Record-Breaking Image Quality with DECam

The circle is the star shown in the imexam plot (on right). The bar shape in the bright stars are the result of interpolating over saturation and bleeding. Image credit: Frank Valdes.

On the night of January 27 2015 Dark Energy Survey (DES) observers Yuanyuan Zhang (U. Michigan), Andrew Nadolski (U. Illinois), and Josh Frieman (Fermilab/U. Chicago) experienced a half-night of excellent image quality, with most images over a period of ~3 hours with FWHM ~ 0.8 arcsec or better. During this time, a 90 second z band image with number 403841, at airmass 1.22, was found to have a FWHM of 0.62 arcsec, as measured from several stars on each of several CCDs by fitting a Moffatt function (beta = 3.5) using IRAF. This is the best ever DECam image quality recorded during the DES, now coming to the end of its Year 2 observing campaign. The star profiles are a little under-sampled given the pixel size of 0.263 arcsec, and different tools can give slightly different answers for the FWHM. Nevertheless, this is close to the instrumental floor of around 0.45 arcsec (optics, CCD diffusion) and it is particularly gratifying that this superb image quality is maintained over the entire focal plane.