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extrasolar planets are currently detected via indirect methods . the radial velocity technique has proved most successful in recent years . however , photometric measurements of planets transiting in front of their central star through the line - of - sight to earth have already provided more than 30 planet detections in the past years . many more transiting planet detections are expected in the very near future by the space missions corot @xcite and kepler @xcite . combining both methods allows the derivation of planetary radii , true masses and thus , the mean density of extrasolar planets . transiting planets are therefore of particular importance to investigate basic planetary parameters . the detection of extrasolar planets via the transit method requires long duration , high photometric quality lightcurves of a large number of stars . the major difficulties for ground - based transit searches are interruptions of the time series , e.g. due to the diurnal cycle or weather conditions , and the earth s atmosphere limiting the photometric signal - to - noise - ratio @xcite . located on the antarctic plateau at @xmath0 southern latitude , provides the potential for long , uninterrupted high - quality photometric time series observations from ground during its winter period . the maximum number of astronomical nights at in comparison to mauna kea has already been calculated by @xcite , including the influence of the sun , the moon and cloud coverage . while has less astronomical nighttime , the percentage of cloud free observing hours is similar to mauna kea . kenyon & storey do not discuss optimizations for long time series observations of target fields . their paper , however , presents a detailed discussion of the expected sky brightness at . they estimate that atmospheric scattering should be exceptionally low , and even the sky brightness due to the moon should be reduced . has reduced zodiacal light , no light pollution and only minor disturbances due to aurora . furthermore , scintillation noise as a major limiting factor of photometric quality is expected to be about 3.6 times lower at dome c compared to chilean sites for long integrations @xcite , which is particularily important for the search of small planets . may therefore be the best site on earth for stable high - accuracy photometry . alternatively , networks distributed favorably in longitude can also provide high duty cycles . a well known example is the global oscillation network group ( gong ) for asteroseismological studies which include six network sites ( four in the northern and two in the southern hemisphere ) . an estimate of the expected duty cycle for spectroscopic asteroseismology from in comparison to gong has been presented by @xcite . they conclude that provides a better or similar performance than gong for observing periods up to 100 days , attaining a duty cycle of up to 87% . the network performs better for observing periods of half a year or more , but with a duty cycle of only about 77% .    in this paper , we include the factors already investigated by mosser & aristidi , like the sun and cloud coverage . in addition we include the moon and the varying extinction of the target fields during observations . furthermore , we discuss not only a comparison of to low - latitude networks , but also a combination of with other sites . a comparison between and the corot mission has already been presented @xcite . the resulting potentials for long - duration time - series photometric observations are then discussed in the context of maximizing the probability to detect extrasolar photometric transits . observations from the ground are limited for various reasons . the main restriction is imposed by the sun for observations in the visible spectral range . dark time can be reduced further by periods of full moon , because during such periods the sky background noise level reduces the photometric accuracy . selected target fields move across the sky , rising and setting and thus further limiting any observation campaign from a single site . after a brief discussion of the underlying time - frame in  [ period ] , the actual determination of observation times from these astronomical constraints will be the subject of section  [ geometricalconstraints ] . in practice , local weather might likewise limit observations , e.g. due to cloud coverage . section  [ clouds ] describes the model used for simulating this effect . finally , the total observing time on a chosen target field as well as the photometric quality obtained crucially depend on its coordinates . in  [ targetoptimization ] we therefore present a simple model designed to provide a quantitative solution to the target field selection process . the selected time - frame for our simulation limits possible observations to a certain period and therefore affects the results systematically . the most appropriate choice is to consider a one year period for each site studied , as the earth s orbit around the sun has the most important impact on observability . after one year has elapsed , time intervals of possible observations would basically show again the same pattern for any given target field . the results presented here correspond to one year ( @xmath1 ) , starting on 1 march 2008 , 0:00h ut .    