LSST CCD Chip Calibration

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LSST CCD Chip Calibration

• Tiarra Stout

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LSST

• Large Synoptic Survey Telescope
• Camera 1.6 m by 3 m.
• 3.2 billion pixels.
• 2800 kg (6173 lbs)
• 10 square degrees of sky

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CCD Array

• Charge-coupled device
• How it works
• Rows of pixels with silicon layer.
• Photon hits silicon layer.
• Energy from the photon causes an electron to
  release.
• Electrical charge stored in pixel.
• Charges are read out vertical shift, then
  horizontal shift.
• Charges converted to digital number.
• Digital number corresponds to number of photons
  it received.
• Image is created pixel by pixel using digital
  number to show intensity of each pixel.

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LSST CCD Array

• LSST array will be much larger than current
  prototype.
• 21 rafts
• Rafts have 9 chips
• Each chip has 16 segments. (M51)
• Total 189 CCD chips

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Calibration

• Single prototype chip currently installed in
  Calypso telescope in Arizona.
• How well does it function?
• Images taken with Calypso.
• Find standard star.
• Photometry (measure of a stars flux or intensity
  of electromagnetic radiation) must then be done
  on the stars to compare the results of the LSST
  chip with known results of well measured objects.

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Photometry

• IRAF Image Reduction and Analysis Facility.
• First had to find good data. (example)
• Point object. Not smeared or elongated.
• Not saturated or too faint.
• Standard star in measured filters.
• Settled on EGGR 102 a.k.a. HIP 66578 and 24
  other names.
• Process images to reduce interference from
  electronic readout noise, thermal electrons,
  pixel-to-pixel variations, optical
  non-uniformities, etc.
• After processing, photometry is done to extract
  instrumental magnitudes.
• This is calculated by measuring the area under
  the radial profile of star (represents light from
  star and background), subtracting out the
  background light, dividing by exposure time, and
  taking the log of the final result.

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Photometry

• Compile catalog with standard star measurements
  to find coefficients to magnitude equations.
• Magnitude equations used to convert instrumental
  magnitudes to absolute magnitudes.
• Want absolute magnitude to characterize the star.
  By measuring the magnitude in different filters,
  we can figure out how far it is, its composition,
  etc.

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My Results

• Found coefficients for magnitude equations in r
  (red) and i (infrared) filter.
• IFIT mi(V-VI) i1 i2 Xi i3 VI
• mi instrumental magnitude
• VV magnitude
• VI color value. Difference between two filters.
• Xi airmass (extinction rate of photons as they
  move through atmosphere))
• i1 -0.02885237
• i2 0.01845326
• i3 0.1401964
• Known magnitude of EGGR 102 in i filter is
  12.979. Our value was 12.978.

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My Results

• RFIT mr (V VR) r1 r2 (Xr 1.326)
  r3 VR
• mr instrumental magnitude
• V V magnitude
• Xr airmass
• VR color value
• r1 -0.2086798
• r2 0.7110448
• r3 2.087208
• Known magnitude is 12.873. Our value is 12.465.
• This equation had to be modified since it did not
  originally converge. Brought the term closer to
  zero to shift the intercept and create higher
  precision by subtracting the average airmass of
  the images from the airmass variable.

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Conclusions

• Chip performing well and accurately so far. Need
  more data, however.
• Future observations need to include more standard
  stars in common filters (ugriz) for calibration
  purposes.
• Attention on image quality.

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