Calibration Considerations

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Calibration Considerations

• Christopher Stubbs
• Department of Physics Department of Astronomy
• Harvard University
• cstubbs_at_fas.harvard.edu

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FeIIIII ?
SII ?
SiII ?
FeIIIII ?
Supernova spectrum (Ia)
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Spectra from 2003
from Tom Matheson
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Sample light curves (I)
z 0.40
z 0.34
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Sample Light Curves (II)
z 0.61
z 0.82
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SN Peak Magnitudes
Tonry et al (2003)
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Current Metrology Chain for Ground-based
Astronomical Flux Measurements.
Celestial Sources
Celestial Calibrator, Vega
Black Body Calibrator
Supernova measurements require knowing relative
instrumental sensitivity vs. l
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Vega is primary celestial calibrator
5000 A 1 mm
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Detector-based Radiometry

• Modern metrology for radiometric measurements is
  based on detectors, not sources.

Larson, Bruce and Parr, NIST special publication
250-41
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Transferring flux standard to photodiodes

• Through 1 intermediate step, this calibration is
  transferred to Si photodiodes

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An alternative calibration approach
Calibrate relative system response
primary corrector optics filter detector rel
ative to Si photodiode. Measure transfer
function of atmosphere with dedicated
spectrograph
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Calibration is a Major source of Systematic Error
in Photometry

• Each pixel sees a sum of sources, with different
  spectral energy distr.
• The challenge is to recover source flux in the
  case where sky background dominates.
• There is, in principle, no single number that can
  correct for R(l)!

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Implications

• Need to map out system response, on an ongoing
  basis, as a function of wavelength
• Mirrors, filters, and detectors as integral meas.
• Requires flat-field screen inside enclosure, plus
  illum. and reference calibrators
• Need to characterize atmospheric transmission(l)
  and emission(l) in order to correct for
  atmospheric effects
• Small spectroscopic telescope as part of
  observatory.
• Need to adjust exposure time calculator to
  include flat-fielding errors.
• Try it out!