and PSF shape measurements in a representative sensor from the Dark Energy Camera (DECam) and a prototype LSST sensor. Applying the method to DECam data recovers known significant sensor effects for which corrections are currently being developed within DES. For an LSST prototype CCD with pixel-response non-uniformity (PRNU) of 0.4%, we find the impact of “improper” flat-fielding on these observables is negligible in nominal .7″ seeing conditions. These errors scale linearly with the PRNU, so for future LSST production sensors, which may have larger PRNU, our method provides a way to assess whether pixel-level calibration beyond flat fielding will be required.

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M. Baumer, C. Davis and A. Roodman
Fri, 23 Jun 17

Comments: 9 pages, 11 figures, submitted to PASP. Code available at this https URL

Detection and Implications of Laser-Induced Raman Scattering at Astronomical Observatories [IMA]

(Abr.) Laser guide stars employed at astronomical observatories provide artificial wavefront reference sources to help correct (in part) the impact of atmospheric turbulence on astrophysical observations. Following the recent commissioning of the 4 Laser Guide Star Facility (4LGSF) on UT4 at the VLT, we characterize the spectral signature of the uplink beams from the 22W lasers to assess the impact of laser scattering from the 4LGSF on science observations. We use the MUSE optical integral field spectrograph to acquire spectra at a resolution of R~3000 of the uplink laser beams over the wavelength range of 4750\AA\ to 9350\AA. We report the first detection of laser-induced Raman scattering by N2, O2, CO2, H2O and (tentatively) CH4 molecules in the atmosphere above the astronomical observatory of Cerro Paranal. In particular, our observations reveal the characteristic spectral signature of laser photons — but 480\AA\ to 2210\AA\ redder than the original laser wavelength of 5889.959\AA\ — landing on the 8.2m primary mirror of UT4 after being Raman-scattered on their way up to the sodium layer. Laser-induced Raman scattering is not unique to the observatory of Cerro Paranal, but common to any astronomical telescope employing a laser-guide-star (LGS) system. It is thus essential for any optical spectrograph coupled to a LGS system to handle thoroughly the possibility of a Raman spectral contamination via a proper baffling of the instrument and suitable calibrations procedures. These considerations are particularly applicable for the HARMONI optical spectrograph on the upcoming Extremely Large Telescope. At sites hosting multiple telescopes, laser collision prediction tools also ought to account for the presence of Raman emission from the uplink laser beam(s) to avoid the unintentional contamination of observations acquired with telescopes in the vicinity of a LGS system.

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F. Vogt, D. Calia, W. Hackenberg, et. al.
Fri, 23 Jun 17

Comments: 17 pages, 11 figures, accepted for publication in Physical Review X

Large amplitude tip/tilt estimation by geometric diversity for multiple-aperture telescopes [IMA]

A novel method nicknamed ELASTIC is proposed for the alignment of multiple-aperture telescopes, in particular segmented telescopes. It only needs the acquisition of two diversity images of an unresolved source, and is based on the computation of a modified, frequency-shifted, cross-spectrum. It provides a polychromatic large range tip/tilt estimation with the existing hardware and an inexpensive noniterative unsupervised algorithm. Its performance is studied and optimized by means of simulations. They show that with 5000 photo-electrons/sub-aperture/frame and 1024×1024 pixel images, residues are within the capture range of interferometric phasing algorithms such as phase diversity. The closed-loop alignment of a 6 sub-aperture mirror provides an experimental demonstration of the effectiveness of the method.

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S. Vievard, F. Cassaing and L. Mugnier
Fri, 23 Jun 17

Comments: Accepted for publication in Journal of the Optical Society of America A (JOSAA)

Spectral Linear Dark Field Control: Stabilizing Deep Contrast for Exoplanet Imaging Using out-of-band Speckle Field [IMA]

Wavefront stabilization is a fundamental challenge to high contrast imaging of exoplanets. For both space and ground observations, wavefront control performance is ultimately limited by the finite amount of starlight available for sensing, so wavefront measurements must be as efficient as possible. To meet this challenge, we propose to sense residual errors using bright focal-plane speckles at wavelengths outside the high contrast spectral bandwidth. We show that a linear relationship exists between the intensity of the bright out-of-band speckles and residual wavefront aberrations. An efficient linear control loop can exploit this relationship. The proposed scheme, referred to as Spectral Linear Dark Field Control (spectral LDFC), is more sensitive than conventional approaches for ultra-high contrast imaging. Spectral LDFC is closely related to, and can be combined with, the recently proposed spatial LDFC which uses light at the observation wavelength but located outside of the high contrast area in the focal plane image. Both LDFC techniques do not require starlight to be mixed with the high contrast speckle field, so full-sensitivity uninterrupted high contrast observations can be conducted simultaneously with wavefront correction iterations. We also show that LDFC is robust against deformable mirror calibration errors and drifts, as it relies on detector response stability instead of deformable mirror stability. LDFC is particularly advantageous when science acquisition is performed at a non-optimal wavefront sensing wavelength, such as nearIR observations of planets around solar-type stars, for which visible-light speckle sensing is ideal. We describe the approach at a fundamental level and provide an algorithm for its implementation. We demonstrate, through numerical simulation, that spectral LDFC is well-suited for picometer-level cophasing of a large segmented space telescope.

