RE: [APML]: Stacking in Photoshop

["Richmann, Michael" <richmann@cmt.anl.gov>]

Rats, rats and double rats.  Seems like there might be a market
then for an image processor that can selectively remove unwanted
color components.  Thanks in any case for the info...

M.

> -----Original Message-----
> From: Chuck Vaughn [SMTP:aa6g@aa6g.org]
> Sent: Friday, January 30, 1998 8:46 AM
> To:   astro-photo@nightsky.com
> Subject:      Re: [APML]: Stacking in Photoshop
> 
> 
> 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>