This image was taken using the Ca K filter module being used with 63mm 840mm FL objective. The filter module still used the original 5mm blocking filter (BF) and so it resulted in severe vignetting as the solar disk would have been 7.7mm in diameter (840/109). You can see the results with the darkened corners of the image. I tried to minimise this with disk placement, and in theory flat fielding could have solved it, though it was pretty severe vignetting for that. I wanted a larger BF and first of all tried the Omega 393nm filter as a BF.