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Photometry and Astrometry: Bright Point Sources

Photometry and Astrometry: Bright Point Sources. May 16, 2006 Cullen Blake. Bright Point Sources. Stars in the 10.5<J<14 range Photometry of individual images Potential pitfalls Astrometry of individual images Expectations and preliminary results. Photometry.

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Photometry and Astrometry: Bright Point Sources

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  1. Photometry and Astrometry: Bright Point Sources May 16, 2006 Cullen Blake

  2. Bright Point Sources • Stars in the 10.5<J<14 range • Photometry of individual images • Potential pitfalls • Astrometry of individual images • Expectations and preliminary results

  3. Photometry • Differential aperture photometry • 2MASS catalog provides excellent set of reference stars 2MASS performance: 2% floor

  4. Results • Scatter of binned measurements well above photon noise • Measurements at single dither position highly correlated • Binned error more like • Individual 7.8s measurements not independent • Scatter at single dither position well described by photon noise • Sources of systematic error unique to undersampled data: • Intra-pixel response • Flat field noise • Problems worse when 40% of light on single pixel

  5. HST J band, center to corner 0.37 mag; Lauer (1999)

  6. Intra-pixel response • Centers of NICMOS3 pixels tend to be more sensitive • Undersampled PSFs are particularly problematic • Net 1% problem in H band

  7. Flat Field Errors • PAIRITEL does not have a cold shutter for taking dark images • Variations in dark or bias very difficult to calibrate out of flat fields • PAIRITEL flats generated from rapidly changing sky • Pervasive 2% noise in flat fields generated with different methods • Likely primary source of residual noise

  8. Long-term Photometric Stability • It has been suggested that “second-order” extinction potential problem • Changes in PWV result in differential extinction • Target and comparison very different in temperature • Spectrum could have strong features, comparison smooth • Spectra convolved with filter and sky change by different amounts • Probably undetectable without knowledge of PWV • Modeled with high-resolution atmospheric spectra from Ken Jucks • High resolution comparison spectra from Pickles, UCDs from Cushing

  9. Long-term Photometric Stability • Seasonal changes of a few mm PWV can lead to large offsets in J • H and K bands only slightly influenced • Composite flux relatively immune to second-order extinction

  10. Astrometry • 2MASS absolute positions accurate to ~80mas • Differential astrometry with longer integrations should be better

  11. Simple Estimate • WCS-tools solutions show 200 mas residuals per image (absolute) • Use 2MASS catalog • Feed SexTractor centroids • Allow polynomial fitting • A single 30 minute integration yields ~100 observations • Assuming images are correlated in sets of 3 • Expect ~35 mas absolute astrometry

  12. Very simple methods yields ~40 mas per 30 minutes spanning 8 months

  13. Next Steps • Focus on differential astrometry • Fix comparison stars • Measure target position relative to centroid of ensemble • Consider geometric distortions to image plane

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