Kennet B. W. Harpsœ, Uffe G. J\orgensen, Michael I. Andersen, Frank Grundahl
The EMCCD is a type of CCD that delivers fast readout times and negligible
readout noise, making it an ideal detector for high frame rate applications
which improve resolution, like lucky imaging or shift-and-add. This improvement
in resolution can potentially improve the photometry of faint stars in
extremely crowded fields significantly by alleviating crowding. This
improvement in resolution is a prerequisite for observing gravitational
microlensing in main sequence stars towards the galactic bulge. However, the
photometric stability of this device has not been assessed. The EMCCD has
sources of noise not found in conventional CCDs, and new methods for
photometric reduction must be developed. We aim to investigate how the normal
photometric reduction steps from conventional CCDs should be adjusted to be
applicable to EMCCD data. One complication is that a bias frame cannot be
obtained conventionally, as the output from an EMCCD is not normally
distributed. Also, the readout process generates spurious charges in any CCD,
but in EMCCD data, these charges are visible as opposed to the conventional
CCD. A simple probabilistic model for the dark output of an EMCCD is developed.
Fitting this model with the expectation-maximization algorithm allows us to
estimate the bias, readout noise, amplification, and spurious charge rate per
pixel and thus correct for these phenomena. To investigate the stability of the
photometry, corrected frames of a crowded field are reduced with a PSF fitting
photometry package. We find that it is possible to develop an algorithm that
elegantly reduces EMCCD data and produces stable photometry at the 1% level in
an extremely crowded field.
View original:
http://arxiv.org/abs/1202.3814
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