Physics PhD Dissertation Defense: Adam Snyder

Aug 06, 2020 - 10:00 am to 11:00 am

Research Advisor: Aaron Roodman
Zoom Meeting Link: https://stanford.zoom.us/j/9800673354
Email for snyder18@stanford.edu for Zoom password.

Characterization of LSST Camera Sensor Effects During Integration and Testing

Integration and testing of the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) Camera is currently being performed at the SLAC National Accelerator Laboratory.  Once completed, the LSST Camera will be used to perform one of the largest wide-field astronomical surveys in the optical, generate an unprecedented amount of data, and allow for cosmological analyses with great statistical power.  In order to realize the full benefit of the increase in statistical precision that the LSST aims to accomplish, it is necessary to have a deep understanding of systematic effects that may negatively impact the accuracy of the survey science results. One particular type of systematic effects are those resulting from aspects or behavior of the charge-coupled devices (CCDs) used for digital imaging.  The LSST Camera focal plane is heavily segmented, consisting of 189 CCDs arranged into 21 stand-alone Raft Tower Modules (RTMs), each of which must be carefully verified and tested to ensure that they adhere to the requirements of the project.  In this thesis I present the results of electro-optical testing of the focal plane electronic cross talk, astrometric and shape distortions due to static pixel area variations, and deferred charge effects. 

In CCDs that possess multiple readout channels, electronic cross talk refers to the appearance of “ghost” images in one segment of the CCD that coincide with bright sources that are imaged in a different segment of the CCD.  Characterization of the cross talk involves its measurement in a number of different regimes: cross talk within a CCD, cross talk between CCDs on the same raft, and cross talk between CCDs on different rafts. I will discuss the measurement methodology and the results for the cross talk characterization during the LSST Camera partial focal plane testing period.

Changes in CCD pixel response that are caused by pixel area variations rather than quantum efficiency effects are of particular concern because of their impact on photometry, astrometry, and measurement of galaxy shapes for weak lensing science. While these pixel area variations can be measured using images of star fields, I present a method to use a custom built optical projector capable of projecting patterns of realistically sized sources on to the Camera focal plane in order to study these effects in the laboratory, prior to on-sky operation.

Finally, in addition to global charge transfer inefficiency that is inherent in the operation of a CCD, a fraction of the CCDs in the LSST Camera exhibit additional deferred charge contributions that will degrade the brighter-fatter correction, the measurement of the optical PSF, and ultimately astrometric and shape measurements used for weak gravitational lensing and galaxy clustering science.  In the second half of this thesis, I present a characterization of the deferred charge effects that was used to determine the sources of additional deferred charge and the development and implementation of a new deferred charge correction algorithm to reduce the impact on the survey science goals.  The results of the application of the correction algorithm to data sets obtained during the partial focal plane testing period are shown, and areas for improvement and future study are discussed.