furthermore , only one field is considered per site to determine the maximum possible time coverage for planet detections . thus , planet detections at large orbital distance was given highest priority , rather than optimizing for detections of very short - period planets by maximizing the number of fields . the sun and moon affect the background noise level depending on their position relative to the observer . in order to decide whether a target is observable at a certain time or not , some criteria are needed . the most common and efficient way is to limit the three local altitudes @xmath2 , i.e. to apply @xmath3    for example @xmath4 calculates observation times without direct sun- or moonlight , as they would be restricted to stay always below the horizon . however , since scattered light might likewise disturb observations significantly , the common limit @xmath5 was used in this work , at which the contamination of the photometry is negligible . furthermore , target fields below @xmath6 altitude are not considered observable due to their poor photometric quality , i.e. @xmath7 .    moreover , the moon s contribution to sky brightness is a function of its phase @xmath8 . therefore the moon criteria in eq . ( [ limit_h ] ) was relaxed by additionally allowing for situations corresponding to @xmath9 i.e. times when the moon altitude is above its limit , but the corresponding phase stays below a specified parameter @xmath10 . furthermore , the distance between moon and target field can be restricted to stay above a certain value , that is @xmath11 where @xmath12 denotes the angle between moon and target on their great circle . based on our observational experience , these last two limits were chosen to be @xmath13 and @xmath14 at which the influence on photometric accuracy starts to be significant .    compared to low - latitude sites , the sky background is expected to be darker at dome c for the same local positions of sun , moon and target due to the low aerosol content of the air @xcite . nevertheless , we refrained from adjusting the limits as this effect is still to be examined more quantitatively . the average cloud pattern at a given site was simulated using a model by @xcite , which requires two input parameters , @xmath15 and @xmath16 , derived from weather statistics . @xmath15 stands for the average fraction of clear skies , whereas @xmath16 denotes the mean duration of cloudless periods . both can vary seasonally .    according to the model , a simple pattern is created that consists of periods that either allow observations ( clear sky ) or not ( clouds ) for any given site and season . the duration of a _ single _ cloudless period is governed by a gamma distribution of the form @xmath17 so that @xmath18 denotes the probability to find an uninterrupted interval of length @xmath19 . in order to meet the given fraction @xmath15 of clear sky , the mean duration of cloudy periods is then clearly given by @xmath20 . substituting this value for @xmath16 in eq . ( [ clouds_p(t ) ] ) , one obtains the corresponding probability distribution of cloudy weather . an alternating sequence of clear and cloudy periods will therefore provide an approximation of real sky coverage . so , periods of potential observations ( calculated using the geometrical constraints ) are further reduced by only selecting clear sky times . as these are based upon random processes , 500 monte carlo runs are performed for each configuration in order to achieve statistically significant results .      for the purpose of site comparisons , the calculation of observational time series is often derived from astronomical night times only , i.e. without including a specific target field . these results form an upper limit on the duty cycle , but are usually not achieved by any _ single _ target . for instance , most fields can only be observed for a fraction of a year due to the earth s annual motion around the sun . however , the outcome of transit search programmes largely relies on how much time a chosen field is being observed . in particular , only long time - series might provide a sufficient detection rate for planets with earth - like orbits . including target field coordinates into the simulation is therefore necessary for an accurate analysis of duty cycles . to select optimized target field coordinates , we posed the question : for a given site and time interval , which target field will provide the longest time coverage ? two parameters are relevant here :    * observation time @xmath21 * mean airmass @xmath22    considering the observation time , fields observable over long periods are favoured . however , the photometric quality is reduced with increasing airmasses . therefore our calculations seek to minimize the mean airmass .    