Techniques d'observation spectroscopique d'astéroïdes

46 CHAPTER 2. WHY SPECTROSCOPY?
tel-00785991, version 1 - 7 Feb 2013
Figure 2.2: The atmospheric transmission above Mauna Kea for the wavelength ranges 0.9 - 2.7 µm with a water
vapor column of 1.6 mm and an air mass of 1 (Source: http://www.gemini.edu/?q=node/10789).
several other components into the instrument called spectrometer. Usually, the designs of spectrometers
incorporate the following basic components: an entrance slit to reduce the overlap
between adjacent wavelengths and to reduce the background noise, collimators to produce parallel
beams of light, a dispersive element, a focusing element to produce focused images of the
slit for different wavelengths of the spectrum, and a detector.
2.2 Spectroscopy and atmospheric transparency
The observation of celestial bodies using different types of ground-based telescopes is possible
in the regions of electromagnetic spectrum for which the atmosphere is transparent. There are
two spectral windows which allow the observation: the optical (V) up to the mid-infrared(the
near-infrared 0.8 - 2.5 µm interval is denoted as NIR) and the radio one. The X-rays and ultraviolet
wavelengths are blocked due to absorption by ozone and oxygen, while the far infrared
radiation is blocked mainly due to absorption by water and carbon dioxide.
While in the optical wavelength region the atmosphere is almost completely transparent, in
the near-infrared there are absorption bands of water vapors making some regions like 1.4-1.5
µm and 1.8-2.0 µm poorly transparent (Fig. 2.2). Because of the effects of the atmosphere, observations
with space telescopes, such as the Hubble and Spitzer telescopes, are very valuable.
Another important difference between the V and NIR spectral intervals is the fact that the
sky is brighter in the NIR region. For example in the J, H, K filters 1 the estimated sky background
has 15.7, 13.6, respectively 13 mag/arcsec 2 . Additional, important variations of the sky
background could be observed in the intervals of tens of arc minutes of the sky.
1 Wide band filters centered on 1.25 µm (J), 1.65 µm (H), 2.2 µm (K)