## 3 Spectroscopic Survey
### Reduction of the Data Obtained with the Galileo Telescope  
The \"longslit\" spectroscopy consists of passing light from an astronomical source through a thin rectangular aperture. A CCD sensor records the light diffracted by a scattering element, the grating. However, the data reduction needs other images for calibration in addition to those of the objects: bias, flat-field (quartz), caliber lamp (e.g., He-Fe-Ar) and spectrum of an any class spectro-photometric star (e.g., Vega, HR153, etc.; this procedure is necessary because the CCD response is measured in counts called ADU(\(x\),\(y\)) (Analog Digital Units), which are related to the intrinsic signal of the plane of the sky I(\(x\),\(y\)) by the relation:  
\[\mathrm{ADU}(x,y)=\mathrm{I}(x,y)\mathrm{FF}(x,y)+\mathrm{BIAS}(x,y)+\mathrm{ DARK}(x,y)+\mathrm{SKY} \tag{1}\]  
where  
* The BIAS(\(x\),\(y\)) is obtained with the shutter closed and an exposure time of 0 s, i.e., 0 photoelectrons collected; in this way, subtracting photo-electrons from the scientific image countings due to instrumental effects are eliminated.  
\begin{table}
\begin{tabular}{l l l l} \hline \hline
**Denomination** & **Lines/mm** & **Blaze Wavelength [Å]** & **Spectral Range [Å]** \\ \hline
300 & 300 & 5000 & 3500–7000 \\ \hline
1200 R & 1200 & 6825 & 5750–7000 \\ \hline
600 V & 600 & 4500 & 4250–6590 \\ \hline
1200 B & 1200 & 4000 & 3820–5020 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Different gratings available for the Boller & Chivens spectrograph at the Galileo Telescope.  
\begin{table}
\begin{tabular}{l l} \hline \hline Longitude & E 11°31′35.138′′ \\ \hline Latitude & N 45°51′59.340′′ \\ \hline Altitude & 1044.2 m above sea level \\ \hline International code IAU-MPC & 043 \\ \hline Primary mirror diameter (outer edge) & 1237 mm \\ \hline Primary mirror effective diameter & 1200 mm \\ \hline Primary mirror thickness (at the edge) & 208 mm \\ \hline Primary mirror focal length & 6000 mm \\ \hline Primary mirror focal ratio & f/5.0 \\ \hline Secondary mirror diameter & 520 mm \\ \hline Equivalent focal length & 12,100 mm \\ \hline Focal ratio & f/10.1 \\ \hline Scale & 17.05 arcsec/mm \\ \hline \hline \end{tabular}
\end{table}
Table 2: The Galileo Telescope’s main technical features.  
* The flat-field FF(\(x\),\(y\)) is obtained by pointing the telescope towards the closed dome illuminated by the uniform light from a lamp (this method is also known as dome flat). Thus, the obtained spectrum is composed of the continuum spectrum from the lamp, the response curve of the CCD and the non-uniform pixel response.
* The DARK(\(x\),\(y\)) is due to the eddy currents that are generated in the CCD caused by thermal agitation. This contribution can be neglected because modern CCDs are kept at very low temperatures (\(-80/90\)\({}^{\circ}\)C).
* Other images used are those of the He-Fe-Ar lamp and the standard star, respectively, for the wavelength calibration and the flux calibration.  
The reduction of the spectra has been made with the IRAF (Image Reduction and Analysis Facility), which allows processing astronomical images; in our specific case, the Pennar pipeline made available by the observatory of Asiago has been used.