Techniques d'observation spectroscopique d'astéroïdes

114 CHAPTER 7. SPECTRAL PROPERTIES OF MAIN BELT ASTEROIDS
Table 7.1: Some characteristics of our observed MBAs: semi-major axis, eccentricity, inclination, absolute magnitude
(H), and orbital period.
Object a e i Orbit. Period H
AU
◦
Days Mag.
(9147) Kourakuen 2.19161 0.106186 5.816 1185.06 13.4
(854) Frostia 2.36832 0.172996 6.091 1331.25 11.8
(1333) Cevenola 2.63344 0.133589 14.641 1560.93 11.5
(3623) Chaplin 2.85105 0.086814 3.071 1758.35 11.9
(10484) Hecht 2.32087 0.079053 5.729 1291.44 13.7
(31569) 1999 FL18 2.31427 0.123171 6.402 1285.94 14.0
Table 7.2: Log of asteroids observations. Asteroid designation, date of observation with the fraction of the day for
the mid time of the observation, apparent magnitude, phase angle, heliocentric distance, the airmass at the mean
UT of each observation, the integration time for each spectrum (ITime), and the number of cycles are presented.
tel-00785991, version 1 - 7 Feb 2013
Asteroid Date V Φ r Airmass ITime Cycles
UT Mag.
◦
UA sec
(9147) Kourakuen 2011-11-15.357 16.1 20.1 1.9596 1.116 120 16
(854) Frostia 2007-03-13.487 14.9 3.6 2.3484 1.070 120 10
(1333) Cevenola 2007-03-12.549 15.6 13.8 2.6696 1.035 120 24
(1333) Cevenola 2007-03-13.575 15.6 14.5 2.6710 1.031 120 10
(3623) Chaplin 2007-03-12.378 17.1 14.7 3.0366 1.085 120 18
(3623) Chaplin 2007-03-13.267 17.2 14.9 3.0371 1.018 120 20
(10484) Hecht 2011-11-16.420 17.0 16.3 2.1986 1.060 120 6
(31569) 1999 FL18 2011-11-16.330 17.1 19.9 2.0294 1.040 120 12
was used in low-resolution mode for these sessions. The observations were made in the 0.8-
2.5 µm spectral interval. A 0.8 arcsec wide and 15 arcsec length slit, oriented North-South,
allowed simultaneous measurements of the object and sky. The nodding procedure described
in chapter 3 was applied. The observing conditions and parameters are given in Table 7.2.
The automatic guiding mode of the telescope was used for these observation. Since, for the
main belt asteroids the relative speed (∆RA/∆t, ∆DEC/∆t) is low, it does not impose difficulties
for the differential tracking. In the moment of observations our objects had speed bellow
0.6 ′′ /min.
The apparent magnitude of the asteroids varies depending on the relative position with the
Earth and the Sun. In the case of main belt asteroids this apparent magnitude variation could
up to four magnitudes. The asteroids described here were observed when they were close to
their brightest apparent magnitudes (at oppositions - Table 7.2).
Another constraint that should be taken into account when scheduling the observations is
the airmass. In order to obtain good SNR for the spectra, I managed to observe all objects at an
airmass smaller than 1.12 (zenith angle below 25 ◦ ) - Table 7.2.
The integration time for each image was 120 second. Depending on the weather conditions,
and considering a basic SNR evaluation of the data, a different number of images for each
object were taken.
Each observed asteroid was preceded by observations of solar analogs located in the vicinity.