We characterize the ability of the Dark Energy Camera (DECam) to perform relative astrometry across its 500 Mpix, 3-deg2 science field of view, and across 4 years of operation. This is done using internal comparisons of ∼ 4 × 107 measurements of highS/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 ≈ 10 µm 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 50–100 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 ≈ 0.7 arcmin−2, e.g. from Gaia, the typical atmospheric distortions can be interpolated to ≈ 7 mas RMS accuracy (for 30 s exposures) with 10 coherence length in 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 (≈ 0.02 pixels, or ≈ 300 nm) on the focal plane, plus the stochastic atmospheric distortion.
|Original language||English (US)|
|State||Published - Mar 5 2017|
- Atmospheric effects
- Instrumentation: detectors
- Methods: data analysis
ASJC Scopus subject areas