The ongoing process of finding an optimized transit search system has to consider several different kinds of aspects. An important aspect is to select the magnitude range of the stars that will be monitored. Transits of bright stars (V < 10 mag) will be preferred targets because spectroscopic analysis of the atmosphere of the planet will be possible as already demonstrated for the planet HD 209458b (Charbonneau 2002, Vidal-Madjar 2003 & 2004). But the number of transiting planets orbiting bright stars will be limited mainly because most of the bright stars are large main sequence stars or giants. Transit signals of these kinds of stars are not detectable from the ground. Furthermore the stellar density of the bright stars is rather low. Thus all-sky surveys seem to be the only way to monitor a high number of bright stars. But the small apertures of these systems will yield high scintillation noise for short exposure times. Longer exposure times will increase the background noise for the all-sky surveys with large pixel scales. Additionally the large pixel scales of these systems will lead to a high number of false alarms due to the high number of blends especially for regions with higher stellar density. But all-sky surveys like PASS or KELT should be able to find some few transits of exoplanets orbiting bright stars.
Another way to find Hot Jupiter transits for bright stars is to monitor bright stars by radial velocity surveys. The spectral types of many bright stars are known and these stars can be selected by these RV surveys. The N2K survey (Fischer et al. 2005) has started to monitor 2000 metal-rich F and K type dwarfs brighter than 10.5 mag. All Hot Jupiters found will be photometrically probed for transit signals. A first transiting planet has already been discovered by this survey: HD 149026 b (Sato et al. 2005). This planet is of Saturnian size (0.725 ± 0.05 Rjup) with a transit signal of 0.3% depth that would be hard to detect in an all-
sky survey like PASS or KELT because of their high minimal noise levels.
If the goal of a transit search is to find as many transiting planets as possible then fainter stars (V > 15) have to observed. Going to deeper magnitudes increases the fraction of stars small enough that transits can be detected from the ground. Large meter-class telescopes (e.g. 1.3m OGLE-III telescope) with mosaic CCDs have to be used to monitor faint stars for at least 4 months to reach a sufficient orbital phase coverage. High logistical and financial efforts to build and operate such systems are hard to justify by the scientific output of statistics for Hot Jupiters. The RV confirmation exceeds high efforts using the largest telescopes of the world. No spectroscopic analyses of the planetary atmospheres are possible with current telescopes and near-future facilities.
Therefore it seems to be a good compromise to monitor stars at medium magnitudes (10 < V < 15). For the brighter objects the RV confirmation can be done with smaller telescopes like the 2m TLS, for the fainter ones only a few RV measurements are necessary with larger telescopes to confirm the planetary character of the transit events. In-transit spectroscopy will be possible with larger telescopes in the near future. And, most important, photometric monitoring is still possible with low-cost telescopes combined with single-chip CCDs, that can be dedicated completely to transit search.
Based on the commercially available 4k CCD technology an optimized system to search for transits is proposed: a 45cm aperture telescope with a focal ratio 1/F = 1 combined with a CCD using the 4k chip KAF 16801 with a FOV of 4.7 degree by 4.7 degrees. It was analyzed how to choose optimal target fields for this proposed OTSS system and other existing or proposed transit search systems. The proper field selection is important to have a high probability of detecting a transit. Therefore field selection has to be optimized for the different instrumental set-ups. Several aspects have to be considered:
- Possible phase coverage from the observational site (observability, technical limitations, photometrical limitations, weather, etc.)
- Content of stars small enough to be able to detect transits from the ground - Crowding of the potential target field (number of undisturbed stellar signals).
An optimized field selection should consider all these aspects to reach high search efficiency. The analysis showed that smaller systems using 2k CCDs like STARE and BEST should preferentially observe target field above the Galactic plane. Target fields in the Galactic plane have a too high stellar density for these systems. Furthermore the number of small stars in fields of the Galactic plane is larger or the same than target fields in the Galactic plane which are dominated by large main sequence stars and giants in the observed magnitude range. Larger search systems with 4k CCD can observe both target fields in and above the Galactic plane with higher detection probabilities. The highest detection probability is reached for the proposed OTSS system with an optimized pixel scale to obtain Nyquist-sampled PSFs without defocusing.
To obtain higher detection probabilities for transiting exoplanets good orbital phase coverage has to be reached. The meteorological constraints on most of the observational sites do not allow the achievement of necessary duty cycles of 80 percent or better. Thus new strategies have to be developed to increase the observational time base. The observations of single target fields for more than one season are proposed for single-instrument surveys. Another way is to build up networks of two or more telescopes to improve the orbital phase coverage.