Workshop on Precision Astronomy with Fully Depleted CCDs November 18, 19 2013 Introduction and Overview Christopher Stubbs Professor of Physics and of Astronomy, Harvard firstname.lastname@example.org. Workshop Objectives.
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Workshop on Precision Astronomy with Fully Depleted CCDsNovember 18, 19 2013Introduction and OverviewChristopher StubbsProfessor of Physics and of Astronomy, Harvardstubbs@physics.harvard.edu
Flux attenuation over 12 pixel
region around chip edge
Coherent lateral electric field at edges of array
1. Generate unwarped image
2. Divide each LSST pixel into micropixels
3. Apply exponential warp to micropixel coordinates
4. Populate integer micropixels with scaled data
5. Crop, bin back from micropixels to LSST pixels
Real and Simulated Rolloff Profiles
Warped Grid of “Stars” (A. Garg, LSST/Harvard)
Ran Source Extractor on original and warped fake stars, both with and without flatfielding
Output aperture mag, PSF mag, FWHM, ellipticity, centroids
We want to know:
- How does warping affect the MAGNITUDE of the star?
- How does warping affect the SHAPE of the star?
- How does warping affect the POSITION of the star?
- What happens if we try to flat-field by a warped flat?
Unwarped Grid of “Stars”
Warped Grid of “Stars”
PSF broadening is
ellipticity is basically unchanged
LSST ellipticity spec:
95% of focal plane e < 0.07
Warped vs. Unwarped Centroids
But astrometric shifts are not limited to these edge effects due to guard rings…
IN2P3 team, LSST sensor data
PSF fitting photometry
PSF corrections applied to weak lensing observations
galaxy color determinations
2-d autocorrelation at 73Ke, half of full well depth, (IN2P3 analysis)
Space charge effects impurities electrode effects/guard rings
How can we cleanly disentangle the contributions of genuine photon detection efficiency from various charge transport/lateral field effects?
What is the optimal pixel sampling to attain best possible measurement precision? There are some characteristic length scales:
Charge diffusion rms footprint
Pixel scale, 10-15 microns
Auto-correlation length in flat fields
Mask step-repeat scale
Correlation length of impurities (tree rings)
Coherence length of PSF shape across the focal plane, due to optics and atmosphere
The size of the PSF (or the size of a resolved object) compared to these scales will determine the systematic error that is introduced.
Do these considerations favor using TDI (drift scanning) systems for surveys, as opposed to staring arrays with a few pixels spanning the PSF?
How can we quantify, prioritize, calibrate and correct for these effects?
WASP project uses highly defocused images to perform differential photometry of sources.
They achieve 0.2 mmag photometric precision (Tregloan-Reed & Southworth, MNRAS 431, 996 (2013)).
0.2 mmag photometric precision (Tregloan-Reed & Southworth, MNRAS 431, 996 (2013)).
Subpixel pointing precision (Bryson et al, ApJLett713, L97 (2010)).
Thanks for taking the time to join us for this gathering.
Also, thanks in advance to our speakers.