Adjusting luminosity and overall color (Continued)
Now, I'm going to make a suggestion that you don't have to follow. You can either lighten the background of the entire image, or you can choose to lighten just the area inside your sketch circle. I prefer to lighten inside the sketch circle only. The reason is that as long as I'm doing this, why not take advantage of the fact that the view through the eyepiece usually shows some glow to the background sky compared to the non-visual part of the eyepiece. (I don't own a Nagler, so I don't know what it means to not have a non-visual area in my eyepiece ;-) I leave it up to you. I think it makes up a little bit for the fact that you no longer have that striking pure black background. I like consolations.
Selecting just the circle can take a little bit of fiddling. First, grab the oval selection tool. Then move your cursor as close to the center of your sketch circle as possible and hold the option/alt and shift keys and start dragging out a circle. (The shift key constrains the oval to a perfect circle. The option or alt key causes the circle to originate from the center rather than diagonally from one side.) Your circle probably isn't perfectly centered, but size it as close to the size of your sketch circle as possible and then drag it around until it is centered. If it is too big or small, you can redraw it, or you can go to the "Select" menu, then to the "Modify" submenu and choose either "Expand..." or "Contract..." and enter 1 or more pixels in the box to enlarge or reduce your circle. Once the selection circle encompasses your sketch circle, you are ready to work some magic on the image. If you aren't sold on the idea of adjusting only the circle, then you just saved yourself a little bit of time. :D
Figure 16: Dragging the selection circle from the center outward
At this point, we have a choice of two prominent means of adjusting the tonal range of your image: Levels or Curves. I prefer to use curves, because they provide an incredible amount of flexibility in how you adjust the values of your image. But because the learning curve (eheh) is heavier, we'll keep the discussion to the use of Levels. In a later tutorial, I'll address curves. They are incredible.
Before I forget--now would be a good time to grab your sketch pad or observation record sheet so you can compare the tonality of the digital image to what you tried to convey with pencil and paper at the telescope. Refer to it as you adjust the image to decide what enhancements you need to focus on.
OK, back to the computer. Go to the "Image" menu to the "Adjustments" submenu, and select "Levels..." You will now see a composite histogram that shows where the tonal values of your image reside. At the bottom of the menu is a shaded bar that goes from dark to light (Fig. 17, A) that represents which side of the histogram represents which shades. In the large window above you will see what looks like a black mountainous profile (Fig. 17, B). If your image is a positive DSO sketch, that mountain should be piled up above the dark portion of that shaded gray bar. The reverse will be true of a negative DSO image. This is telling you what proportion of the pixels in your image are black, white and whatever shade of gray in between. The taller the black mountain, the more pixels there are at that part of the tonal range.
Figure 17: Levels dialog window
The first thing we want to do is change the low value in the "Output Levels" section (Fig. 17, C). Right now it says "0". This means that the darkest dark possible is a perfect black of zero. I would suggest raising it to a value between 20 and 30. If you are working on an image with nebulosity, you may be pleased to see an immediate improvement in the visibility of dimmer portions of the image. You have just brightened the darkest darks a small amount, allowing any slight increase in brightness from absolute black to be easier to see. Depending on how light or dark your overall scan was, you may need to lower these numbers. You don't want your sketch to look like it is plagued with light pollution.
Figure 18A: Shadow output level set to 10
Figure 18B: Shadow output level set to 20
Figure 18C: Shadow output level set to 30
The next step is to play with the three arrows beneath the histogram. We will treat them separately below.