Techniques d'observation spectroscopique d'astéroïdes

CHAPTER 3. OBSERVING TECHNIQUES 61
atures. To remove this noise, an exposure is taken of similar length as the useful images,with
the dome and shutter closed. These dark images can also be used to find dead
or hot pixels. If dark frames are used, the CCD bias is contained within them and separate
bias corrections are not necessary. Similarly, multiple dark exposures can be averaged to
reduce the random readout noise by averaging them. This is called a "master dark file".
Dark current is more significant in infrared arrays, and it may not be linear and scalable
from different exposures [McLean, 2008].
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Flat - Sensitivity variations from pixel to pixel arise as the result of fabrication processes and
also due to optical attenuation effects such as microscopic dust particles on the surface
of the CCD. A flat field image to correct for this effect is usually obtained by observing
inside of the telescope dome (if it is matt or white) or place a huge white card on the dome.
The dome is illuminated with a projector lamp. In this case the telescope is completely
out of focus which ensures that the field is uniformly illuminated. For faint objects it is the
light of the sky that dominates, and so it is better to try to use the sky itself as a flat-field.
In other cases, as in photometry for instance, the flat field could be done using a sky region
in the day light time (at the beginning and end of the night).
Arc lamp - "arc" images are used to determine the pixel to wavelength correspondence, more
exactly to make the wavelength calibration. Typically, the lamps used contain helium,
neon, xenon, argon or a combination thereof. The emission lines from the spectrum of the
arc lamp are at known wavelengths and can be identified. For the IRTF/SpeX a lamp with
argon is available (Fig. 3.2).
Standard star - A solar-like standard star spectrum taken at similar airmass is required to
correct the atmospheric effects and to remove the signature of the Sun’s spectrum in order
to have only the signature of the asteroid surface. The G2 stars are used with magnitudes
(usually between 5 to 12) that allow to obtain a high SNR (signal to noise ratio) spectrum
with a short integration time (a few seconds). If the star is too bright it will saturate the
CCD, while a fainter star will require an unacceptably long integration time.
The steps required for data reduction are figured in Fig. 3.2. These steps are described
below:
Acquisition of the images containing the spectra. The most important aspect that should be
taken into account is the variation of the sky background. This effect is caused in principal by
the chemical reaction of combination/recombination in ionosphere 2 . There are two techniques
used to avoid this unwanted effect. First, the images are taken with an exposure time less than
120 sec. It is known that the variation of the sky background in an interval lower than 120
sec. could be neglected. Images with longer exposure time can be obtained by combining
individual images with shorter exposure time. Second, the spectra are obtained alternatively
2 the most common name of these phenomena is airglow