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Title: Astrometric Calibration and Performance of the Dark Energy Camera

Abstract

We characterize the ability of the Dark Energy Camera (DECam) to perform relative astrometry across its 500 Mpix, 3 $deg^2$ science field of view, and across 4 years of operation. This is done using internal comparisons of $~ 4 x 10^7$ measurements of high-S/N stellar images obtained in repeat visits to fields of moderate stellar density, with the telescope dithered to move the sources around the array. An empirical astrometric model includes terms for: optical distortions; stray electric fields in the CCD detectors; chromatic terms in the instrumental and atmospheric optics; shifts in CCD relative positions of up to $$\approx 10 \mu m$$ when the DECam temperature cycles; and low-order distortions to each exposure from changes in atmospheric refraction and telescope alignment. Errors in this astrometric model are dominated by stochastic variations with typical amplitudes of 10-30 mas (in a 30 s exposure) and $$5^{\prime}-10^{\prime}$$ arcmin coherence length, plausibly attributed to Kolmogorov-spectrum atmospheric turbulence. The size of these atmospheric distortions is not closely related to the seeing. Given an astrometric reference catalog at density $$\approx 0.7$$ $$arcmin^{-2}$$, e.g. from Gaia, the typical atmospheric distortions can be interpolated to $$\approx$$ 7 mas RMS accuracy (for 30 s exposures) with $$1^{\prime}$$ arcmin coherence length for residual errors. Remaining detectable error contributors are 2-4 mas RMS from unmodelled stray electric fields in the devices, and another 2-4 mas RMS from focal plane shifts between camera thermal cycles. Thus the astrometric solution for a single DECam exposure is accurate to 3-6 mas ( $$\approx$$ 0.02 pixels, or $$\approx$$ 300 nm) on the focal plane, plus the stochastic atmospheric distortion.

Authors:
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Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States); Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
Contributing Org.:
DES Collaboration
OSTI Identifier:
1361381
Alternate Identifier(s):
OSTI ID: 1346375; OSTI ID: 1369280
Report Number(s):
arXiv:1703.01679; FERMILAB-PUB-17-057-AE
Journal ID: ISSN 0004-6280; 1516279
Grant/Contract Number:
AC02-07CH11359; AC05-00OR22725; AC02-76SF00515; AST-1615555; SC0007901
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Publications of the Astronomical Society of the Pacific
Additional Journal Information:
Journal Volume: 129; Journal Issue: 977; Journal ID: ISSN 0004-6280
Publisher:
Astronomical Society of the Pacific
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; 46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY

