What is Extinction

1
Extinction P. Thompson, B. Gilfus, J. Neeley, A.
James, E. Moyer P. Enrique da Silva, A. Kanaan,
S. Kanbur

• What is Extinction?
• When light from a star passes through the
  Earths atmosphere, part of it is absorbed and
  part is dispersed. Consequently the light
  received by the telescope is weaker than the
  light the telescope would have received in the
  absence of the atmosphere. This process is
  called extinction.
• Because the atmosphere can change from night to
  night, we only estimate extinction using
  observations from a single night.
• Because different filters observe different
  wavelength windows, we only estimate extinction
  using observations made through the same filter.
• Airmass
• The extinction of an observation depends chiefly
  on the amount of atmosphere through which the
  light passes. As seen below, the extinction of a
  higher-altitude observation (1) will be less than
  that of a lower-altitude observation (2).

• 2nd Order Catalog Extinction
• User input night, filter, catalog (list of
  stars with known m0s)
• Extinction equation m m0 a k(airmass)
  k(airmass)(color)
• color is computed with respect to the reference
  band, i.e. color V - I
• Output coefficients a and k
• Method
• Query the database for a list of stars in the
  given catalog for which we have observations on
  the given night through the given filter.
• Make three lists x, y, and z with one element for
  each observation, where xi airmassi, y
  colori, and zi mi m0i and for the ith
  observation.
• Compute a, k, and kprime so as to minimize the
  RSS S(zi a kxi kxiyi)2.
• Make one graph for each color index and stack
  them in ascending order, portraying the
  three-dimensional curve z a kx kxy.
• Compute the correlation coefficient, r2. 0 r2
  1, where 0 indicates the worst possible
  correlation and 1 a perfect correlation.
• Return a, k, k and r2 in a dictionary.
• Graph to the right ?

Acknowledgement The authors thank NSF OISE award
0755646 and the 2009 Chretien award of the
American Astronomical Society.