High Frame-rate Imaging Based Photometry, Photometric Reduction of Data from Electron-multiplying Charge Coupled Devices (EMCCDs)

摘要

The EMCCD is a type of CCD that delivers fast readout times and negligiblereadout noise, making it an ideal detector for high frame rate applicationswhich improve resolution, like lucky imaging or shift-and-add. This improvementin resolution can potentially improve the photometry of faint stars inextremely crowded fields significantly by alleviating crowding. Alleviatingcrowding is a prerequisite for observing gravitational microlensing in mainsequence stars towards the galactic bulge. However, the photometric stabilityof this device has not been assessed. The EMCCD has sources of noise not foundin conventional CCDs, and new methods for handling these must be developed. We aim to investigate how the normal photometric reduction steps fromconventional CCDs should be adjusted to be applicable to EMCCD data. Onecomplication is that a bias frame cannot be obtained conventionally, as theoutput from an EMCCD is not normally distributed. Also, the readout processgenerates spurious charges in any CCD, but in EMCCD data, these charges arevisible as opposed to the conventional CCD. Furthermore we aim to eliminate thephoton waste associated with lucky imaging by combining this method withshift-and-add. A simple probabilistic model for the dark output of an EMCCD is developed.Fitting this model with the expectation-maximization algorithm allows us toestimate the bias, readout noise, amplification, and spurious charge rate perpixel and thus correct for these phenomena. To investigate the stability of thephotometry, corrected frames of a crowded field are reduced with a PSF fittingphotometry package, where a lucky image is used as a reference. We find that it is possible to develop an algorithm that elegantly reducesEMCCD data and produces stable photometry at the 1% level in an extremelycrowded field.