The extreme brightness range of nebulae 1451 Simple techniques

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One of the challenges of deep-sky photography is that nebulae and galaxies are typically much brighter in the center than at the edges. Careful adjustment of levels or curve shape helps bring out the faint regions without turning the bright regions into featureless white. A good tactic in Photoshop is to adjust the Levels slider several times in small, careful steps.

If that's not enough, you can stack exposures of different lengths, thereby combining their dynamic range. For scientific work, this is the best way to cover an extreme brightness range because all parts of the image are processed alike. If your goal is only to bring out detail, though, read on.

14.5.2 Layer masking (Lodriguss' method)

A very powerful way to combine different exposures is to use layer masking in Photoshop. This technique has been perfected and popularized by Jerry Lodriguss (www.astropix.com). Figure 14.10 shows what it accomplishes. The key idea is that the shorter exposure replaces the longer one in the parts of the latter that are overexposed.

I'm going to give quick instructions for this technique here, but if you are not very familiar with Photoshop, you may also need to consult additional documentation.

(1) Start with two differently exposed images of the same deep-sky object, one with appreciably more exposure than the other.

(2) Align them, preferably using astronomical software (see p. 154), but do not stack them. Instead, save the aligned images in a form that you can open in Photoshop.

(3) Make an extra copy of the image with the longer exposure. This is the one you will edit.

(4) Open all three in Photoshop. The image you are going to edit contains a copy of the longer exposure.

(5) Working as described on p. 181, paste a copy of the shorter exposure on top of the image you are editing, in a new layer. Do not merge it down or flatten the image, nor adjust its opacity.

(6) Now you are going to create a layer mask, also known as an alpha channel. That is an invisible layer containing a black-and-white image whose only function is to control the opacity of a visible layer.

To do this, see Figure 14.11. With Layer 1 (the pasted image) selected, click to create a mask. Then hold down Alt and click on the mask itself (currently an empty white box). Now you are editing the mask instead of the image.

(7) Copy the shorter exposure and paste it into the mask. It will appear in black and white.

(8) Blur and darken the mask so that it looks something like Figure 14.12. The light areas are those where the shorter exposure will predominate in the finished product.

(9) By clicking and Alt-clicking as shown in Figure 14.11, you can switch back and forth between editing the image and the mask. Adjust the mask until its effect suits you.

Figure 14.10. Combining different exposures with layer masking. (a) Orion Nebula, 1 minute, 8-inch telescope at f /6.3, Canon XTi, ISO 400. (b) Same, 2 minutes. (c) Combined, using a layer mask to take the central region from the shorter exposure.
Figure 14.11. How to add a mask to a layer in Photoshop and how to work with it.

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Figure 14.12. The layer mask used in Figure 14.10.

(10) Finally, flatten the image and make other adjustments, such as levels and unsharp masking.

You can combine any number of images in this way, and the results can be impressive (Figure 14.13).

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