如何调整F20光路系统拍摄高分辨HAADF像

How to see the Ronchigram in crystalline samples. Usually I would apply plasma cleaning before TEM My sample is Al-Cu alloy and precipitates with Cu layers sperated by Al layers would be expected. I usually find it difficult to center the C2 aperture (smallest size) due to lack view of Ronchigram, Can you give some practical tips for this operation.

Of course HRTEM would also work for the purpose. But the contrast would also depend on defocus value. HAADF-STEM seems much less dependent on defocus and is easier for interpretation.

Thanks!