Citation Formats

Bernstein, G. M., Armstrong, R., Plazas, A. A., Walker, A. R., Abbott, T. M. C., Allam, S., Bechtol, K., Benoit-Lévy, A., Brooks, D., Burke, D. L., Rosell, A. Carnero, Kind, M. Carrasco, Carretero, J., Cunha, C. E., Costa, L. N. da, DePoy, D. L., Desai, S., Diehl, H. T., Eifler, T. F., Fernandez, E., Fosalba, P., Frieman, J., García-Bellido, J., Gerdes, D. W., Gruen, D., Gruendl, R. A., Gschwend, J., Gutierrez, G., Honscheid, K., James, D. J., Kent, S., Krause, E., Kuehn, K., Kuropatkin, N., Li, T. S., Maia, M. A. G., March, M., Marshall, J. L., Menanteau, F., Miquel, R., Ogando, R. L. C., Reil, K., Roodman, A., Rykoff, E. S., Sanchez, E., Scarpine, V., Schindler, R., Schubnell, M., Sevilla-Noarbe, I., Smith, M., Smith, R. C., Soares-Santos, M., Sobreira, F., Suchyta, E., Swanson, M. E. C., and Tarle, G. Astrometric Calibration and Performance of the Dark Energy Camera. United States: N. p., 2017. Web. doi:10.1088/1538-3873/aa6c55.
Bernstein, G. M., Armstrong, R., Plazas, A. A., Walker, A. R., Abbott, T. M. C., Allam, S., Bechtol, K., Benoit-Lévy, A., Brooks, D., Burke, D. L., Rosell, A. Carnero, Kind, M. Carrasco, Carretero, J., Cunha, C. E., Costa, L. N. da, DePoy, D. L., Desai, S., Diehl, H. T., Eifler, T. F., Fernandez, E., Fosalba, P., Frieman, J., García-Bellido, J., Gerdes, D. W., Gruen, D., Gruendl, R. A., Gschwend, J., Gutierrez, G., Honscheid, K., James, D. J., Kent, S., Krause, E., Kuehn, K., Kuropatkin, N., Li, T. S., Maia, M. A. G., March, M., Marshall, J. L., Menanteau, F., Miquel, R., Ogando, R. L. C., Reil, K., Roodman, A., Rykoff, E. S., Sanchez, E., Scarpine, V., Schindler, R., Schubnell, M., Sevilla-Noarbe, I., Smith, M., Smith, R. C., Soares-Santos, M., Sobreira, F., Suchyta, E., Swanson, M. E. C., & Tarle, G. Astrometric Calibration and Performance of the Dark Energy Camera. United States. doi:10.1088/1538-3873/aa6c55.
Bernstein, G. M., Armstrong, R., Plazas, A. A., Walker, A. R., Abbott, T. M. C., Allam, S., Bechtol, K., Benoit-Lévy, A., Brooks, D., Burke, D. L., Rosell, A. Carnero, Kind, M. Carrasco, Carretero, J., Cunha, C. E., Costa, L. N. da, DePoy, D. L., Desai, S., Diehl, H. T., Eifler, T. F., Fernandez, E., Fosalba, P., Frieman, J., García-Bellido, J., Gerdes, D. W., Gruen, D., Gruendl, R. A., Gschwend, J., Gutierrez, G., Honscheid, K., James, D. J., Kent, S., Krause, E., Kuehn, K., Kuropatkin, N., Li, T. S., Maia, M. A. G., March, M., Marshall, J. L., Menanteau, F., Miquel, R., Ogando, R. L. C., Reil, K., Roodman, A., Rykoff, E. S., Sanchez, E., Scarpine, V., Schindler, R., Schubnell, M., Sevilla-Noarbe, I., Smith, M., Smith, R. C., Soares-Santos, M., Sobreira, F., Suchyta, E., Swanson, M. E. C., and Tarle, G. Tue . "Astrometric Calibration and Performance of the Dark Energy Camera". United States. doi:10.1088/1538-3873/aa6c55.
@article{osti_1361381,
title = {Astrometric Calibration and Performance of the Dark Energy Camera},
author = {Bernstein, G. M. and Armstrong, R. and Plazas, A. A. and Walker, A. R. and Abbott, T. M. C. and Allam, S. and Bechtol, K. and Benoit-Lévy, A. and Brooks, D. and Burke, D. L. and Rosell, A. Carnero and Kind, M. Carrasco and Carretero, J. and Cunha, C. E. and Costa, L. N. da and DePoy, D. L. and Desai, S. and Diehl, H. T. and Eifler, T. F. and Fernandez, E. and Fosalba, P. and Frieman, J. and García-Bellido, J. and Gerdes, D. W. and Gruen, D. and Gruendl, R. A. and Gschwend, J. and Gutierrez, G. and Honscheid, K. and James, D. J. and Kent, S. and Krause, E. and Kuehn, K. and Kuropatkin, N. and Li, T. S. and Maia, M. A. G. and March, M. and Marshall, J. L. and Menanteau, F. and Miquel, R. and Ogando, R. L. C. and Reil, K. and Roodman, A. and Rykoff, E. S. and Sanchez, E. and Scarpine, V. and Schindler, R. and Schubnell, M. and Sevilla-Noarbe, I. and Smith, M. and Smith, R. C. and Soares-Santos, M. and Sobreira, F. and Suchyta, E. and Swanson, M. E. C. and Tarle, G.},
abstractNote = {We characterize the ability of the Dark Energy Camera (DECam) to perform relative astrometry across its 500 Mpix, 3 $deg^2$ science field of view, and across 4 years of operation. This is done using internal comparisons of $~ 4 x 10^7$ measurements of high-S/N stellar images obtained in repeat visits to fields of moderate stellar density, with the telescope dithered to move the sources around the array. An empirical astrometric model includes terms for: optical distortions; stray electric fields in the CCD detectors; chromatic terms in the instrumental and atmospheric optics; shifts in CCD relative positions of up to $\approx 10 \mu m$ when the DECam temperature cycles; and low-order distortions to each exposure from changes in atmospheric refraction and telescope alignment. Errors in this astrometric model are dominated by stochastic variations with typical amplitudes of 10-30 mas (in a 30 s exposure) and $5^{\prime}-10^{\prime}$ arcmin coherence length, plausibly attributed to Kolmogorov-spectrum atmospheric turbulence. The size of these atmospheric distortions is not closely related to the seeing. Given an astrometric reference catalog at density $\approx 0.7$ $arcmin^{-2}$, e.g. from Gaia, the typical atmospheric distortions can be interpolated to $\approx$ 7 mas RMS accuracy (for 30 s exposures) with $1^{\prime}$ arcmin coherence length for residual errors. Remaining detectable error contributors are 2-4 mas RMS from unmodelled stray electric fields in the devices, and another 2-4 mas RMS from focal plane shifts between camera thermal cycles. Thus the astrometric solution for a single DECam exposure is accurate to 3-6 mas ( $\approx$ 0.02 pixels, or $\approx$ 300 nm) on the focal plane, plus the stochastic atmospheric distortion.},
doi = {10.1088/1538-3873/aa6c55},
journal = {Publications of the Astronomical Society of the Pacific},
number = 977,
volume = 129,
place = {United States},
year = {Tue May 30 00:00:00 EDT 2017},
month = {Tue May 30 00:00:00 EDT 2017}
}

