Digital Image Enhancement in Astrophotography
A revolution has been occurring in traditional film-based astronomical photography during the past few years. It is now possible to extract even more information from an astronomical negative than was possible using the time-consuming darkroom image enhancement processes described in detail elsewhere on this web-site. By scanning a negative and digitizing the information in the film image, one is able to perform all of the traditional darkroom image manipulations using a computer and sophisticated image processing software. The software not only allows the photographer to accomplish traditional manipulations faster and easier, but it can so efficiently increase the signal-to-noise ratio that it permits one to extract information from the negative that was impossible to obtain before. Furthermore, and perhaps the most important aspect of this digital technology, is that the photographer can enhance the subtle information hidden in the shadows of the image while maintaining and even enhancing detail in the highlights. This problem has plagued astrophotographers ever since the first photons hit a film emulsion. Any one who has spent hours in a photographic darkroom trying to work with astronomical negatives knows this only too well!
Two pioneers in the use of digital image enhancement techniques as they apply to astronomical negatives, Jerry Lodriguss and Chuck Vaughn, have excellent articles on their web-sites (SEE LINKS), which go into great detail on how to apply this exciting technology to your own astrophotos. They are a must-read for all interested photographers! Tony and Daphne Hallas of Hallas Digital Services provide professional state-of-the-art image processing for your astronomical negatives. Their web-site can also be found by clicking on LINKS.
Following is a brief description of the method I am currently using to digitally process astronomical images. Every image appearing on this Web-Site has been digitally enhanced using the techniques described below.
The original astronomical negatives are scanned at maximum resolution, using either a Polaroid Sprintscan Plus for 35 mm negatives (2700 dpi) or a Polaroid Sprintscan 45 (2000 dpi) for medium format (6X7) negatives, to produce high resolution TIFF files. The Polaroid scanner software is used prior to the final scan to optimize the contrast and tonescale range as well as the color balance and saturation of the pre-scan image. A Power Macintosh G3 with 320 mB of RAM is used to power the Polaroid and Adobe Photoshop 5.0 software.
The high resolution scans are imported into Adobe Photoshop 5.0, saved as IBM-PC TIFF files, then imported into the Picture Window 2.0 program (Digital Light & Color) for digital compositing. Since Picture Window requires Windows-based software, I use a PC with a 1GHz Pentium III processor and 512 mB of RAM to run the program. Two to four unique images are combined to produce a single final image with the Composite command (Transformation menu) using the BLEND operation. This "averaging of pixel values" serves to improve the signal-to-noise ratio by dramatically reducing film grain (as the square root of the number of negatives). The darkroom equivalent of this operation is performing a double exposure; i.e., exposing each negative sequentially onto a single sheet of photographic paper. I sometimes use the FILTER operation for extremely dim objects: One first optimizes each individual scanned image for brightness and contrast. Then using the Invert command in Photoshop, each image is converted to a negative. The two "negatives" are then "stacked" in Picture Window using the Composite (Transformation menu) command with the FILTER operation selected and the overlay amount set at 100%. This is the digital equivalent of sandwiching two negatives in the darkroom and exposing onto a single sheet of photographic paper. The result is a much more contrasty and saturated image when the final negative is reverted back to a positive. There is some grain reduction with this method of digitally combining images, although not as good as when merging is done using the BLEND operation. The final composited image file is imported back into Adobe Photoshop 5.0 (on the Macintosh) for adjustments to the tonescale (gamma curve), hue and saturation of the images, un-sharp masking of the star images, Gaussian blur to reduce film grain, removal of film flaws and cropping.
35mm transparencies and negatives can be prepared for output display. These are produced using a Polaroid ProPalette 7000 Digital Film Recorder directly from the digital image files.
CCD Images are discussed elsewhere on this web site.