Deep Sky Photography: Darkroom Techniques

Enhanced Color Astrophotography

Conventional darkroom techniques as described below were originally used to enhance nearly all of the the photographic images appearing on this web-site.  The advent of digital image processing of astronomical negatives has essentially rendered the photographic amplification method obsolete.  Consequently, each image displayed on these web pages has been digitally enhanced using techniques described under Digital Image Processing.   I include a description of the original photographic process not only for historical interest, but for those who, for their own personal reasons, may wish to work up their astronomical negatives in the darkroom instead of on the computer monitor.


I do all of my own film processing and image enhancement work in a small darkroom I have constructed from a walk-in closet in my home. This includes processing of all negatives using C-41 chemistry. Kodak Flexicolor Chemistry is used to process the original astronomical negatives.  I use Unicolor C-41 chemistry  for all subsequent steps in the process which involve large format negatives and significant quantities of processing solutions.  The Unicolor C-41 process is faster and less expensive than Kodak Flexicolor and is very reproducible.

Negatives and prints are rotary processed in a JOBO CPP-2 with Lift which produces very reproducible results in a semi-automated fashion. I use a high quality 4X5 enlarger. For best results with the enhancement process described here, it is very important to use high quality ( i.e.,expensive!), fast, apochromatic enlarging lenses. Only these lenses will provide pin-point stars during the multiple steps where light is passed through sandwiches of negatives. It is also necessary to use enlarging lenses with a longer focal length than normal for a given film format to avoid parallax at the edges of the film sandwiches. For example, a 150mm f/4 apo enlarging lens should be used to print through a sandwich of 6X7 negatives. Furthermore a high power lamp is needed in the enlarger to provide enough light to work with after passing through two layers of film.

Exposure times at the telescope on hypered Fuji SHG-400 were 50 to 60 minutes at f/7. Exposure times using the hypered Kodak PPF are half of this (i.e., 25 to 30 minutes). Two identical exposures are obtained of the desired object (preferably on the same night, but this is not mandatory). These original astronegatives are then carefully sandwiched in register. One negative is taped to a light box. The second negative is gently dusted with lithographers anti set off powder (to prevent Newton’s rings) and carefully placed on top of the first negative. Using a 20X achromatic loupe, a star is chosen at each end of the negative and placed in exact register. The process is repeated about 5 times on each side until registration is perfect. The top negative is then fixed to the bottom negative with silver mylar tape. In this way a permanent film sandwich can be made. The process is identical for all subsequent film sandwiches in the enhancement process.

The principal behind the stacking of negatives was first described and perfected for amateur astronomical photography by Tony and Daphne Hallas in 1989:  At each stage of stacking and copying, the color saturation and contrast are enhanced. The trade-off is image sharpness being compromised at each step thereby limiting the number of times one can do it. The original astronegative sandwich is placed in the enlarger and held securely using a glass negative carrier. This is critical to prevent buckling of the film during the long exposures (up to one minute) in the enlarger. The film sandwich is exposed onto a 4X5 sheet of Vericolor Print Film (VPF), Kodak 4111. These 4X5 sheets are processed in C-41 chemistry using the JOBO processor. This produces a color transparency. Exposures and filter packs are chosen such that when two of these transparencies are sandwiched together and viewed on a color- corrected light box, they produce a neutral black sky which is about one-half stop darker than you would ordinarily choose to view a single, properly exposed, transparency. (This also means that the single VPF should be about one stop less than you would normally expose it.) This will ensure good color saturation when producing the internegative. The next step is to sandwich the pair of VPF’s together, as described previously,then prepare an internegative by contact-copying onto a 4X5 sheet of Kodak Commercial Internegative Film (4325). This is done in a standard glass copy proofer. Care must be taken at all stages of copying, with both 4111 and 4325, that the sheets are in exact register to facilitate later stacking into sandwiches. To achieve proper color balance in the final internegative (i.e., to avoid crossover), the correct exposures and filter packs must be used when producing internegatives. For example, the internegative film should not be overexposed, just to lay down more density. Or, if the highlight region of the object (i.e. Andromeda) tends to print toward the cyan, one would add more red to the filter pack when exposing the internegative. I have found that the recommended filter packs and exposure times listed by the manufacturer gets me pretty close with only minor adjustments being needed.

Objects with a uniformly low surface brightness are the easiest to work with and thus give the best results for beginners to the technique. A good example of such a nebula would be the North American and Pelican nebulae. More challenging are nebulae containing large variations in brightness, with areas of high density on the negatives typical of galaxies and some reflection nebulae, such as the yellow nebula around Antares. In these cases, burning in regions of the transparencies and dodging the same areas on the internegatives is necessary to produce the most pleasing results.

Most of the commonly photographed astronomical subjects are best printed from a single internegative. Otherwise the object is too contrasty and over-saturated, decreasing the aesthetic appeal of the image. However, many of the objects presented on this Web-Site are so dim that they hold up quite well to sandwiching the final internegatives to make high-contrast, highly color-saturated transparencies and prints. Again, in preparing internegatives to be used in a sandwich, it sometimes works best if these internegatives are exposed about one-stop less than you would if you were to work from a single internegative. This seems to produce superior prints from the internegative sandwiches.  Prior to the advent of digital image processing, all of the Sharpless and van den Bergh objects displayed on this web site were prepared from internegative sandwiches.

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