Using Photoshop to Process DSLR Exposures
By Clif Ashcraft
On November 29th, 2007, I tried out my new (to me) Digital Single Lens Reflex Nikon D40 camera from my observatory in central New Jersey. Here below is a step-by-step tutorial showing how I processed the raw exposures to achieve the final result.
The image of Comet Holmes 17p that I got with my DSLR was obtained using a 50mm f/1.8 lens and the "no flash" preset on the Nikon D40. When this preset is used under dim light conditions, the camera opens the lens up all the way and takes a time exposure until sufficient image density is obtained. When pointed at the Perseus region of Comet Holmes 17p, I believe the component of the subject which limited the exposure was a thin wispy cloud layer, illuminated by the red tail lights on the nearby NJ Turnpike. My exposures were about 5 to 6 seconds in length. I took several exposures and after eliminating the ones with the colorful trails of planes passing overhead, I had 5 "good" ones to work with. These all had a pervasive red haze over the whole field of view, but brighter in places where the cloud layer was thickest. One of these "raw" images is shown below.
Note that the comet is quite prominent in the image.
If we now pull up the levels histogram in Photoshop we see the following:
Note the broad peak on the right in the RGB levels. This turns out to be almost completely caused by the red background in the image from the light pollution illuminated clouds:
By moving the left slider up into this peak, most of the red background goes away:
Small adjustments made to the green and blue histograms:
Gave this result:
Finally, adjusting the right slider on the RGB histogram like so:
Produced this result:
Only the densest portions of the clouds remain as red blobs in the image.
I did this procedure on all five of the images and then copied them onto a black background in a new Photoshop file giving me a five layered image. I then shifted each image until the stars were precisely aligned. Stacking can be accomplished in Photoshop in one of two ways. In the "Normal" mode you adjust the opacity of each layer to 1/n. This means the first layer is 100% opaque, the second layer is 50%, the third is 33%, the fourth is 25% and the fifth is 20%. At that point the "Merge Visible" command in the Layers menu is used and the layers collapse into a single stack. Alternatively, the "Lighten" mode can be selected and all layers left at 100% opacity before merging. I am not sure which is the best way, they both seem to work. After some more playing with the details of the levels histogram, the following resulted:
Note that the clouds blurred out a lot because they were drifting by and were different in each image.
I then cropped the region containing the comet:
and then added the annotation using the Text tool in Photoshop:
These results were published in the December 2007 issue of the Asterism newsletter.