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Boër, M. 2000, in ASP Conf. Ser., Vol. 216, Astronomical Data
Analysis Software and Systems IX, eds. N. Manset, C. Veillet, D. Crabtree (San Francisco: ASP), 327
The TAROT-2 Project
M. Boër
Centre d'Etude Spatiale des Rayonnements (CESR/CNRS),
BP 4346, F 31028 Toulouse Cedex 4, France
Abstract:
This paper presents the TAROT-2 project an automatic,
autonomous observatory. Its main objectives are the study
of cosmic Gamma-Ray bursts and the detection of extrasolar
planets. The telescope will feature a 1.5m SiC mirror,
and will have the ability to slew to any position at a
speed of up to 60 deg/s. TAROT-2 will be completely
autonomous from the request for observation management and
scheduling, to the processing of observational data.
Automated telescopes have seen much development in
recent years. Many of them were built to respond to
specific requirements of a particular scientific project: this is
the case of the Télescope à Action Rapide pour les
Objets Transitoires (TAROT, Boër et al. 1999; Boër et
al. 2000), an automated telescope mainly devoted to the
observation of cosmic gamma-ray bursts. This telescope has
achieved a high degree of autonomy, being able to process
the requests, establish its own schedule, and process the
observations to produce a source list matched with several
catalogs.
However, prompted by the need to reach a better
sensitivity level, we decided to begin the study of a new, larger
instrument. In this paper we present the scientific objectives,
the materials and techniques we plan to use, as well as the
foreseen characteristics of this project, named TAROT-2.
Cosmic gamma-ray burst are probably the most violent
explosions in the universe, other than the big-bang itself.
These objects are located at cosmological distances with
redshifts on the order of unity. For an isotropic expansion about
ergs are
released in few seconds. Their
origin remains largely unknown, with models
invoking the coalescence of a binary neutron star, or
neutron star - black hole system, the core collapse of a
super-massive star, and a variety of other combinations.
This energy is released mainly at gamma-ray wavelengths,
where GRBs were discovered, and are still mainly observed in
this domain. However, quite recently they have been
detected as multi-wavelength sources, i.e. they have been
shown to emit at all wavelengths, including, radio,
optical and infrared. Their emission may be separated in
two phases. In the burst phase, most of the energy is
released, mainly at gamma-ray and X-ray wavelengths. A
bursting optical counterpart has been observed in only one
case (Akerlof et al. 1999) out of approximately 20
contemporaneous optical burst observations. The bursting
phase is followed by the afterglow phase, where the
luminosity of the source decreases exponentially with an
index on the order of 1. This phase has been observed at
optical wavelength in approximately one third of the
observed sources, and may last several months until the
luminosity is too faint to be detected.
The TAROT-2 goals for the observation of GRBs are the following:
- Detection, and localization
of the optical burst associated with the GRB, i.e.
during the gamma-ray emission, upon reception of a position from
a satellite, e.g. CGRO/BATSE, HETE-II, SWIFT, GLAST, INTEGRAL or
BALLERINA;
- Study of the transition between the burst and the afterglow
regimes;
- Detection and study of ``orphan'' optical transient or afterglow events,
possibly
associated with undetected GRBs.
Objective 1 implies a fast moving telescope, objective 2 a
good sensitivity, and objective 3, in addition, the
monitoring of large areas over the sky.
About 20 extra-solar planets have been discovered to date.
These systems are frequent, and appear to be quite different
from our solar system. These discoveries have all been
made by measuring the radial velocity of
selected stars, which introduces an important selection bias.
The goal of TAROT-2 is to detect extra-solar planets
using the transit method. In this method the light curves
of a large number of stars are monitored for the variation
caused by the partial occultation of the light by the
planet transiting the star. It has the advantage of being able to derive
directly the planet size, and may potentially lead to a
larger number of detections. The main difficulty is to
reach a photometric precision better than
for a
Jupiter like planet. This accuracy is easily reachable by the
current generation of CCD detectors.
The other difficulty is the
continuous monitoring of a large number of stars, implying
a large field of view and a high duty cycle. This last
feature implies an automatic telescope de facto.
Short CCD readout times, and fast telescope slews
between fields are required.
The two examples given above show the diversity of objectives an
automatic telescope can achieve. We have plans to observe active
galactic nuclei in conjunction with very high energy detectors,
detection of supernovae and monitoring of X-ray transient sources.
Several objectives are connected to the solar system, as is
the case with the detection and study of Kuiper objects. TAROT-2
will also be able to eventually detect Earth-grazing asteroids
and to follow space debris.
These objectives lead to a telescope of
1.5m in size. In order to achieve the high slewing speed
(60 deg/s) we require that the telescope be as light as
possible. We need to accommodate to the
large size of the GRB error boxes which may be provided by
several spacecraft, hence the field of view will be 2
degrees. We plan to have a companion telescope, or time on
another automated telescope, in order to be able to perform
spectroscopy on selected sources.
TAROT-2 will be a prototype telescope in three ways:
- We plan to make use of a SiC ceramic for the mirror,
the secondary mirrors, and the connecting structure (the tube).
The advantages of this material are the following:
- A large Young modulus resulting in a high stiffness; the
telescope elements will be almost unaffected by gravity.
- Excellent thermal conduction resulting in a reduction of
the polishing process duration, the reduction of thermal
gradients and avoiding the need for any thermal control.
Since the structure and the optical parts are made of the
same material, the design, as well as the realization of
the various parts are greatly simplified. Because of the
stiffness of the SiC we will be able to avoid several
parts, like the barrel, actuators, enabling a further
reduction of the weight and complexity. The telescope
structure will weigh a total of 100kg, and will be
quite compact to reduce the different moments of
inertia. This will result in increased reliability
because of the simplicity of the design.
- 2
- The second major feature of TAROT-2 will be its high level
of autonomy, and the introduction of an imaging and
spectrographic instrument under the same functional observatory,
while the instruments can actually be separated by thousands of
kilometers. TAROT-2 will be able to process the observations from
the request level sent by an astronomer, sitting anywhere
around the globe, to the pre-processing of frames, including
source extraction and separation, light curve analysis, etc. This
last point implies that we will need to develop specific methods in
order to maintain a high level of accuracy and reliability,
with no human intervention, and in a very
constrained timeframe(1 frame/minute).
- 3
- Finally the huge amount of data, the complexity of the
system, and its networked aspects imply that TAROT-2 will have to
be accessible from the WWW in a flexible and comprehensive way,
and that high level tools to navigate on the data have to be
developed.
TAROT-2 will need a large
number of developments beyond the use of SiC. It will be the
first networked observatory, linking an
imaging/photometric telescope, a spectroscopic telescope,
and the already working TAROT-1 telescope. In addition,
other automatic observatories may be added, e.g. to
perform continuous observation of a given object without
interruption. Also, the efficient
use of the instrument network, the scheduling algorithm
being responsible of sending the right program to the right
telescope (e.g. bright sources to TAROT-1 and fainter
sources to TAROT-2), and to schedule eventually follow-up
observations of interesting objects using the
spectroscopic telescope.
The TAROT-2 project is at present under study and review by
several institutions in France, and we hope to begin the
development in 2000.
Acknowledgments
TAROT-2 is at present supported for a study by the
Ministère de l'Education, de la Recherche et de la
Technologie and the Université Paul Sabatier, Toulouse.
References
Akerlof, C., et al. 1999, Nature, 398, 400
Boër, M., et al. 1999, A&AS, 138, 579
Boër, M., et al. 2000, this volume, 115
© Copyright 2000 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA
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