High-contrast adaptive optics systems, such as those needed to image extrasolar planets, are known to require excellent wavefront control and diffraction suppression. The Laboratory for Adaptive Optics at UC Santa Cruz is investigating limits to high-contrast imaging in support of the Gemini Planet Imager. Previous contrast measurements were made with a simple single-opening prolate spheroid shaped pupil that produced a limited region of high-contrast, particularly when wavefront errors were corrected with the 1024-actuator Boston Micromachines MEMS deformable mirror currently in use on the testbed. A more sophisticated shaped pupil is now being used that has a much larger region of interest facilitating a better understanding of high-contrast measurements. In particular we examine the effect of heat sources in the testbed on PSF stability. We find that rms image motion scales as 0.02 λ/D per watt when the heat source is near the pupil plane. As a result heat sources of greater than 5 watts should be avoided near pupil planes for GPI. The safest place to introduce heat is near a focal plane. Heat also can effect the standard deviation of the high-contrast region but in the final instrument other sources of error should be more significant.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.