Combining Exposures with Layer masks

 

Step 1

After taking your picture you may notice that to get faint detail, you loose detail in the bright areas. Figure 1

Figure 1

What I do is save two images, one like Figure 1 and another with low contrast, like figure 2.

Figure 2

See how this brought out hidden detail in the center, including a hidden supernova just to the right of center.

 

Step 2

Now take these two images to photoshop to do a layer mask.

Open both images, put the low contrast on the left, and the high on the right.

Click on the left image, then click on select, then click all, then click edit then copy.

Now click on the right image, then click on edit and then paste. This will put the first exposure on top of the second.

You want the layer menu open, do this by clicking on Windows then click on Show layers. Figure A

Figure A

Now click on Layer 1 in the layer window, it will turn blue. Then click on the mask icon (that's the dashed circle in a square) at the bottom of the layers window. You will then see a white box. Figure B

Figure B

Now click on the long exposure to make it active, then click on the background layer in the layer window. Click on select then click all then go to Edit and click on Copy.

Now hold down the keyboard ALT and click on the white box in the layer display. The image will turn blank.

Now click on Edit and then Paste. You can now close the image on the left, you no longer need it. Figure C

Figure C

Now click on View and then New View and place this window on the left. Figure 3

Click on the right image, the image on the left will not look right, ignore that for now.

Figure 3

Now click on the mask icon, what was the white box in the layer window.

Click on Filter then click Blur and then click Gaussian Blur. I set my radius to 10, but you can play around to find what works best for you. Figure 4

Figure 4

Now you will need to tweak the mask a bit. Click on the right image and then click on the image menu and select Adjust then click Curves. Place your arrow on the black dot on the lower left side of this window, left click and hold to move this point. You want to get the background black. Then click OK. Figure D

Figure D

Now click on the small arrow on the upper right corner of the layer window and choose Flatten image.

You can now save your image.

 

You can see now see detail in the arms, as well as detail in the center, dust lanes, and imbedded stars. This exposure was taken at F3.3, 20 exposures at 30 seconds each and stacked making one exposure of 10 minutes. Figure 5

Figure 5

Some notes. If you use the same image with different contrast, there is no reason to line up the images. If you are using two different exposures, such as a 5 minute exposure of the Orion nebula, and then want to use a 15 second exposure of the Trapezium, you will need to align the images first.