Techniques d'observation spectroscopique d'astéroïdes

62 CHAPTER 3. OBSERVING TECHNIQUES
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Figure 3.3: The raw spectra of an asteroid and standard star. The twos spectra are modulated by the absorption
bands of the Earth atmosphere (essentially telluric water bands).
at two separate locations along the slit (close to top - "A" and close to bottom - "B"). This is
call the nodding procedure [Nedelcu, 2010]. In the low resolution mode of SpeX, spectra are
acquired only in a band of 512x100 pixels of the CCD.
Flat field corrections.
The flat field images are made using a lamp based on Quartz-
Tungsten-Halogen (T=3200 K). This procedure is applied at the beginning and at the end of
the observing session, by taking 10 images each time. If a pixel value is greater than 10%
of the neighboring pixels, it is considered as a bad pixel. Such pixels are replaced in all images
(object images, flat fields and arc images and standard star images) with a value obtained
from interpolation of neighboring pixel values. A "master flat" is obtained by combining and
averaging all flat field images. The master flat field is subtracted from all images.
Removing the sky background. The consecutive images A and B are subtracted (A-B and
B-A) resulting new images containing two spectra: one with positive pixel values and another
with negative pixel values (Fig. 3.2.3).
Wavelength calibration. The wavelength calibration is made by identifying the emission
lines of an Argon lamp. Thus, it results a correspondence between the pixel position on the
x-axis and the wavelength (Fig. 3.2.3).
Combining the images. The two spectra (corresponding to both positive and negative pixel
values ) are identified in each image. The images are cut, only the positive spectrum being kept.
A final spectral image (Fig. 3.2.4) is obtained for each object by gathering all its corresponding
images (before summing all images they are aligned by the brightest trace).
Extraction of the raw spectrum. The final point of this stage consists in summing the value