Re: [APML]: Stacking in Photoshop

[Chuck Vaughn <aa6g@aa6g.org>]

Richmann, Michael wrote:
> 
> Long as we're on the topic of Photoshop, does the latest PC
> version (4.0 if I remember correctly) have the ability to selectively
> remove parts of the color spectrum?  An example of what I'm
> looking for would be the case of a photo which would otherwise
> be ok except that the sky was loaded pretty nicely with one
> or more Hg lines from local lighting.  Can Photoshop, after
> presenting the color histogram, remove any of those components?

If you're asking if it can remove an emission line source the answer
is no but you can modify the color cast caused by one. The finest color
control Photoshop has is to modify red, yellow, green, cyan, blue and
magenta separately along with the color attributes of saturation and
lightness. This is done with the Hue/Saturation control. The Hue is
based on the 360 degree color wheel with color mentioned above to be
60 degrees apart. Any color you select can be adjusted +/- 60 degrees.
For example if you select cyan, all predominately cyan pixels can be
shifted as much as -60 degree (pure blue) or +60 degrees (pure green).

This might do what you want in some cases but if you had a light pollution
line very near a nebula emission line and shifted that color both 
would be shifted.

I mentioned above that only the pixels that one color dominates are
shifted. If a pixel is 80% red and 10% each green and blue, only
selecting red will change the hue (maybe a small effect selecting
magenta or yellow). If you select green, cyan or blue and change the
hue, there is no effect on that pixel.

Right now I'm looking to see if there is a way to change the values
of the non-dominate color components of each pixel. So far I don't know.
Maybe invert and then adjust the hue....

Chuck  <aa6g@aa6g.org>