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  • We characterize the variation in photometric response of the Dark Energy Camera (DECam) across its 520~Mpix science array during 4 years of operation. These variations are measured using high signal-to-noise aperture photometry ofmore » $>10^7$ stellar images in thousands of exposures of a few selected fields, with the telescope dithered to move the sources around the array. A calibration procedure based on these results brings the RMS variation in aperture magnitudes of bright stars on cloudless nights down to 2--3 mmag, with <1 mmag of correlated photometric errors for stars separated by $$\ge20$$". On cloudless nights, any departures of the exposure zeropoints from a secant airmass law exceeding >1 mmag are plausibly attributable to spatial/temporal variations in aperture corrections. These variations can be inferred and corrected by measuring the fraction of stellar light in an annulus between 6" and 8" diameter. Key elements of this calibration include: correction of amplifier nonlinearities; distinguishing pixel-area variations and stray light from quantum-efficiency variations in the flat fields; field-dependent color corrections; and the use of an aperture-correction proxy. The DECam response pattern across the 2-degree field drifts over months by up to $$\pm7$$ mmag, in a nearly-wavelength-independent low-order pattern. We find no fundamental barriers to pushing global photometric calibrations toward mmag accuracy.« less
  • We characterize the ability of the Dark Energy Camera (DECam) to perform relative astrometry across its 500 Mpix, 3more » $deg^2$ science field of view, and across 4 years of operation. This is done using internal comparisons of $~ 4 x 10^7$ measurements of high-S/N stellar images obtained in repeat visits to fields of moderate stellar density, with the telescope dithered to move the sources around the array. An empirical astrometric model includes terms for: optical distortions; stray electric fields in the CCD detectors; chromatic terms in the instrumental and atmospheric optics; shifts in CCD relative positions of up to $$\approx 10 \mu m$$ when the DECam temperature cycles; and low-order distortions to each exposure from changes in atmospheric refraction and telescope alignment. Errors in this astrometric model are dominated by stochastic variations with typical amplitudes of 10-30 mas (in a 30 s exposure) and $$5^{\prime}-10^{\prime}$$ arcmin coherence length, plausibly attributed to Kolmogorov-spectrum atmospheric turbulence. The size of these atmospheric distortions is not closely related to the seeing. Given an astrometric reference catalog at density $$\approx 0.7$$ $$arcmin^{-2}$$, e.g. from Gaia, the typical atmospheric distortions can be interpolated to $$\approx$$ 7 mas RMS accuracy (for 30 s exposures) with $$1^{\prime}$$ arcmin coherence length for residual errors. Remaining detectable error contributors are 2-4 mas RMS from unmodelled stray electric fields in the devices, and another 2-4 mas RMS from focal plane shifts between camera thermal cycles. Thus the astrometric solution for a single DECam exposure is accurate to 3-6 mas ( $$\approx$$ 0.02 pixels, or $$\approx$$ 300 nm) on the focal plane, plus the stochastic atmospheric distortion.« less
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  • We have built a reliable and robust system that takes as input an astronomical image, and returns as output the pointing, scale, and orientation of that image (the astrometric calibration or World Coordinate System information). The system requires no first guess, and works with the information in the image pixels alone; that is, the problem is a generalization of the 'lost in space' problem in which nothing-not even the image scale-is known. After robust source detection is performed in the input image, asterisms (sets of four or five stars) are geometrically hashed and compared to pre-indexed hashes to generate hypothesesmore » about the astrometric calibration. A hypothesis is only accepted as true if it passes a Bayesian decision theory test against a null hypothesis. With indices built from the USNO-B catalog and designed for uniformity of coverage and redundancy, the success rate is >99.9% for contemporary near-ultraviolet and visual imaging survey data, with no false positives. The failure rate is consistent with the incompleteness of the USNO-B catalog; augmentation with indices built from the Two Micron All Sky Survey catalog brings the completeness to 100% with no false positives. We are using this system to generate consistent and standards-compliant meta-data for digital and digitized imaging from plate repositories, automated observatories, individual scientific investigators, and hobbyists. This is the first step in a program of making it possible to trust calibration meta-data for astronomical data of arbitrary provenance.« less
  • We describe a new method which achieves high-precision very long baseline interferometry (VLBI) astrometry in observations at millimeter (mm) wavelengths. It combines fast frequency-switching observations, to correct for the dominant non-dispersive tropospheric fluctuations, with slow source-switching observations, for the remaining ionospheric dispersive terms. We call this method source-frequency phase referencing. Provided that the switching cycles match the properties of the propagation media, one can recover the source astrometry. We present an analytic description of the two-step calibration strategy, along with an error analysis to characterize its performance. Also, we provide observational demonstrations of a successful application with observations using themore » Very Long Baseline Array at 86 GHz of the pairs of sources 3C274 and 3C273 and 1308+326 and 1308+328 under various conditions. We conclude that this method is widely applicable to mm-VLBI observations of many target sources, and unique in providing bona fide astrometrically registered images and high-precision relative astrometric measurements in mm-VLBI using existing and newly built instruments, including space VLBI.« less