From: Perry Holcomb <hpholcomb@yahoo.com>

Date: November 28, 2007 6:51:21 AM MST

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] Re: Mars Fri Nov 23

May a newbie break in here??

I am simply amazed, astounded, and ATR (all-the-rest)

at this discussion on Schiller.  I find it educational

and enlightening.  Most of all, I appreciate the

knowledge and passion of those contributing to my

learning.

One quick question re this Schiller astrophoto:  What

is the approximate size of the smallest detail that

can be eyeballed in it??  Info like that is also

intriguing, at least to me, but generally not

discussed.

Thanks from an astronomy kindergartner, but who is

learning!!

Perry

--- Duane <macastronomer@mac.com> wrote:

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

     ____________________________________________________________________________________

Get easy, one-click access to your favorites. 

Make Yahoo! your homepage.

http://www.yahoo.com/r/hs