Photometry, PSF Fitting,Astrometry

AST443, Lecture 8Stanimir Metchev

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Administrative• Project 2:

– finalized proposals due today• Project 3:

– 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

• Photometry– point-source centering– aperture

• background• flux calculation• SNR

• PSF-fitting– photometry

• Astrometry

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Centering of Point Sources

• centroid– chapter 5.1.1. of Howell– sub-pixel precision possible– IDL Astronomy Library: cntrd.pro

• 2D profile fitting– gaussian (gcntrd.pro)– modified Lorentzian, Moffat– PSF fit (revisit later)

!

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Aperture Photometry

• object flux = total counts – sky counts• estimation of background

– Npix, bkg > 3 Npix, src

– use rbkg >> FWHM, whenever possible• enclosed energy P(r)

– “curve of growth”

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PalomarAO PSF

Hayward et al. (2001)

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Aperture Photometry

• object flux = total counts – sky counts• estimation of background

– 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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Aperture PhotometryCookbook

• determine object centers– option 1:

• 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 ofinterest– aper.pro– apply appropriate aperture corrections

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Absolute vs. DifferentialPhotometry

• 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 vs. DifferentialPhotometry

• 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 staror

– 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

• limiting precision– δr ~ FWHM / SNR– unatainable in practice

• systematic effects– focal plane curvature, distortion– differential atmospheric refraction– pixel sampling

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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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Hayward et al. (2001)

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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

– relevant plots (curve of growth, radii for optimum SNR)– tables (photometry with aperture and PSFs)– your code.