Photometry, PSF Fitting, · PDF file 2009. 10. 7. · Photometry • absolute...

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Transcript of Photometry, PSF Fitting, · PDF file 2009. 10. 7. · Photometry • absolute...

lecture8.ppt2
– see at end – due in class on Wed, Oct 14
• Midterm: Monday, Oct 26 • Reading:
– chapter 5 of Howell: photometry and astrometry • Get acquainted with IDL Astronomy packages
– download ATV (http://www.physics.uci.edu/~barth/atv/) – IDL Astronomy Users Library:
• object finding, centering • photometry • PSF fitting (DAOPHOT-type procedures)
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Outline
!
– Npix, bkg > 3 Npix, src
– use rbkg >> FWHM, whenever possible • enclosed energy P(r)
– “curve of growth”
– Npix, bkg > 3 Npix, src – use rbkg >> FWHM, whenever possible
• enclosed energy P(r) – “curve of growth”
• optimum aperture radius r – SNR(r) first increases, then decreases with r
• Fig. 5.7 of Howell – dependent on PSF FWHM and source brightness
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• approximately from ATV • precisely with gcntrd.pro
– option 2: • find automatically and center precisely: find.pro
• determine curve of growth from brightest star – aper.pro – get aperture corrections
• find aperture size for optimum SNR on objects of interest – aper.pro – apply appropriate aperture corrections
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• absolute photometry: – requires aperture correction – requires non-variable photometric standard stars
• similar time and location on sky as science targets (same airmass) • ideally, with identical color (e.g., B–V) as science targets
– requires photometric weather conditions – best attainable accuracy ~1% – example applications:
• color-magnitude diagrams • supernova flux measurements
10source: Kitt Peak National Observatory
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• absolute photometry: – requires aperture correction – requires non-variable photometric standard stars
• similar time and location on sky as science targets (same airmass) • ideally, with identical color (e.g., B–V) as science targets
– requires photometric weather conditions – best attainable accuracy ~1% – example applications:
• color-magnitude diagrams • supernova flux measurements
• differential photometry: – usually, with respect to stars of known brightness in the same field
• identical time and airmass – subject to variability of reference stars – best attainable accuracy ~0.001% (space), ~0.05% (ground) – example applications:
• searches for transiting planets
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PSF-fitting Cookbook • DAOPHOT I, II, III (P. Stetson 1987, 1991, 1994) • Implemented in IDL:
– getpsf.pro - step 1, determining the PSF – rdpsf.pro
– pkfit.pro - step 2, fitting the PSF to a single star or
– group.pro - step 2, simultaneous PSF fitting to – nstar.pro groups of stars
– substar.pro - step 3, subtracting stars to check residuals • produces accurate positions, photometry
– especially in crowded fields
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Astrometry: Pixel Sampling
• r = FWHM / (pixel size) • r < 1.5: under-sampled • Nyquist sampling: r ~ 2 (r=2.355, precisely)
– optimal SNR, error rejection, positional precision • r > 2 desirable for best photometry,
astrometry on bright point sources
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Project 3 • Finish the data reduction on the science exposures from Project 1
– create sky frames • median-combine without aligning the individual science object pointings of identical exposure
times – reduce the individual science exposures
• subtract sky, flat-field – align the reduced science exposures, and median-combine them
• e.g., in IDL: gcntrd + rot or correl_optimize • Perform aperture photometry on the point sources
– determine curve of growth from brightest source (aper) – find optimum aperture for the faint and bright sources (aper) – do aperture photometry and apply aperture corrections (aper)
• Perform PSF-fitting photometry on all sources – fit PSF to brightest source, using output from aper above (getpsf, group, nstar) – compare outputs for magnitudes and positions of all sources between the aperture and
PSF-fitting photometry • Submit a 1-page write-up, appended by