A good use for cheap red filter

[pdalek (Patrick)]

With many dslr sensors the blue pixels also weakly record red light. The blue channel will have a weak copy of the red image on top of the blue data.
Smart software can handle this, but a simple white balance can get the colours wrong, especially between stars and nebulosity.

Take a flat through the red filter. Do a bias correction for best results. Suppose you find the blue channel is x% of the red. This gives a correction factor to apply to your images.

When processing future images, you can subtract x% of the red channel from the blue. (Use little less than x% to be safe). This should be done after stacking but before messing with the white balance. There are better ways, but this is very simple.

[bkm2304 (Richard Brown)]

Originally Posted by pdalek

With many dslr sensors the blue pixels also weakly record red light. The blue channel will have a weak copy of the red image on top of the blue data.
Smart software can handle this, but a simple white balance can get the colours wrong, especially between stars and nebulosity.

Take a flat through the red filter. Do a bias correction for best results. Suppose you find the blue channel is x% of the red. This gives a correction factor to apply to your images.

When processing future images, you can subtract x% of the red channel from the blue. (Use little less than x% to be safe). This should be done after stacking but before messing with the white balance. There are better ways, but this is very simple.

Hi Patrick,
I would always filter cheap reds- you never know what the winemaker has put in 'em!!!!

Seriously, this is a clever piece of advice so thanks for the info. Another example of expertise shared on this forum..

Thanks.

Richard