Photometry, PSF Fitting, Astrometry. AST443, Lecture 8 Stanimir Metchev

Transcription

1 Photometry, PSF Fitting, Astrometry AST443, Lecture 8 Stanimir Metchev

2 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 ( IDL Astronomy Users Library: object finding, centering photometry PSF fitting (DAOPHOT-type procedures) 2

3 Outline Photometry point-source centering aperture background flux calculation SNR PSF-fitting photometry Astrometry 3

4 Centering of Point Sources centroid chapter of Howell sub-pixel precision possible IDL Astronomy Library: cntrd.pro 2D profile fitting gaussian (gcntrd.pro) modified Lorentzian, Moffat PSF fit (revisit later) 4

5 Aperture Photometry object flux = total counts sky counts estimation of background N pix, bkg > 3 N pix, src use r bkg >> FWHM, whenever possible enclosed energy P(r) curve of growth 5

6 Palomar AO PSF Hayward et al. (2001) 6

7 Aperture Photometry object flux = total counts sky counts estimation of background N pix, bkg > 3 N pix, src use r bkg >> 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 7

8 Aperture Photometry Cookbook 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 of interest aper.pro apply appropriate aperture corrections 8

9 Absolute vs. Differential Photometry 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 9

10 source: Kitt Peak National Observatory 10

11 Absolute vs. Differential Photometry 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 11

12 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 12

13 Astrometry limiting precision δr ~ FWHM / SNR unatainable in practice systematic effects focal plane curvature, distortion differential atmospheric refraction pixel sampling 13

14 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 14

15 Hayward et al. (2001) 15

16 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. 16