Originally intended for the HAT network, HATsim calculated detection probabilities for simulated exoplanetary transits as function of orbital periods. For a given number of locations (eg. observing sites in Australia, Namibia and Chile); clear statistics; observing fields, cadence and length one can find the probability of detecting exoplanets. Detection criterion was the observation of at least three planetary transits in a given period of time. The program was written in Python by the inspiration of Dr. Gaspar Bakos in February 2007, and was never actually used. Later it was improved for scheduling observing runs for robotic telescopes and now used as an ephemeris program.

The original problem required the precise calculation of rise and set times of the Sun and any celestial objects (Ra, Dec) at any location on Earth. This was done with high precision almanach data also considering the effects of atmospheric refraction and the hight of the observing site.

There is a parameter file which stores the data for observatories and targets, as shown in Figure 1 and Figure 2 shows the output calculated for August 9, 2011.

Figure 1.: HATsim parameter file. In the ephemeris mode only the observatory's location, longitude, latitude, altitude and time zone is used. Target rise and set times are calculated for all the observatories listed for the target. Date, or observing session is specified in the command line. Comments here used for spectral type and brightness, these are propagated to the ephemeris.

Figure 2.: HATsim ephemeris output for Aug 09, 2011. Output is sorted according to observatory id and meridian times. G191B2B is observed from every location. Times are local for every observatory. (Columns are: sset=sunset; srise=sunrise; rise=target at 30 degrees altitude, rising; meridian=maridian pass; set=target at 30 degrees altitude, setting; oleng=length of feasibility; mam=airmass on meridian; moon=moon phase.)

Some of the references used: