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LSST CCD Chip Calibration

LSST CCD Chip Calibration. Tiarra Stout. 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. CCD Array. Charge-coupled device How it works: Rows of pixels with silicon layer. Photon hits silicon layer.

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LSST CCD Chip Calibration

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  1. LSST CCD Chip Calibration Tiarra Stout

  2. 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

  3. 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.

  4. 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

  5. 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 star’s 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.

  6. 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.

  7. 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.

  8. 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 • V=V 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.

  9. 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.

  10. 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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