High and Low times for 555 oscilator are inverted
When the output is high the capacitor charges through R1 (and D1) so the high time is proportional to R1. 

When the output is low the discharge pin is also low, so the capacitor discharges through R2 (and D2, if present). 

If you want high_time/low_time = 0.5/1 then R1/R2 = 0.5, so R2 should be double R1. 



During astable operation the capacitor charges and discharges between Vcc/3 and 2Vcc/3. When first powered, the capacitor can be discharged to 0V or something near to that so the initial cycle tends to be considerably longer.