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.
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:
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.
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:
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.
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.
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