Load small sample of indicator into robot? Possible?

Hi,

In my robot I am using the following lines of code to load in a BoillingerBand indicator data:

TimeFrame bbdsTimeFrame = API.TimeFrame.Minute15;
_bbds = Indicators.BollingerBands(MarketData.GetSeries(bbdsTimeFrame).Close, 20, 2, MovingAverageType.Simple);

This takes quite a long time to load as I am guessing it is loading all the historic data into the variable. Is there a way to simply load in the last 20 mins say of historic rather than all of it? I only need the current realtime values moving forward and maybe one timeframe of historic.

Thanks in advance