AstroImaging Technical Note: “Mask Processing in Photoshop”
Ken Charlwood

(Ken was the winner of the Uncooled Digital Imaging Competition at the 2004 South Pacific Star Party. Here he describes how his winning image of M42 was captured and processed.)

My images were captured with an uncooled digital camera - a Fujifilm FinePix S2 Pro SLR [1]. The camera was set to an equivalent “film speed” of ISO1600, with auto white balance. All other in-camera settings were switched off and the images were captured as 12MB raw files.

The final M42 image (Figure 1) was created by combining a long and short time exposure, plus a layer mask using a blurred black and white copy of one of the original images. A similar process is described by Jerry Lodriguss in his e-book Photoshop for Astrophotographers [2]

The method can be used on any extended object with a bright centre or core where the range of brightness levels is difficult to capture with a single exposure. M42, in particular, has such a large range of levels that a long, medium, and short exposure could be used. Galaxies with a bright core where detail is lost in long exposures are also excellent candidates for this method.

The process can be used with cooled CCD, digital or scanned film images, but the long exposure needs to have a sufficiently long exposure to detect all the faint detail in the outer extremes of the image. Basic image calibration using darks and flats should first be applied to your images if necessary.

I used a home-made 18" f4.5 Newtonian on a fork mount with a Dob driver, hand-guided via a 100mm refractor. A telescope with wide aperture and short focal length (leading to a low f number) produces a wider field and nice tight star images. These can be very difficult to obtain with a long focal length of say, 2,000 mm or more.

Each long exposure image (Figure 2) is a 4 minute exposure. Four of these were aligned using the layers "normal" method to reduce noise - not to increase brightness or contrast. Brightness can be adjusted later in levels or curves. This produced a composite single long exposure (Figure 3).

While there is little apparent difference between these images, the “noise” in the stacked image will have been reduced by a factor of 2 compared to the single image. Noise reduces in proportion to the square root of the number of stacked images.

Reduced noise levels enable the brightness and contrast of the imaged object to be increased without the noise becoming a problem.

This composite image has a total exposure time of 16 minutes at ISO1600. If you are using film or cooled CCD cameras, it may be easier to use a long single exposure.

It is interesting to note however, that the amount of light gathered in this “stacked 16 minute” image is equivalent to over two hours of exposure using an ISO200 Emulsion Film (arguably more if reciprocity failure is taken into account).

The short exposure image (Figure 4) was only 10 seconds, however this could have been be a little shorter to show the Trapezium stars even more clearly.

As described by Jerry Lodriguss, the procedure is:

1. Open the images in Photoshop
2. Copy and paste the short exposure (Figure 2) on top of the composite long exposure (Figure 4) in a layer.
3. Create a layer mask (Figure 5) from the short exposure.
4. Paste the long exposure into the mask.
5. Open a new view of the image to see effects of manipulating the mask.
6. Blur the mask (Figure 6).
7. Adjust the density and contrast of the mask with curves.
8. Tweak the colour of the core to match the rest of the image.
9. Flatten the image to combine the layers - save the final image (Figure 1).

Figure 1 - The Final Image of M42	Figure 2 - A Single 4-Minute Exposure
Figure 3 - The Long Four 4-Minute Exposure	Figure 4 - The Short 10 Second Exposure
Figure 5 - The Black & White Mask	Figure 6 - The Blurred Mask

References:
[1] http://www.dpreview.com/reviews/fujis2pro/
[2] http://www.astropix.com/PFA/PFA.HTM