From: "Duane" <macastronomer@mac.com>

Date: November 27, 2007 5:46:11 PM MST

To: Astro_IIDC@yahoogroups.com

Subject: Re: Mars Fri Nov 23

I use Photoshop's curves. Curves are actually pretty simple, but very powerful.

The shortcut on a Mac is Apple-M, PC is Cntrl-M I think. To remember that, just remember 

how great curves are and think, "mmmmm... Cuuurves!"

When you open curves, you'll see a grid. On the left is the output (how dark a given pixel 

will appear) and on the bottom is the current density (how dark it is now). From bottom to 

top is dark to light, and from left to right is dark to light (however, this can be reversed).

If you click around on your image while in curves, you'll see a circle on the curves grid that 

represents the pixel you are over.

Now for the meat of it...

There is a diagonal line (lower left to upper right) which cross-references the current 

shade with the output shade. Black represent black and so on. If you were to click on this 

part of the line and raise it up, Black would now be represented by a lighter shade. Moving 

the line moves several shades at a time, so all of these shades are represented and 

altered.

The diagonal is a 1:1 ratio in density to output. But in any place where you make the line 

more vertical, or should I say increase the slope, you are stretching the detail. Any place 

where the line become more horizontal, it flattens and loses contrast.

The main goal is to locate the area you want to bring out detail and pull the line more vertical in that region.

For somebody new to curves, I have 4 simple ways to start using it:

UP) If your detail is in the dark shades, grab the middle of the curve and pull it up a bit 

(detail pops out on the low end but is suppressed on the high end). It has an overall affect 

of lightening the image.

DOWN) If your detail is in the lighter shades, grab the middle and pull it down (the 

opposite of above).

S) If your detail is in the center, click once on the middle of the line (it will lock it in place 

with a black dot), then pull the line from a quarter of a way from the  dark end, down. You 

may want to pull the lighter quarter of the line up. This makes an S shaped curve (detail 

pops out in the middle grays but is suppressed at the extremes).

Reverse S) If your detail is toward both extremes but not in the middle (rarely happens), 

put the lock in the middle and push the darker quarter slightly up and the lighter quarter 

slightly down (detail pops out at the extremes but the middle is suppressed).

Now for the posterization issue:

Click in the area where the shade changes due to the sphere of the planet. This will show 

you the region on the line where you should make it more vertical. It won't take too much

—just season it to taste. Be careful you aren't flatting any important data by keeping an 

eye on the important detail in the image.

I hope I wrote that clear enough, it should be a good primer for anybody who wants begin 

to understand curves. There is much more to curves, especially as a color balance tool. I 

hope I get time to add some of this to my webpage someday.

Please let me know if that was helpful. I know it's tough without graphical aid.

Duane

--- In Astro_IIDC@yahoogroups.com, "Christoph Bosshard" <chboss@...> wrote:

Hi Duane

Thanks for your comment.

Can you point me to some detailed sources regarding stretching the

curves? Would you do this in IIDC or in Photoshop?

Hopefully there will be more good material to work with in the comming

weeks. I am still doing some changes to my equipment at the moment to

be well prepared.  :-)

I will certainly post the results to get some more tips on how to 

improve. I think it is a great way of learning.

regards

Chris

Hi Chris,

Thanks for the feedback. I think that left right alignment is

actually a bit of bubbling (disk 

size due to atmospheric refraction) but reprocessing may allow me to

get a better pick on 

the photos that are accepted. I rather loath picking them by hand,

but it is another option.

Nice shot too. I always figure that if I can tell which side of Mars

is showing that it was 

worth while night. It looks to me like you have the Mare Serinum side.

There's a little bit of posterization on your image that can be

avoided using curves to stretch the area in the histogram that

contains Mars' data before doing too much 

sharpening or other processing.

Keep shooting and sharing. I find it really helpful to see what

others are doing.

Tonight, major wind and jetstream issues here. Time to get some sleep :)

Duane