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Flat Fielding

All observed frames must be divided by a normalized flat frame in order to correct the sensitivity difference among detector pixels. This procedure is called the flat fielding. Flat frames are obtained by subtracting median OFF-flat frame from median ON-flat frame. While OFF-flat frames are taken when the continuum light source is turned off, ON-flat frames are taken when the light source is turned on with a uniform brightness distribution. The OFF-flat frames include background thermal noise and dark noise. The ON-flat frames must have sufficient intensity level with a high signal-to-noise ratio. Median OFF- and ON-flat frames are made of combining several frames with median filtering to exclude cosmic ray events and reduces noise. Figures 29 and 30 show the median OFF- and ON-flat frames of $ H$ and $ K$ bands. The background thermal noise, which is more dominant in $ K-$band, and dark noise are excluded by subtraction median OFF-flat frame from median ON-flat frame.

For image frames, the flat fielding is completed by dividing object frames by the median flat frame. For spectral frames, however, one has to proceed further in order to make a flat frame due to the following reasons.

  1. Spectral frames have inter-order regions where the light from the light source does not arrive except for scattering light or ghosts. The pixel values of such regions are generally low. These regions would have very large pixel values close to infinity if the flat fielding is done using a median flat frame. In order to avoid this, inter-order regions in flat frames must be set to unity.
  2. The median flat frame has an illumination profile along the wavelength reflecting the characteristics of the lamp (temperature, spectral lines, etc.). The illumination profile must be removed by fitting a proper polynomial to the median flat frame.
  3. The illumination profile along the wavelength also varies with the blaze functions of the echelle and the cross-disperser. The blaze functions are removed from the median flat frame by fitting polynomials.
The next subsections explain these procedure in detail.

  

  

Figure 29: Frames for making $ H-$band flat frame. (left) OFF flat frame and (right) ON flat frame.
\begin{figure}
\lq
\begin{center}
\includegraphics[width=6in]{Flat_H_ONOFF.eps}
\end{center}
\end{figure}

Figure 30: Frames for making $ K-$band flat frame. (left) OFF flat frame and (right) ON flat frame.
\begin{figure}
\lq
\begin{center}
\includegraphics[width=6in]{Flat_K_ONOFF.eps}
\end{center}
\end{figure}



Subsections

Tae-Soo Pyo
2003-05-29