for that , both parameters are calculated as a function of potential equatorial target coordinates @xmath23 throughout the whole sky for each site . in order to optimize them simultanously , they must subsequently be weighted against each other by defining a function @xmath24 . using such a weighting mechanism then provides a sky map from which we can easily derive a maximum value . although the choice of a specific function @xmath25 is somewhat arbitrary , it should in general show a reasonable dependency on its two variables @xmath21 and @xmath22 : firstly , @xmath25 should be monotonically decreasing in @xmath22 , as the photometric quality can only be reduced with larger airmasses . secondly , only a linear dependency on the observing time @xmath21 is meaningful . finally , we have already excluded target fields with airmasses @xmath26 by setting @xmath27 , thus it is consequent to demand @xmath28 for @xmath29 . these requirements are well fulfilled by the following approach : @xmath30 the parameters @xmath31 and @xmath32 have been chosen to reproduce the dependency of photometric accuracy on the airmass of observations , as illustrated in fig . [ fig : am_weighting ] : in the region @xmath33 we get @xmath34 , which accounts for the usually negligible variation of the signal - to - noise ratio of photometric data recorded at these airmasses . beyond that the photometric quality is significantly reduced , resulting in a strong devaluation of observing times with an average airmass close to 2 . this procedure yields an efficient choice of target coordinates for site networks . we then maximized the total time of observation from at least one site and decided to observe from whichever site provided the smallest airmass whenever simultaneous observations were possible . target coordinates found by this procedure obviously take into account astronomical configurations only . additional factors are required to optimize the field for an exoplanet transit search programme . in particular , the target field must contain a large number of dwarf stars . the required high number has to be weighted against crowding effects in dense star fields . therefore , this optimization step depends on the fov and the spatial scale of the instrument chosen . it has to be performed for a particular instrument and is therefore not within the scope of this paper which concentrates on the astronomical boundary conditions in terms of optimized duty cycle . as already mentioned , we aim to compare the window function obtained from antarctica with one or more low - latitude sites as well as a combination of with such locations . in addition to , three sites with similar low - latitudes are considered for a potential southern hemisphere network . the location of the best ii telescope ( erikson _ et al . _ , in prep . ) at cerro armazones was chosen in chile . in africa a site next to the gamsberg in namibia was included in our calculations , whereas the anglo - australian observatory ( aao ) was selected on the australian continent . the geographical coordinates are listed in table [ localcoordinates ] . .local coordinates of considered sites [ cols="<,<,<,<",options="header " , ]      the two parameters used by the weather model , @xmath15 and @xmath16 , can exhibit strong seasonal and geographical variability . for the simulation of free - sky windows , monthly mean values are used for each site in table [ localcoordinates ] . figure [ fig : weather_compare ] shows the dependency of the assumed average clear sky fraction @xmath15 on time and location . for , winter observations from @xcite with mean values of 6.8 consecutive clear days and 84% overall clear skies were extrapolated to the whole year . in the case of chile , cloudiness data from the european southern observatory ( eso ) paranal were averaged for 1983 - 2006 , whereas publicly available _ surface synoptic observations _ ( synop ) from 2004 - 2007 were used for the aao . finally , an eso site survey provided values for @xmath15 at close - by gamsberg in namibia @xcite .    for australia , the average duration of clear periods in the data set was found to be @xmath35 . for chile and namibia we assumed @xmath36 according to @xcite . however , the parameter @xmath16 only affects the duration of individual observations and is thus of minor importance . for example , using values @xmath37 for chile does neither change the total observation time @xmath21 nor the time coverage of planetary transits . we first validated the model against previous studies . calculated celestial coordinates agree with results from the publicly available _ _ jpl horizons _ _ ephemeris software to within 30 arcseconds for the years 1900 - 2100 . in addition to this , the results of @xcite for available dark time at and mauna kea were confirmed when we adopted their slightly differing limits for sun- and moonlight . then , optimal target fields have been simulated for all single sites as well as for three possible combinations of the sites . the various configurations are summarized in table [ tab : results ] .      