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5 Spectral extraction

One of the advantages of the Super-COSMOS machine is that it scans the plates with a direction parallel to the longtitudinal axis of the spectra. Thus, our spectra are parallel to a coordinate axis. The success of the DETSP procedure is that it detects all the spectra at the same common-wavelength zero-point at 5400 Å. This zero-point (0.000 mm) corresponds to our pixel scale (1-128) at 10 pixels.

After the spectral detection, a new procedure starts, responsible for the extraction of spectra (EXTSP). The spectral length contains 128 pixels. These are: the zero-point plus 118 pixels on the right of zero-point plus 9 pixels in the left of zero-point. For a better signal-to-noise ratio, the actual extraction of the spectrum is performed by means of rectangular weighted "slit'' sliding on data (Balestra et al. [1990]). Its width and shape are either fixed or determined by the average fit on the transversal sections of the spectrum.

The new zero-point defined by DETSP at 5400 Å$\;$ had to be added to the dispersion curve of the objective prism P1 (Nandy et al. [1977]). The parallel displacement in mm of zero-point gives new distance measurements for various features. The results are shown in Fig. 2 and Table 2.

\psfig{,height=65mm,width=88mm} }
\vspace{5mm} \end{figure} Figure 2: Dispersion curve for objective prism P1


Table 2: Details for the features on objective prism P1
${\rm Feature}$ $\lambda(\mbox{\AA})$ ${\rm Distance}~(\mbox{mm})$ ${\rm Pixel~Num.}$
$\mbox{Zero-Point}$ $\mbox{5400}$ $\mbox{0.000}\pm\mbox{0.005}$ $\mbox{10}\pm\mbox{1}$
$\mbox{TiO}$ $\mbox{5000}$ $\mbox{0.100}\pm\mbox{0.005}$ $\mbox{20}\pm\mbox{1}$
$\mbox{H}_{\beta}$ $\mbox{4861}$ $\mbox{0.150}\pm\mbox{0.005}$ $\mbox{25}\pm\mbox{1}$
$\mbox{TiO}$ $\mbox{4800}$ $\mbox{0.160}\pm\mbox{0.005}$ $\mbox{26}\pm\mbox{1}$
$\mbox{H}_{\gamma}\mbox{+G}$ $\mbox{4340,~4300}$ $\mbox{0.320}\pm\mbox{0.005}$ $\mbox{42}\pm\mbox{1}$
$\mbox{CaI}$ $\mbox{4227}$ $\mbox{0.340}\pm\mbox{0.005}$ $\mbox{44}\pm\mbox{1}$
$\mbox{H}_{\delta}$ $\mbox{4101}$ $\mbox{0.430}\pm\mbox{0.005}$ $\mbox{53}\pm\mbox{1}$
$\mbox{H}\mbox{+H}_{\epsilon}$ $\mbox{3970}$ $\mbox{0.500}\pm\mbox{0.005}$ $\mbox{60}\pm\mbox{1}$
$\mbox{CaII+K}$ $\mbox{3936,~3934}$ $\mbox{0.520}\pm\mbox{0.005}$ $\mbox{62}\pm\mbox{1}$
$\mbox{MgI+FeI~blend}$ $\mbox{3820}$ $\mbox{0.570}\pm\mbox{0.005}$ $\mbox{67}\pm\mbox{1}$
$\mbox{FeI+H~blend}$ $\mbox{3730}$ $\mbox{0.640}\pm\mbox{0.005}$ $\mbox{74}\pm\mbox{1}$
$\mbox{FeI~blend}$ $\mbox{3580}$ $\mbox{0.740}\pm\mbox{0.005}$ $\mbox{84}\pm\mbox{1}$

The extracted spectra are stored in a two-dimensional file $n \times 128$, where n=426 is the number of detected spectra. Every row of this file is an independent normalized spectrum with length 128 pixels (Fig. 3).

\psfig{,height=100mm,width=40mm} }
\vspace{5mm} \end{figure} Figure 3: A sample of $426 \times 128$ spectra

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