The vertical lines you see are common defects of the CCD chip, called "bad columns" that cannot be removed with calibration, but only in post-processing during stacking or through interpolation. These are just columns of pixels that do not work properly.
Columns defects are an inherent problem of the manufacturing of high-end CCD chips, like the one used in some of our cameras (KAF 16803). There are multiple reasons for a column to be bad, from electronic to mechanics. In some cases the columns are completely dead (dark), and cannot therefore be fixed in the calibration stage. In many cases these columns have some constant added to what they should normally show, and can therefore be fixed during calibration. A good CCD usually have no more than a couple of them. All of our CCDs either show zero or up to 2 columns.
The two techniques to remove them are:
- Use a proper dithering pattern during the observing run. Then, during the post-processing while stacking, such artefacts will be removed.
- If this doesn't work or it's not possible as you don't have enough images, then you have to interpolate with neighbouring columns. If you use PixInsight, you can use the tool "CosmeticCorrection" to correct the defect.