Astrophotography exposure setting for noise reduction

I was on a hiking for a week into the dense forest of Western Ghats with my Sony slt a58.

I am into Astrophotography.

The place was absolute black with millions of stars. The perfect place for shooting stars and milky-way.

I was getting shot with following setting:

Exposure:

Shutter Speed: 30 seconds

f/number: f/3.5

ISO : 1600

Now the image was with more noise and then i have to decrease either ISO or Shutterspeed.

I have tried using ISO-800, 30s and it was less noisy but bit more darker.

So in this condition what should be more feasible to decrease ISO or Shutterspeed? and why? to get perfect less noisy star photograph?

Note: There is also online calulator that will calculate exposure depending on your camera gear - http://www.lonelyspeck.com/milky-way-exposure-calculator/