Extending Dynamic Range in Lunar Imaging with Luminosity Masking

Tony Gondola, 5-07

 
Anyone who images the moon for any length of time will soon realize that it is impossible to capture detail across the full range of available brightness levels with today's 8 bit cameras.  If you try and eliminate all clipping the final exposure is often too dark to really bring up the areas of interest. The usual compromise, especially when shooting near the terminator is to accept some clipping in limited areas such as sun facing crater rims in order to get a reasonable exposure of the rest of the field. This paper presents a simple technique for extending dynamic range thus making it possible to create lunar images with detail extending from the dark area near the terminator areas to the brightest sun facing crater rims and rayed craters. The technique is especially useful for large mosaics and full disk images where the range of brightness is especially wide.
 
The first step in the process is to collect the extended range data. To do that you'll need to collect two avi files for every field. One at a normal exposure level allowing some clipping of limited areas and another under exposed by one stop. Because gain settings can be non-linear it's best to make the adjustment by cutting the integration time in half to achieve the one stop reduction. Here's what a typical example field looks like, normal exposure is on the left, one stop under on the right:
 
Stack and sharpen all your fields as you normally would. If you're making a mosaic then you'll need to assemble one for both exposure levels. In the example used here avi's were captured for the creation of a full disk image. Here are the starting mosaics:
 
Load both images into Photoshop. For this example I'm using version 5.5 but any version including Elements should work.

Select the full frame of the under exposed image and paste it onto the brighter image. Bring up the layers view, the brighter image should be at the bottom labeled as Background.

Before the mask can be created the two images will need to be aligned. Set the opacity of Layer 1 to 50%, zoom in 200% and manually drag the layer into position. Reset opacity back to 100%, change the layer mode from Normal to Luminosity and re-select the Background layer. To create the selection mask press CTRL+ALT+Tilde (looks like this ~). The usual selection "marching ants" will appear around the highlight areas of the image. Depending on your image you'll see a lot of selected areas or very little but don't worry about that and proceed with the next step.

 

Re-select Layer 1 and click on the Add Layer Mask at the bottom of the layers box. It's the gray square containing a white circle. The formally dark image will brighten as the mask is applied:
 
At this point you can click on the mask (right of the paper clip symbol and make small level changes if needed but it's best to keep any changes small.  Once you're satisfied go ahead and flatten layers. 

To bring up the terminator area a bit more select the color picker tool and invoke Select/Color Range menu option. Pick an area of very dark gray that's just inside the terminator. The marching ants will return outlining the near terminator area. Feather the selection area by 5 pixels or so. While the selection area is still active bring up the Levels control and move the midpoint triangle slightly to the left. Usually going to 1.05 or 1.10 will be enough. Click the Preview box on and off to assess the effect. Go easy here, all you're after is a slight brightening of the near terminator area that will still blend in with the rest of the image. Once you're satisfied de-select the area. Here's the result:

To finish up apply final levels and curves adjustments and in this case a bit of sharpening. Here's the final extended dynamic range result: