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Mapping of the PSF to large radii

Contact: Emanuel Bertin
Contact: Gary Bernstein

We stack star images with SNRs>100 (20 in the u-band) in each channel (excluding CCD # 61) using reduced images DECam_00180620-25, DECam_00180721-32, and normalizing all exposures to the flux in a ring with diameter 6"<⌀<9". The central 5" are affected by saturation so we merge the stacked star images with the PSFEx model obtained from the concatenation of DECam central CCDs (#28 and #35); basically all pixels at radius <3" are replaced with the PSFEx model while the outer pixels are from the stacked star images, scaled to match the PSFEx model in the ring with diameter 6"<⌀<9". The results are "average" PSF images with a 0.263" pixel scale, normalized to 1 inside a disk with diameter ⌀=6", and with a dynamic range that reaches about 15 magnitudes (~120dB):

channel u g r i z Y
PSF image (N is up, E is left) Stacked PSF model in the u band Stacked PSF model in the g band Stacked PSF model in the r band Stacked PSF model in the i band Stacked PSF model in the z band Stacked PSF model in the Y band
PSF isophotes [mag.arcsec⁻²] Isophotes of the stacked PSF model in the u band Isophotes of the stacked PSF model in the g band Isophotes of the stacked PSF model in the r band Isophotes of the stacked PSF model in the i band Isophotes of the stacked PSF model in the z band Isophotes of the stacked PSF model in the Y band
FITS file psf_u.fits.gz psf_g.fits.gz psf_r.fits.gz psf_i.fits.gz psf_z.fits.gz psf_Y.fits.gz
Seeing 0.99" 0.92" 0.83" 0.75" 0.77" 0.77"
Mag offset from ⌀=60" to ⌀=6" 0.0061 0.0049 0.0041 0.0040 0.0116 0.0516
Surface brightness μ as a function of distance d Stacked PSF profile in the u band Stacked PSF profile in the g band Stacked PSF profile in the r band Stacked PSF profile in the i band Stacked PSF profile in the z band Stacked PSF profile in the Y band
μ at d = 30" [mag.arcsec⁻²]* 15.69 16.15 16.42 16.57 14.92 13.33
μ at d = 60" [mag.arcsec⁻²]* 17.80 18.49 18.94 19.13 16.90 16.15

* Extrapolation based on best-fitting bi-Moffat model.

We believe that this feature is not caused by the optics, but rather to diffraction from the front-side channel stops (in one direction) and other gate structures (for the other direction). These rectilinear structures form diffraction gratings and produce the orthogonal features seen here. These feature are only noticeable at long wavelengths (i.e. Y-band) where the photons are able to penetrate the full thickness of the CCD. Similar features have been seen by Tony Tyson et al. when testing the LSST CCDs (STA1920A).