November 2, 2017
Dr. Chris Johnson, STScI, will present "Crowded Field Photometry Using Image Subtraction and PSF Fitting: From the Galactic Bulge to Massive Open Clusters". Free pizza and everyone welcome to attend.
Crowded field photometry is vital when analyzing dense stellar regions of space that contain thousands up to tens of millions of stars per square degree. Aperture photometry fails in these dense regions since stellar neighbors can be so close that they contribute unwanted light in to your aperture and give false magnitudes. Stars appear as point sources of light when imaged with a telescope and their light can be modeled as a Gaussian function. The light spreads out radially from the bright center (much like a bell curve) and we refer to this function as a "Point Spread Function" or PSF. We can then fit a basic PSF model to all the stars in the field, build up a better average PSF model by iterating through the image and rejecting bad pixels, variable stars, bad images, etc. and create a final master PSF. We then subtract this master from each individual star in each image again and the residuals left behind will tell us something about the source. I have extended the techniques that I used from graduate school to look at X-ray binaries in the Galactic Bulge to uncovering eclipsing binary populations in very young open clusters.