[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
RE: [APML] Another image processing challenge for snowy nights-M45
This
was an interesting challenge. I processed it as if it were my own image, so it
reflects my own preferences. The result is at:
I
would resize it down to about 1200 pixels wide to deal a little better with the
signal-to-noise (graininess) issues. My experience is 99% CCD, so it will be
interesting to compare notes with others on techniques. I have this theory that
many of the issues are the same, so I appreciated a chance to manipulate a film
image.
Here's what I did to it:
*
Created a mask in Photoshop to fit the brighter central portion. Then I darkened
the overall background (Image | Adjust | Levels} until it matched the value in
the corners. I did this just in case there was anything to save in the corners,
but I ultimately elected to crop so this step wasn't
necessary.
* I
noticed a slight green cast to the image, presumably from light pollution, so I
used Image | Adjust | Curves to balance the color visually -- trying to get it
close. I discovered a red gradient running left to right, so I created a mask
and filled it with a gradient that would allow me to use Image | Adjust | Curves
to remove the gradient. This left me with a very faint top to bottom gradient,
which seemed to cross color channels, so another mask, another gradient removal.
I finished up by using Levels and Curves to get the histogram where I wanted it
-- dark but not black background, and brightness is the nebula without
over-emphasizing the brightness that occurs around the bright
stars.
* At
this point I found the grain standing out quite a bit, so I did a three-stage
smoothing using Gaussian blur. I selected the darkest background -- what I would
call the "true background" -- and blurred it using a setting of 1.0 pixels.
next, I select this background and the outer edges of the nebulosity, and
applied a lighter smoothing, about 0.6 I think. Then I selected nearly
everything, all but the brightest stuff, and applied the barest smoothing, 0.3
pixel.
* I
did a final review of the histogram and did a few small adjustments. I found
some background areas that I could not tell if they were nebulosity or just
noise.
At
this stage, I usually take stock of what I've done, and then I will go back to
the original image and perform essentially the same steps but with much more
care as to balance of color, avoiding emphasizing noisy portions of the image,
building masks for gradient removal much more carefully, etc. So this image is
really a sketch that will allow me to develop a more technical plan of attack
for the image. If this were my image, I'd probably shoot more images to try to
develop better S/N before doing final processing.
So
that's an avid CCDer's processing approach. How would someone approach this
image with a film background?
Ron Wodaski
The New CCD Astronomy
http://www.newastro.com
Hi All!
If any of you would like to try another raw
image, I have a 2 shot composite of M45 at 2048x1371 pixels. The image size is 8Mb and you can download it
at
Be sure to right click the above hyperlink and
select Save Target As
This one is 2 30 minute exposures on Fuji Superia
800 Extra. There is no processing done at all. No levels,
nothing. I'd love to see what the pros can do to my image!
Thanks!
