I had a tracking mount. Exposures around 15min were good though this was in NYC so the sky was a tad bright! Used Tri-X and did some experiments with Litho film. It turns out that emulsion speed isn't that important for astrophotography. Low reciprocity failure and high contrast are the most important characteristics. If you have a very high contrast emulsion and expose enough that the sky background gets you up past the toe of the curve you get the highest detective quantum efficiency. The litho experiments were interesting - developed under the safelight you'd see an amazing stellar image appear and then, in a few seconds, the whole film went black. Needed to slow the development down.

The high contrast, slow emulsion approach was used extensively by astronomers before silver halide plates were replaced by CCDs. I was a practicing astronomer in the '80's and Kodak IIIa plates were the king. We'd expose to get the sky up to a density of about 1.0. In a dark place like Chile that would take about 75min at the f/2.7 prime focus of the 4m telescope. if you're interested I recently scanned one of the plates and put on my facebook page (https://www.facebook.com/allan.wirth).

Good luck with your lens!

Allan