## 3 Spectroscopic Survey
### Observing Campaign  
Observing the Moon with the Galileo Telescope is not trivial; the telescope is designed to observe faint objects. There are some issues to deal with and to focus on, which will be outlined below as follows:  
* The Moon's brightness is the first relevant driver. To avoid saturation of the image, the exposure time needs to be properly reduced. The saturation occurs when the countings exceed this value 2\({}^{\rm{mbit}}\), i.e., 65,536 counts; however, the sensor loses linearity beyond 40,000 counts while under 30,000 counts, the signal to noise ratio (SNR) increases. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Star** & **RA [hh mm ss]** & **Dec [\({}^{\circ}\)\({}^{\prime\prime}\)\({}^{\prime\prime}\)]** & **V Mag** \\ \hline Land (SA) 93–101 & 01 53 18.0 & +00 22 25 & 9.7 \\ \hline Hyades 64 & 04 26 40.1 & +16 44 49 & 8.1 \\ \hline Land (SA) 98–978 & 06 51 34.0 & \(-\)00 11 33 & 10.5 \\ \hline Land (SA) 102–1081 & 10 57 04.4 & \(-\)00 13 12 & 9.9 \\ \hline Landolt (SA) 107–684 & 15 37 18.1 & \(-\)00 09 50 & 8.4 \\ \hline Land (SA) 107–998 & 15 38 16.4 & +00 15 23 & 10.4 \\ \hline
16 Cyg B & 19 41 52.0 & +50 31 03 & 6.2 \\ \hline Land (SA) 112–1333 & 20 43 11.8 & +00 26 15 & 10.0 \\ \hline Land (SA) 115–271 & 23 42 41.8 & +00 45 14 & 9.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Typical stars defined as solar analogs (same spectral class of the Sun) used as the divisor in the reduction of “relative flux” spectra. * Exposure time below the second might give rise to some errors due to the calibration of the mechanical shutter being set on poses of several minutes. During a full Moon, it is necessary to act on the diaphragm aperture, which has to be rightly closed to minimize faults that affect the flux results and to extend the exposure time; because of this, a good spatial resolution for the guide camera could be lost. * The Moon's proximity to the Earth causes a rapid variation in the Moon's coordinates, and robotically or differential tracking are not possible; for example, the [PERSON] (the robotic telescope of the INAF Astronomic Observatory of Padua), in addition to not pointing at the Moon, cannot even approach it. Nonetheless, the Galileo Telescope has the ability to perform manual tracking at three different speeds; in this way, we can follow the Moon's motion seamlessly and we have freedom of choice in the observable areas. This is not a common feature but thanks to that, the tracking of the selected bright zone has been performed in a meticulous way. * The guide camera does not have a high resolution due to air turbulence and it works poorly in a high illumination condition; so, positioning the slit on the right zone and navigating on the lunar surface is not always an easy issue. This phenomenon of image distortion is augmented when approaching polar latitudes due to the Moon's sphericity. * Another important aspect is bound up with the subtraction of the sky's emission. This operation can be performed in different positions depending on the objective of the observations; in most instances, a compromise is necessary. For example, the Moon has an exosphere up to 500 km thick, which does not need to be considered for a lunar surface spectroscopy analysis. This is ensured by the length of the slit (8.5 arcminutes on the celestial plane, about 950 km of field of view in width) that allows extracting the lines of the sky without \"contaminating\" the final spectrum. Moreover, it is necessary to bear in mind that not all of the Moon's visible area can be analyzed due to the apparent diameter of the Moon but the \"longslit\" spectroscopy makes all the observations near the Moon's edge or terminator easier. As emerged in the last sentence, the terminator reveals a more observable area and so the better nights are not the ones with the full Moon but those during the waxing descent or crescent phase, even it could seem counter intuitive; Figure 6 gives a better comprehension of this concept.