as a first step , we investigated the limitations imposed on time - series observations by each of the constraining factors considered individually . figure [ fig : effects ] shows calculated values of @xmath21 taking into account firstly only sunlight , then gradually including moonlight , target field visibility and finally cloud coverage .    at a single site like best ii , the maximum dark time is 163 days per year due to the diurnal cycle ( @xmath38 , dark bars in fig . [ fig : effects ] ) . at , the dark time per year is somewhat less ( 135 days ) due to its high latitude . on the contrary , a low - latitude network can in principle observe all year round . however , effects like moonlight , target field selection and clouds further reduce the maximum available observing time .    the target field selection does not impose a strong restriction on available observing time from , because many fields are above @xmath39 almost continuously during the polar night . at low latitude sites , the full available dark time can only be filled with observations of several target fields over the year . for one field it is , of course , significantly reduced ( compare third bars from left for each site in fig . [ fig : effects ] ) . in addition , there are also severe effects by clouds and the final performance is therefore significantly reduced in comparison to the ideal case ( fourth bar ) . due to the unequal impact of limitations upon different sites it is therefore necessary to consider all four effects simultaneously . lcccrrrcc dome c & @xmath40 & @xmath41 & without & @xmath42 & @xmath43 & @xmath44 & & 1.03 + & & & with & @xmath45 & @xmath46 & @xmath47 & & 1.03 + best ii & @xmath48 & @xmath49 & without & @xmath50 & @xmath51 & @xmath52 & & 1.33 + & & & with & @xmath53 & @xmath54 & @xmath55 & & 1.41 + network & @xmath56 & @xmath57 & without & @xmath58 & @xmath59 & @xmath60 & & 1.33 + & & & with & @xmath61 & @xmath62 & @xmath63 & & 1.40 + dome c + best ii & @xmath64 & @xmath65 & without & @xmath66 & @xmath67 & @xmath44 & & 1.33 + & & & with & @xmath68 & @xmath69 & @xmath70 & & 1.34 + dome c + network & @xmath71 & @xmath72 & without & @xmath73 & @xmath74 & @xmath44 & & 1.33 + & & & with & @xmath75 & @xmath76 & @xmath77 & & 1.34 +    in table [ tab : results ] potential target coordinates @xmath23 with maximum @xmath78 are given for each combination of sites . also the corresponding values of total observation time @xmath21 and mean airmass @xmath22 are given  with and without clouds , respectively . furthermore , the mean and maximum duration of single continuous periods of observations is shown as @xmath79 and @xmath80 .    the window function itself is illustrated in fig . [ fig : domec ] for . the long observing periods during the polar night are clearly visible . however , continuous observations do not exceed 23 hours due to the sun rising above @xmath81 at noon even around winter solstice . nevertheless , the task of observing the _ same _ target from a single site as long as possible can best be fulfilled from antarctica . even when weather statistics are taken into account , clearly outcompetes even an excellent low - latitude site such as best ii . under realistic conditions , the mean total duration of observations of a target field with cloud coverage is 85 days from antarctica which is more than twice as long as from chile ( 38 days ) .    in order to achieve a similar performance without , the combination of at least three low - latitude sites would be necessary . figure [ fig : network1 ] shows the performance of a network consisting of chile , namibia and australia . this would in principle allow for very long continuous observations taken in turns , peaking in mid - winter at a maximum period close to one month between full moons . but interruptions due to cloud coverage limit long runs and reduce the total observation time to 98 days . however , the network would still yield about two weeks more than in terms of observing duration .    beyond the direct comparison of with one or more low - latitude sites , network configurations including antarctica deliver very interesting prospects . as is restricted to observations during the winter months only , low - latitude sites can also monitor targets located on the summer night sky around @xmath82 . with the additional choice of @xmath83 , such a field would always remain more than @xmath6 above the horizon at . the consequence is a significantly increased time statistics , as the overlap of observations from antarctica with the other sites would be marginal . figure [ fig : network2 ] illustrates the excellent duty cycle expected from a network consisting of all four considered sites including , whereas fig . [ fig : target_plot ] shows the functional dependency of the total observation time on field coordinates .        combining only and best ii , it would be possible to observe for a total of 123 days per year under realistic conditions , thus exceeding the duty cycle of the three - site low - latitude network by 25% . finally , increasing the total obervation time further to 165 days by including namibia and australia into the network would provide an outstanding performance for an earth - bound project .          