Per product description this lens does not flatten the FOV on my 127mm ES APO. The outside corners still had elongated stars with round stars in the middle 2/3 of the image. Camera used is the ASI 1600mm Pro cooled mono camera. Also noted vignetting on corners of the processed image. I would think E.S. would do a better job of testing their lenses.Don't buy this for 127mm and larger APO's.