28 Feb
2009
28 Feb
'09
11 a.m.
In combining, I think the sum method would be better if I had been more careful to pick out the bad pixels. But without taking care in the pixel department, when I summed all the hot pixels showed up in the finished product. That's why I went with "median" -- but at other times sigma clip might be best. This is one part where I'm hazy. I think it's a matter of testing for each situation. -- Joe
--- On Sat, 2/28/09, Jim Gibson <jimgibson00@yahoo.com> wrote:
From: Jim Gibson <jimgibson00@yahoo.com>
Subject: [Utah-astronomy] Patrick's 2nd HH data (long post)
To: "Utah Astronomy" <utah-astronomy@mailman.xmission.com>
Date: Saturday, February 28, 2009, 8:23 AM
I gave Patrick’s 2nd batch of the HorseHead data a shot.
http://www.slas.us/gallery2/main.php?g2_itemId=1173
ONLY READ ON IF you are interested in MaxIM DL and PhotoShop image processing.
I will try to give full disclosure here. First of all, obviously Tyler is the
target; in other words he has set the standard. So I downloaded a copy of his HH
image processed data as contained in Patrick’s (his data) gallery site:
http://www.slas.us/gallery2/main.php?g2_itemId=1025 . I used that image as a
model just to look at and see if I could come close to it with Patrick’s 2nd
batch of data.
I used Joe’s way of processing as follows.
1 - I took each of the clear photos and subtracted the dark from it.
2 - Then I used a "remove bad pixel" feature of MaxIm DL to get rid
of bad pixels, some of which were in more
than one of the frames. The feature keeps a map of bad pixels so you can apply
it
to other views from the same session.
3 - Next I picked out the clear that seemed to show the best range and used an
"equalize screen stretch" button to
make all the clears about the same.
4 - I combined them using various methods to see which was the best. I think
the best one turned out to be
taking a medium of the images.
5 - I did the same sort of thing for the colors, except that one -- I think it
was green -- had only one image so I didn't combine it with another green.
6 - Finally I Color Combined LRBG without playing with curves because it seemed
about right as it was. My last operation was to adjust the
brightness and contrast with MaxIm DL (Max).
<End Joe’s Way>
I did 3 additional things in Max before saving the image as a TIFF. If the
image is saved in Max at that point and brought into PhotoShop (PS) then you
have a black image. The image has to be stretched using curves and levels to see
anything because of the way Max and PS approach things differently. So, I used
Max to stretch the data first because when comparing the two I felt Max did a
better job for the initial stretch than PS. Here is what I did.
(I am going into detail because this is a learning thing. Also if there is a
better way, maybe Tyler can set us straight.)
1 – In the Screen Stretch box I manually stretched the right had carrot as
far to the right as I could without clipping any star data.
2 – In Max I selected menu item Process > Stretch and set these parameters
Linear Only, Screen Stretch, and 16-bit and clicked OK.
3 – Again in Max I selected menu item Process > Stretch again and set
these parameters Gamma (0.5), Max Pixel, and 16-bit and clicked OK.
Then I saved the image as a TIFF for Photoshop.
>From here on it gets fuzzy because I did a lot of playing around.
In PS the image will still be a little dark. So Using Tyler’s image as a
guide I used curves and levels in the RGB to bring up the contrast and
brightness in moderate steps keeping an eye on the histogram to be sure I
wasn’t clipping data. When you see the combing effect in the histogram you
know you are stretching the data pretty far. The noise will also increase as you
boost the brightness and contrast. When I got the image to about what Tyler’s
brightness looked like then I went into the Blue level and just stretched it a
bit to give the background a bit more of a blue hue than straight black.
Now I checked Tyler’s image and noticed his stars were smaller than mine.
Size matters, so I created a mask for just the big stars by :
1. Creating a duplicate image and doing a Hi-pass filter with threshold = 1
2. Did a Gaussian blur with threshold = 1
3. Went into Image > Adjust > Threshold and played with the slider till
mostly the big stars were left. I cloned out anything I didn’t want till I had
the mask I wanted.
4. Then I did a menu Select >All, then Edit > Copy.
5. In the original image I created a duplicate layer of the original image,
then went to the channels tab and created a new channel. I pasted the mask into
the new color channel. I then clicked on the eye of the RGB channel to reveal
the image, then went back to the adjustment layer and clicked on it. From there
I went to the menu items. I clicked on Select > Load Selection and
lo-and-behold all my big stars were selected.
6. With the big stars selected I enlarged the image to get a closer look at the
selection. The selection wasn’t big enough so I went to menu item Select >
Mofiy > Expand and increased the size of the selection by 1 pixel.
7. Satisfied with the selection I went to menu Filter > Other > Minimum
and it shrank the stars.
If you look at some of the stars you will see a halo around them. I don’t
have this process down perfect yet.
I felt my image was looking pretty good at this point but Tyler’s image was
much smoother. Being this close I had to go all the way, so I went out on the
web and bought NeatImage as a plug-in to PS. Once I applied NeatImage I felt
it smoothed out the noise very well and can give Tyler a run for his money.
That may have sounded boastful but it was meant as a thank you for helping us
overcome the huge learning curve in image processing. And no less thanks to
Patrick and Joe and all of you who have added your input to help us fledglings
along.
Jim
_______________________________________________
Utah-Astronomy mailing list
Utah-Astronomy@mailman.xmission.com
http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy
Visit the Photo Gallery: http://gallery.utahastronomy.com
Visit the Wiki: http://www.utahastronomy.com