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Digital Aperture Photometry

Digital Aperture Photometry. ASTR 3010 Lecture 10 Textbook 9.5. Photometry. How bright is the object? In an object frame… measured brightness = source + background background (or “sky”) : all unwanted light not originated from the source foreground + background scattered light

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Digital Aperture Photometry

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  1. Digital Aperture Photometry ASTR 3010 Lecture 10 Textbook 9.5

  2. Photometry • How bright is the object? • In an object frame… measured brightness = source + background background (or “sky”) : all unwanted light not originated from the source • foreground + background scattered light • glow of the atmosphere • radiation 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.

  3. Digital Apertures • Aperture : a circular area centered on the centroidof 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 pixel’s area inside the aperture) • Estimate the value of the sky emission per pixel • Subtract the sky emission from the total count. A=0.05 A=1.0 A=0.7

  4. When 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 image and the source brightness will be the summation of “scaled” PSF pixel values.

  5. PSF fitting versus Aperture photometry • PSF fitting ≈ infinite aperture size Aperture PSF fit background level Gaussian + constant How do we choose the right aperture size?

  6. Best aperture size? Large Aperture • include more light  larger S • more contamination • added noise from the sky  lower S/N • Small Aperture • less contamination • losing source signal •  lower S/N

  7. Good Aperture Size • Typical choice of apertures: 0.75 to 4 times FWHM • Best S/N  about 2 times the HWHM (or 1 FWHM) radial profile of the object (i.e., 1D PSF)

  8. Python HW #3 (x2 weight) • Using one of FITS files from HW#2, create a Python script that generates a radial profile.

  9. Measuring Sky • Sky 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).

  10. Measuring Sky • Typically measuring the sky level from a sky annulus • inner sky radius : as small as possible yet large enough away from the source • outer sky radius : large enough to include significant # of pixels in statistics, but not too far from the source • Not the mean pixel value: • Or from a dedicated sky region mean median mode

  11. Measuring Sky • What about the case like Super Nova embedded in a rapidly varying background?

  12. How about non-variable objects in the non-uniform sky?

  13. How about non-variable objects in the non-uniform sky? • Find all sources with PSF fitting • Remove detected sources • heavily smooth the residual image • subtract the smoothed residual from the source image • do photometry original image source subtracted image Bright sources are not perfectly subtracted. Why?

  14. Signal and noise in an aperture see textbook pp313-317 for a detailed derivation

  15. CCD equation readout noise limited faint star log (time) sky-limited faint star bright source log (SNR)

  16. In summary… Important Concepts Important Terms photon-noise limited sky-limited readout noise limited • CCD equation (consult textbook!) • Aperture photometry • PSF fitting photometry • Chapter/sections covered in this lecture : 9.5

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