Films For Astrophotography


Spring of 1997 will bring a visit from one of the biggest and brightest comets in history. Comet Hale-Bopp is coming, and promises to display the most awe inspiring celestial fireworks since Comet West in 1976. Skyshooters by the thousands will be out each night, trying to capture the wonder of this visitor with their cameras to capture images that they can show their friends and future generations in years to come. But to record this historic event accurately and with the best possible chance of success, the correct selection of both camera and film will be essential. The right choice will accurately record the memory for years to come, the wrong choice can spell disappointment, and an opportunity lost forever.

In the past few years, the choices for films by astrophotographers has improved dramatically. Modern emulsion technology has produced faster and finer grained films, with excellent color balance in longer exposures than any time in the past. Never before has it been easier for budding skyshooters to record the skies with such clarity and ease. A number of new films by the giants Fuji and Kodak along with the old standbys can make comet photography routine and highly successful. Lets consider for a moment what a good film for shooting comets would be like. First off, a good comet film must have a good spectral response in the blue and yellow parts of the spectrum. A comets long and usually thin gas tail is a subtle sky blue color, and usually contains most of the detail in a comets tail. The dust portion of the tail however, displays a creamy pastel yellow color, and is usually the last feature to develop as the comet nears the sun, when the nucleus becomes more active. The dust tail is usually quite featureless, but constitutes the most spectacular part of a comet as it nears the sun. The long gossamer tails of the great comets were all dust types, and a film responding to the yellow part of the spectrum is an advantage here.The films contrast is very important as well.

Depending on what features you are trying to record on film determines what contrast we wish to use. For example, if the comet boasts a long, yet low surface brightness tail such as the recent Comet Hyakutake did, the best emulsion to choose would be a high contrast film, that would bring out the faint details in the heavenly appendage. On the other hand, if you wish to record fine details in the comets head such as jets and inner nuclear fine points, you would not use a contrasty film because the core would be burned out, hiding the inner details. A medium to low contrast film is best here, and will show the full extent of the coma and many of the inner core details. A finer grain is extremely important for extended objects such as comets. Coarse grained films are fine for stellar objects such as clusters and wide angle milky way images where the coarse grains simply look like stars, and the eye is fooled into seeing an acceptable image. For extended objects such as nebula and comets, a fine grain is a must, the delicate gradations from brilliant to dark can be found in such objects, and these will show grain as a very sandy texture to the recorded image. Avoiding grainy films such as Konika SRG3200 is a good idea for the most satisfactory comet images.

Finally, film speed is a consideration in comet photography. Too slow of a film and the required long exposure times to blur a fast moving comet during the exposure. This may requiring you to guide on the comets usually too faint nucleus. This is usually a rather difficult task, and although Hale Bopp already has a nucleus bright enough to guide on by seasoned skyshooters with large telescopes, novices are rarely set up with such equipment. The best solution is a compromise in grain and speed suitable for the type of lens being used. For most wide angle and telephoto work, which is what most amateurs will be using, a moderate speed fine grained film is preferred over a fast grainy emulsion. Prime focus shooters will appreciate the faster speed, and necessarily coarser grained emulsions to keep the exposure times tolerable.

Today's Films

Three options face the comet photographer when planning a shooting session. The choice of dramatic black and white negatives, brilliant color slides, or high impact color prints from color negatives are sometimes difficult to decide upon. Many skyshooters prefer slides so they can be projected in front of large groups, family and friends. Some like the rewards of the simple darkroom when selecting black and white prints for the final result. And most people today choose color negative films that produce colorful prints that can be duplicated in quantity, and make the best enlargements for the home or office. For black and white negatives, two films stand out clearly in the comet shooting arena. Kodak Tmax 400, and Technical Pan emulsion - 2415 for short. A sensible replacement for the old grainy standby of Tri-x in past generations, the Tmax emusisions are much finer grained than their predecessors, and have the desired blue response needed for good results with the blue gas tail of most comets. A good choice of speed for fine grained results is the Tmax 400 film, readily available in most camera and department stores.

For the do-it-yourself types, better results can be achieved with hypered 2415 Technical Pan film. The film has ultra fine grain, a speed equivalent to most 400 speed emulsions for astrophotography, and can be developed for any contrast desired. Though most will choose to develop it in the contrasty D-19 developer for enhanced shots of the tail, there are many other choices for developers that can be used for any contrast situation. Close up shots of the nuclear region may be better accomplished by development in a slower developer called Technidol, a formulation by Kodak for extremely fine grain and normal contrast for everyday scenes. Technidol can be obtained in most photo supply stores that carry tech pan and darkroom supplies. The disadvantage is the slower film speed, but hypering will make shots of a bright coma practical.

