#1 Tom Glenn
- Posts: 3,944
- Joined: 07 Feb 2018
Soyuz
topic starterPosted 01 April 2020 - 04:46 AM
This image was captured on March 30, 2020 (03:30UT time corresponds to the next day), with my 6" Newtonian scope and ASI183mm with 610nm long pass filter. In the post below the image, I will outline a few observations related to color space and tone curves that I find interesting. Whether or not anyone finds it useful is another story!
https://flic.kr/p/2iKR8By
#2 Tom Glenn
- Posts: 3,944
- Joined: 07 Feb 2018
Soyuz
topic starterPosted 01 April 2020 - 05:03 AM
I’m often asked about processing lunar images, particularly with regards to restoring natural shadow detail along the terminator. Here are a few interesting observations, that may or may not be useful.
One source of confusion, I think, is that people are being misled by the way a raw image of the Moon taken with an astronomy camera displays on your screen, such as when you open it is Photoshop. These images always appear very dark, with unnatural contrast. The stacked output file from AS!3 is maintained as a linear file, and doesn’t have any embedded color profile. Your display device has a gamma associated with it, typically 2.2. In order for the raw images to display properly (representative of what the camera recorded), they have to be encoded with the reciprocal gamma curve in order to restore the linear image.
Without an embedded color profile, Photoshop will usually assign either sRGB or Adobe RGB for color images, or Gray Gamma 2.2 for grayscale images. When you assign one of these profiles, Photoshop assumes that the image has been gamma encoded, with approximately a 2.2 gamma. But it has not. So what we see is the effect of the display gamma being inappropriately applied to the unadjusted linear image. Shown below is an example.
The second image below is the same as the first, and the only difference is I have created a custom color profile designating the input gamma as 1.0 (linear image), instead of 2.2. Notice how the image is much brighter, but the histogram remains unchanged. That’s because the histogram is still showing the linear data, but Photoshop is now applying the proper gamma to the input image so that the monitor gamma brings the image back to the original linear state.
The third image is again the same image, but this time after converting the color profile to sRGB after first opening it with the custom gamma=1 profile shown above. Note that the second and third images look identical, but the histograms are completely different. That is because the first histogram is linear, and the second has been gamma transformed, but represents the identical data. There is a large difference between the effects of “assigning” and “converting” color profiles in Photoshop.
So what’s the point of all this? This information doesn’t necessarily help with processing lunar images, because you can (and should) always adjust the tone curves by eye to achieve both a good look on your monitor, and a histogram that makes sense. However, I consider the information useful because there do appear to be misunderstandings about why the raw images look so dark. When you take a properly exposed image with a DSLR or mirrorless camera, you never actually see the raw linear file if you simply open in a raw editor like Adobe Camera Raw, even if you “process” your raw images. Those images have already been gamma encoded by the raw editor before you see them. Images from the astronomy cameras have not. Final editing decisions are always personal, but people might make different decisions if they knew that opening the raw images in the default working space in Photoshop is assigning a strong unnatural tone curve to the original image, and so you are starting from behind when it comes to shadow recovery. It’s apparent in image 2 and 3 above that the raw image doesn’t have anything approaching black clipping, including the background sky, unlike the raw image when opened as default (image 1).
