I had to switch from the original dovetail nose piece that came with my STXL16200 camera to a custom nose piece in order to be able to achieve focus on my various imaging trains. Since making this switch I have been noticing that the stars in the upper left corner of my images are elongated. I need some advice on how to try to solve this problem. I have attached a copy of one of my recent images. I have checked the flatness of the custom adaptor on a surface plate and it seems to be OK. I don't see any burrs or nicks on any of the surfaces that might be causing uneven contact. Note that there were no shims included with the original dovetail nose piece that came with the camera (Serial # X18040006).