Color slides are very popular for general sky shooting, despite their drawbacks. The visual impact of a sharp color slide projected on a large screen cannot be underestimated, however there are a few guidelines when using slide films that must be adhered to for best results. Most important is sky fog. In most areas to some degree, the color of the night sky is easily recorded as a green or pink background in longer astronomical exposures. In the times of past such as in 1976 when the Great Comet West made its appearance, the local lighting was almost totally dominated by mercury vapor lamps. This gave the sky a greenish cast, contributing to the already greenish hue of airglow of the natural dark sky. The result was nearly all exposures of deep sky objects on slide films were marred by green backgrounds, and since the slide is the final result, you had to live with it.

Prints on the other hand can be color corrected to some degree by the printer, and does not have this disadvantage.

Today, most outdoor lamps which cause the most intense light pollution are of the high pressure sodium variety, and impart a pinkish cast to the sky - almost exactly opposite of the green color of past times. There is an inexpensive way to combat this color cast during the exposure. For areas that the sky background is dominated by mercury vapor lamps or areas far from city lights that have the green airglow color to the sky, shooting the exposures through a weak magenta color correction filter available at most photo stores will be a great help. Correction can vary, but 10 to 30cc of color correction is typical. For areas of much high pressure sodium pollution, such as is typical near large cities of today, a weak green filter can help. Again, 10 to 30cc of cyan and yellow color correction filters sandwiched together will yield green, and correct the color to an aesthetic neutral dark gray. If filters of any type are used, don,t forget the rule of thumb that for every 30cc of filtration doubles the exposure.

Slide films of choice are few, but excellent results have been obtained with two types, Kodak Elite 400, and Ektachrome P1600. Both are far less green sensitive than past Ektachromes and Fujichromes of past years, and have recorded details in the blue gas and yellow dust tails very well in recent comets. Color negatives offer the best selection for high speed films today. As a benefit from competition among film manufacturers for the fastest color negative films, the astronomer has a wider selection of high speed color negative film than ever before. There are three films in our opinion that have far surpassed the others in fineness of grain and excellent color response for shooting comets. These are Fuji Super G 400 Plus, Super G 800 Plus, and Kodak Pro 400 ppf films. The Fuji 800 speed film is fast, has unusually fine grain and is responsive to both the blue gas and yellow dust tails in comets. The contrast is good, and meets nearly all of the requirements for the ideal comet shooting film. Many very excellent images from instruments ranging from simple 50mm cameras mounted on stationary tripods to large aperture astrographs have been obtained with this emulsion and without hypering! Some of the very best images we have seen of the comet were taken with this film with prints revealing cold electric arc blue gas ion tails trailing off to one side of a gossamer golden dust tail. This film is recommended over any other for shooting Hale Bopp.

A finer grained version of the above film is available, and is known as Fuji Super G 400 Plus. This film too has excellent color response in the blue and yellow portions of the spectrum for comets, but has lower contrast overall than its faster brother. This can be used to an advantage for close-ups of the coma region of the comet where higher contrast films would yeild only a featurelees white burned out image. The unique contrast characteristics of this film make it ideal for bringing out inner nuclear jets and the yellow colors of the bare nucleus itself. Both films respond well to hypering, yielding a 2 - 3x speed increase when hypered for 18 hours at 50 degrees Centigrade. Both films can be easily printed to the correct color balance with neutral gray backgrounds by the photo finisher if they are given a sample balanced print to use as a guide. Select one from Astronomy Magazine that has a pleasing color rendition, and provide the photofinisher with it for reference.

Kodaks new Professional film, Pro 400 ppf is also rapidly becoming a top choice for shooting comets. The films superb red and blue response, fine grain and higher contrast make it a fine choice for wide angle lenses to record both the dust and gas tails in excellent detail. When hypered for 6 to 8 hours at 50 degrees Centigrade, the film speed doubles, and the blue response increases dramatically. The films then becomes ideal for prime focus astrophotography, with a speed and contrast superior to the Fuji Super G 400 Plus. Negative films to avoid Not all films are suitable for astrophotography, and a few that promise rich and varied colors or high speed may be the worst of all. When Kodaks Royal Gold series came out recently, many skyshooters rushed out to try the new emulsions, hoping for rich colors and good contrast of deep sky objects and comets. But the sad fact is they are poor in this regard, and nearly all users report a deep brown background to their prints that is difficult to remove by the photofinisher without causing improper colors in the subjects. To avoid this heartache, much better results can be had with Kodaks Gold series of films, which cost a bit less, and give a reasonable color balance in the backgrounds and subject. Some very excellent results were obtained on the recent Comet Hyakutake and Comet Kopf with Kodak Gold 400 and 1000. These two films can be rated as a distant second choice to the harder to find Fuji Super G and Kodak Pro emulsions, but can be quite serviceable.

Conclusion The stage is set, and the player, Comet Hale Bopp will be acting its part in the spring of 1997. With some preliminary planning and the correct choice of camera and film, you can capture this historic event to be remembered vividly for years to come.

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