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

•Differential aperture photometry

•2MASS catalog provides excellent set of reference stars

2MASS performance: 2% floor

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

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σ / N /3

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

Intra-pixel response

•Centers of NICMOS3 pixels tend to be more sensitive

•Undersampled PSFs are particularly problematic

•Net 1% problem in H band

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

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

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

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σ =0.015

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χ 2 = 0.91

Astrometry

•2MASS absolute positions accurate to ~80mas

•Differential astrometry with longer integrations should be better

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

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

Next Steps

•Focus on differential astrometry

•Fix comparison stars

•Measure target position relative to centroid of ensemble

•Consider geometric distortions to image plane