Rotang can either be run from the command line as described here or through the alfa_ang GUI . ... To run rotang at Arecibo: . Start IDL . ... rotang, Date , RA , Dec , Time , /choice . Here Date takes the format YYMMDD (e.g. 041128 for the 28 th of November, 2004). ... Rotang also calculates the Azimuth and Zenith Angle fir every source. If the Zenith Angle is greater than 20 degrees, then the program will return that the source is down and will not calculate an optimum ALFA angle. ...