Digital Aperture Photometry
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Life in the Universe
Digital Aperture PhotometryASTR 3010
1PhotometryHow bright is the object?
In an object frame
measured brightness = source + background
background (or sky) : all unwanted light not originated from the sourceforeground + background scattered lightglow of the atmosphereradiation from the telescope, etc.
In this lecture, we will focus only on the tasks of separating signals from background and getting uncertainty of the measurement.Digital AperturesAperture : a circular area centered on the centroid of the object
Three computations in the aperture photometry:Add up all pixel values inside the aperture: take into account of fractional pixels (A is a fraction of pixels area inside the aperture)
Estimate the value of the sky emission per pixel
Subtract the sky emission from the total count.
A=1.0A=0.7A=0.053When does the Aperture Photometry fails?when star images (i.e., PSFs) seriously overlap
PSF fitting photometry is better in this case! fitting a PSF to each star imageand the source brightness will be the summation ofscaled PSF pixel values.
PSF fitting versus Aperture photometryPSF fitting infinite aperture size
PSF fitAperturebackground level
How do we choose the right aperture size?Gaussian + constantBest aperture size?Large Apertureinclude more light larger Smore contaminationadded noise from the sky lower S/NSmall Apertureless contaminationlosing source signal lower S/N
Good Aperture SizeTypical choice of apertures: 0.75 to 4 times FWHMBest S/N about 2 times the HWHM (or 1 FWHM)
radial profile ofthe object (i.e., 1D PSF)
Python HW #3 (x2 weight)Using one of FITS files from HW#2, create a Python script that generates a radial profile.
Measuring SkySky measurement :Need to measure the sky level at the location of the source impossible.So, we assume that the sky does not change with location (i.e., homogeneous sky).Measuring SkyTypically measuring the sky level from a sky annulusinner sky radius : as small as possible yet large enough away from the sourceouter sky radius : large enough to include significant # of pixels in statistics, but not too far from the sourceNot the mean pixel value: Or from a dedicated sky region
Measuring SkyWhat about the case like Super Nova embedded in a rapidly varying background?
How about non-variable objects in the non-uniform sky?
How about non-variable objects in the non-uniform sky?Find all sources with PSF fittingRemove detected sources heavily smooth the residual imagesubtract the smoothed residual from the source imagedo photometry
original image source subtracted image
Bright sources are not perfectly subtracted. Why?Signal and noise in an aperture
see textbook pp313-317 for a detailed derivation
log (time)log (SNR)bright sourcesky-limited faint starreadout noise limitedfaint starIn summaryImportant ConceptsCCD equation (consult textbook!)Aperture photometryPSF fitting photometryImportant Termsphoton-noise limitedsky-limitedreadout noise limitedChapter/sections covered in this lecture : 9.5