WHEN IS THE BEST TIME TO DO COSMETIC CORRECTION AND LINEAR DEFECT REMOVAL?

Artefacts are a common issue in subframes obtained from high-end CCDs like the one Telescope Live uses.

I asked the Telescope Live community "when is the best time to do cosmetic correction and linear defect removal?" to investigate the post-processing treat. 

Using Pixinsight, the verdict is, Cosmetic Correction should be the first thing you do, even before alignment, and it should be done on each subframe. 

That will take care of most hot and cold pixels and can take care of vertical lines (bad columns), but we’ll tackle those separately. 

Both CC and Line Defects must be done before Star Alignment. Because, after Star Alignment the bad columns will be rotated or scaled or interpolated and nearly impossible to remove as a straight column. 

While there are several ways to remove bad columns, my favorite is the included script in PixInsight. 

Two scripts. One to identify the columnar defects and the second to process and remove them. 

You can find them at Script>Utilities>LinearDefectDetection and LinearPatternSubtraction. 

You run them in that order. 

The first develops a map of the offending pixels, including partial columns, if desired (and who wouldn’t). The second applies the defect map and removes the lines. 

I find it best to start with the defaults and fiddle with the settings after reading the hover-tips or after more in-depth reading. 

Applying anything to your frames before doing Cosmetic Correction and Linear Defect work, results in less-than-optimal images. It’s tedious going frame by frame, but worth the extra effort.