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O. Guyon, K. Miller, J. Males, et. al.
Fri, 23 Jun 17

Comments: 12 pages, 8 figures, submitted to PASP

The All-Sky Automated Survey for Supernovae (ASAS-SN) Light Curve Server v1.0 [SSA]

The All-Sky Automated Survey for Supernovae (ASAS-SN) is working towards imaging the entire visible sky every night to a depth of V~17 mag. The present data covers the sky and spans ~2-5~years with ~100-400 epochs of observation. The data should contain some ~1 million variable sources, and the ultimate goal is to have a database of these observations publicly accessible. We describe here a first step, a simple but unprecedented web interface that provides an up to date aperture photometry light curve for any user-selected sky coordinate. Because the light curves are produced in real time, this web tool is relatively slow and can only be used for small samples of objects. However, it also imposes no selection bias on the part of the ASAS-SN team, allowing the user to obtain a light curve for any point on the celestial sphere. We present the tool, describe its capabilities, limitations, and known issues, and provide a few illustrative examples.

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C. Kochanek, B. Shappee, K. Stanek, et. al.
Fri, 23 Jun 17

Comments: 8 pages, 9 figures, submitted to PASP

A Very Fast And Angular Momentum Conserving Tree Code [IMA]

There are many methods used to compute the classical gravitational field in astrophysical simulation codes. With the exception of the typically impractical method of direct computation, none ensure conservation of angular momentum to machine precision. Under uniform time-stepping, the Cartesian fast multipole method of Dehnen (also known as the very fast tree code) conserves linear momentum to machine precision. We show it is possible to modify this method in a way that conserves both angular and linear momenta.

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D. Marcello
Thu, 22 Jun 17

Comments: 13 pages, 1 figure, accepted for publication by AAS Journals

MinXSS-1 CubeSat On-Orbit Pointing and Power Performance: The First Flight of the Blue Canyon Technologies XACT 3-axis Attitude Determination and Control System [IMA]

The Miniature X-ray Solar Spectrometer (MinXSS) is a 3 Unit (3U) CubeSat designed for a 3-month mission to study solar soft X-ray spectral irradiance. The first of the two flight models was deployed from the International Space Station in 2016 May and operated for one year before its natural deorbiting. This was the first flight of the Blue Canyon Technologies XACT 3-axis attitude determination and control system — a commercially available, high-precision pointing system. We characterized the performance of the pointing system on orbit including performance at low altitudes where drag torque builds up. We found that the pointing accuracy was 0.0042\degree\ – 0.0117\degree\ (15$”$ – 42$”$, 3$\sigma$, axis dependent) consistently from 190 km – 410 km, slightly better than the specification sheet states. Jitter was estimated to be 0.0073\degree\ (10 s$^{-1}$) – 0.0183\degree\ (10 s$^{-1}$) (26$”$ (10 s$^{-1}$) – 66$”$ (10 s$^{-1}$), 3$\sigma$). The system was capable of dumping momentum until an altitude of 185 km. We found small amounts of sensor degradation in the star tracker or coarse sun sensor. Our mission profile did not require high-agility maneuvers so we are unable to characterize this metric. Without a GPS receiver, it was necessary to periodically upload ephemeris information to update the orbit propagation model and maintain pointing. At 400 km, these uploads were required once every other week. At $\sim$270 km, they were required every day. We also characterized the power performance of our electric power system, which includes a novel pseudo-peak power tracker — simply a resistor that limited the current draw from the battery on the solar panels. With 19 30\% efficient solar cells and an 8 W system load, the power balance had 65\% of margin on orbit. We present several recommendations to other CubeSat programs throughout.

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J. Mason, M. Baumgart, B. Rogler, et. al.
Thu, 22 Jun 17

Comments: 11 pages, 16 figures, submitted to Journal of Small Satellites