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Re: [APML] Tricolor Image to Work With
Ron I had just finished my version of Chuck's tri-color set when I got your
e-mail. I kind of "flew by the seat of my pants" on those blue halos. And
the red ones........I didn't even notice them <g>. But thank you for a very
detailed report on fixing them. I am saving the report for when I finally
get my AP155 EDF because I do plan on shooting some TP with it.
Here's my version of Chuck's SH2-157:
http://www.koyote.com/users/bobm/Chuck'sSH2-157.htm
Chuck I expect a strict critique of this.
Bobby Middleton
----- Original Message -----
From: Ron Wodaski <ronw@nwlink.com>
> Those stellar halos were a real bear to deal with. I used about a dozen
> different techniques to control them, and in the end I decided to leave
some
> remnant of them. Otherwise, you have to remove too much of the object
> itself, or risk making an arbitrary selection that could remove signal as
> well as the halos.
>
> My best technique involved making a half-dozen different masks, and then
> adding and subtracting them in a sequence to wind up with just the area I
> really wanted selected.
>
> For example (Photoshop), you can select a color range in RGB channel and
> click on one of the blue halos. The blue channel stars are MUCH bigger
than
> the green and red (and the red are quite a bit bigger than the green), so
> you can't _just_ select the halo, or it will still be too big. Plus, you
get
> rid of any other similar blue areas, not just in the halos.
>
> So the first job is to carefully select the portion of the blue stars that
> you want to keep: the center of the stars in the blue channel, using a
> brightness cutoff that yields a star image about the size of the green
> stars. (Note: the contrast drop off in the green channel is very tight;
the
> stars have a very hard edge. One could try using the green channel as a
> mask, but then you lose any legitimate blue coloring at the edge of the
> stars that really are blue. Tradeoff here.)
>
> Now you can't just dim the rest of the blue image, because there is blue
> signal in the nebula and the background. You also need to select a
> transition zone around the selection you just made, for how fast to drop
> off. So you'll need a selection of the object, and the blue channel's
shadow
> pixels (Color Range) are a pretty good combination of both the background
> and the object in the blue channel. So: you create a mask, and it needs to
> have pure white from the center of the stars in the blue channel; it needs
> to have a transition zone that consists of the selected halos; and you
need
> to be able to subtract the object from the halos. To make the halos a
> transition zone, create a mask of just the halo selection, and then blur
it
> enough so that when you do the darkening, it will make the visible zone of
> the halos. Depending on the quality of your Color Range for selecting the
> halos, you may need to expand or shrink this selection before subtracting
> the object selection from it. Then subtract the background, so that you
> don't remove any blue that is legitimately in the background.
>
> What you are left with is a mask that has pure white in the center of the
> stars, gray in the halos, and black in the object and the background. You
> would then need to decide if you need to do a little blurring or
sharpening
> in order to get the degree of color removal you want.
>
> At every step, you need to stop and experiment with what you've got, maybe
> go back a step or two to adjust what you've done. sometimes, you realize
> that you have to back ten steps! Take good notes so you can recreate with
> whatever variations you require. Photoshop records history, but I don't
> think it can tell you what settings you used at each step.
>
> See? It's just a matter of an hour or so, and you are all done and ready
to
> start on the red halos! <g> Then you can start the actual image processing
> for the object. Nothing to it! By the time I finished my first round, my
> head was spinning. I apologize if my description makes no sense; I got
lost
> more than once trying to find ways to get rid of those halos. After a few
> more tries, I might think of something a little simpler.
>
> Ron Wodaski
> The New CCD Astronomy
> http://www.newastro.com
>
> -----Original Message-----
> From: owner-astro-photo@seds.org [mailto:owner-astro-photo@seds.org]On
> Behalf Of Bobby Middleton
> Sent: Friday, February 02, 2001 2:51 PM
> To: astro-photo@seds.org
> Subject: Re: [APML] Tricolor Image to Work With
>
>
> You guys get some nasty halos with those expensive refractors <g>. If
> someone will give me some hints on getting rid of them then I'll march on
> with this work Chuck has assigned us. I know it will involve a mask of
some
> kind but I'm just too green to figure it out yet. I tried an inverted blur
> from the blue channel but it didn't make any diference (if I did all the
> steps right??)
> Bobby Middleton
>
>
>
> ----- Original Message -----
> From: Chuck Vaughn <aa6g@aa6g.org>
> To: <astro-photo@seds.org>
> Sent: Thursday, February 01, 2001 11:41 PM
> Subject: [APML] Tricolor Image to Work With
>
>
> > List,
> >
> > Since we seem to be into this right now, I've posted RGB
> > grayscale images of SH2-157 that are ready to be put
> > together into a tricolor image. This is a faint and not very
> > spectacular nebula that I photographed over two years ago.
> > It is not on my web site. If you'd like to experiement with
> > it you can download the parts here. Each is about 1.1MB.
> >
> > ftp://shell3.ba.best.com/web2/aa6g/red.tif
> >
> > ftp://shell3.ba.best.com/web2/aa6g/green.tif
> >
> > ftp://shell3.ba.best.com/web2/aa6g/blue.tif
> >
> > The only processing done was in the prescan to get the
> > blue and green channels scaled to match the red and equal
> > contrast stretch in each channel to get the most out of my
> > scanner. The images were aligned in Photoshop but no
> > processing was done there except to eliminate a few dust
> > specs.
> >
> > I put the image together and did a single Curves adjustment.
> >
> > ftp://shell3.ba.best.com/web2/aa6g/sh2_157.jpg (340KB)
> >
> > Maybe you can reduce the blue halos, reduce grain and even
> > out the background which becomes a problem as you stretch
> > this faint nebula.
> >
> > Warning: My web site has limited daily download. If it's
> > exceded a message will probably pop up. I get whatever
> > download amount was used 24 hours ago back at the top of
> > each hour so try then.
> >
> > Have fun,
> >
> > Chuck <aa6g@aa6g.org>
> >
> >
> >
> >
> > -- APML Archives at <http://www.system.missouri.edu/apml/> ---
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> >
>
>
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