in the following , the efficiency of transit detection is examined . figure [ fig : cov3_cld ] shows a comparison of observational coverage based on the simulated window functions . simulations have been performed as a function of the orbital period and for three or more transit events , which is usually the minimum requirement for detection . that is , a coverage of 100% means at least three events of a potential transiting planet will be observed for a given orbital distance . note that we only take into account the time sampling here , so the actual detection probabilities will be reduced further e.g. by the signal - to - noise ratio or geometric orientation . all curves in fig . [ fig : cov3_cld ] show peaks and dips caused by interruptions in the data sampling e.g. due to the diurnal cycle , full moon or cloud coverage . periodic data gaps lead to a reduced observational coverage for orbital periods which are multiple of interruption frequencies .    can provide continuous observational coverage for planets with up to about two weeks orbital period , extending to 30 days with a probability of 50% . in contrast to that , it is very unlikely to detect planets with periods significantly longer than 15 days for a single low - latitude site like best ii in chile within one year . however , an almost equal performance to could be achieved by the combination of three low - latitude sites .    combining a telescope at with one in chile extends the full observational coverage to about three weeks and leads to a significantly improved performance for larger periods compared to alone . finally , the combination of with a network including chile , namibia and australia could , in addition to a good coverage of small orbits , in principle still detect planets with periods of more than 60 days at a reasonable coverage . we have investigated the prospects for long - duration uninterrupted photometric time series from and other low - latitude sites . contrary to previous studies we _ simultaneously _ took into account the limitations of the sun and moon , clouds and target field extinction variations .    due to the long winter , has an outstanding performance in observing a target field over a long time period from a single site . furthermore , the effective observing time period can be extended if is combined with other sites which can provide data during the summer period . for example , the combination of with a site in chile extends the observational coverage significantly . combining with a low - latitude network of sites at chile , namibia and australia provides prospects for observational duty cycles that can be improved upon only by observations from space . to predict quantitative transit detection rates , the photometric quality of must be investigated directly . however , for this we have to await the results of photometric site testing projects like the antarctica search for transiting extrasolar planets ( astep , @xcite ) for a detailed study . nevertheless , the extensively long observational time baseline for photometric variability studies clearly demonstrates the extremely encouraging prospects of this site in antarctica . baglin , a. , _ et al . _ 2006 , in : _ the corot mission : pre - launch status _ , esa sp-1306 , 33 - 38 borucki , w.  j. , _ et al . _ 2003 , in : _ earths : darwin / tpf and the search for extrasolar terrestrial planets _ , esa sp-539 , 69 - 81 fressin , f. , _ et al . _ 2007 , eas pub . ser . , 25 , 225 - 232 hill , f. & newkirk , g . , jr . 1985 , , 95 , 201 kenyon , s.l . & storey , j.w.v . 2006 , , 118:489 - 502 kenyon , s.l . , _ et al . _ 2006 , , 118:924 - 932 mosser , b. & aristidi , e. 2007 , , 119 , 127 - 133 rauer , h. & erikson , a. 2007 , in : _ extrasolar planets _ , ed . r. dvorak , wiley - vch , 207 - 240 rauer , h. , fruth , t. & erikson , a. 2008 , _ proceedings of second arena conference _ , eas pub . ser . , submitted sarazin , m. 1994 , _ technical note on namibian gamsberg site _ , part i , eso - mpia 1994 - 1995 site survey
the detection of transiting extrasolar planets requires high - photometric quality and long - duration photometric stellar time - series . in this paper 
 , we investigate the advantages provided by the antarctic observing platform for planet transit detections during its long winter period , which allows for relatively long , uninterrupted time - series . 
 our calculations include limiting effects due to the sun and moon , cloud coverage and the effect of reduced photometric quality for high extinction of target fields . 
 we compare the potential for long time - series from with a single site in chile , a three - site low - latitude network as well as combinations of with chile and the network , respectively . 
 is one of the prime astronomical sites on earth for obtaining uninterrupted long - duration observations in terms of prospects for a high observational duty cycle . 
 the duty cycle of a project can , however , be significantly improved by integrating into a network of sites .