Removing the Fringes from Space Telescope Imaging Spectrograph Slitless Spectra

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Removing the Fringes from Space Telescope Imaging
Spectrograph Slitless Spectra

• Malumuth et. al. (2003)
• Presenter Tijmen van Wettum

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Overview

• Fringing
• Model
• Results
• Summary

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Fringing

• Rapidly varying sensitivity as a function of
  wavelength from interference of the incident and
  reflected light within the CCD
• Slit-spectroscopy solved with fringe flat from
  a tungsten continuum lamp.
• Slitless-spectroscopy Unable to get a fringe
  flat.
• Predict Fringing from physical and chemical
  structure of CCD.

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Fringing

• 1) A tilt
• 2) Kinks and wiggles
• 3) Vertical washed out areas
• Need to determine thickness of detection layer
  pixel by pixel

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Model

• Fresnell equations
• Unknowns 8 thickness 7 roughness (medium
  substrate assumed infinite thus 13 left)
• Index of refraction extinction coefficient
  assumed known
• IDL procedure to fit

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Model

• Model can reproduce typical fringe pattern
• Need to solve for thickness of each pixel on the
  CCD

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Model

• 51 data sets. Spectra from internal calibration
  source
• Only solve thickness of detection layer

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Model

• Method works well except for a few pixels.
• Result of small error in index of refraction
• Solved by assuming index of refraction of 9000A
  to be accurate, solve the thickness from that,
  then use the solved thickness from the lower
  wavelengths to correct for the index of refection
  for these wavelengths

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Results
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Results

• Good results with Slit spectroscopy
• Old method better with these kind of
  observations

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Results

• Slitless Spectroscopy
• In the paper 3 examples where fringing obscured
  important stellar features

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Summary

• Solved physical and chemical structure of CCD to
  correct for fringing.
• Results are good, but not as good as defringing
  with calibration-measurements
• Very useful in Slitless-spectroscopy.