WIDEFIELD ASTROPHOTOGRAPHY 2 By Chris Schur In this lecture, we will discuss the best methods for testing short focus lens systems photographically, how to evaluate the results of those tests, what the perfect astrofilm for wide field work should do for us, and finally, what films are in the real world. Once a lens is chosen as a potential candidate for skyshooting, it must be tested and evaluated for suitability. Two simple tests will reveal the optical condition a lens: The stop down test, and the trailed focus test. For the first test, mount the camera with the prospective lens system on the back of a driven telescope. Put a fast color film, preferably slide film, in the camera. Aim the scope/camera combination at a bright star field, and engage the drive. Allow a few minutes for the drive to take out the slack in the system. You will not need to guide on any particular star for this test. With the lens all the way open, say at f/2 for a typical 50mm. lens set at infinity focus, adjust the shutter speed to " B ". Open the shutter and expose for one minute. Next advance the film, and close down the lens one stop. Repeat the one minute exposure. Continue stopping down the lens and exposing until you get to f/5.6 or f/8. The final result will reveal which focal ratio will be acceptably sharp. For the second test, turn off the drive or mount the camera on a stationary tripod. Aim the camera up at a similar star field, and open the lens up all the way. Open the shutter and let the stars trail for two minutes. Now very carefully cover the lens with a dark cloth or lens cap, and using a dim light to read the lens, rotate the focus ring on the lens about half a millimeter as seen on the distance vernier toward the close focus position. Uncover the lens for another two minutes and recover it. Repeat this process on the single frame until five or six steps are completed. Close the cameras shutter and repeat the whole test if desired. The end result here will determine just where best stellar focus will be achieved. Anytime colored filters are used in front of the lens, especially red ones, this procedure will have to be repeated. When the film is processed, carefully study the slides with a minimum of a ten power magnification. A 25mm. eyepiece turned around and used as a magnifier works well. Don't use a slide projector for your evaluations because they introduce their own distortions. For the first test, examine each slide in the sequence carefully, starting with the fastest focal ratio first. Note the appearance of the stars in the center, and in the corners of the frame. There will be a point where the stars in the center will appear very sharp and the images in the corners very close to pinpoints. This will be your best compromise between photographic speed and sharpness. This occurs in most lenses between f/2.8 and f/4. You must decide if a small amount of coma at the edge of the frame is acceptable, or if it must be razor sharp to the edge at the expense of a much longer exposure. A good compromise is one stop down from wide open for good performance on extended objects and reasonably sharp images at the edges of the field. When examining the results of the second test, look for a progressive enlargement of the width of the trailed star images. That will be only true if the lens is at best focus in it's infinity position. However in half the lenses you may encounter, especially the older used ones, they will focus in a slightly different position. Here you will see the trail narrow after one or two steps, and then become wider again. Check your notes for the position of the lens that corresponded to the narrowest trail. Mark this position on your lens by either scribing a fine line or arrow into the focus ring, or by putting tape on the ring, and marking it in pen. This will be the position of true focus, at any focal ratio and this setting will be used for astrophotography. Films. There is a wide selection of films on the commercial market suitable for various forms of terrestrial photography, but only a small handful of them are suitable for skyshooting. What are the characteristics of a good film suitable for wide field astrophotography, and what should we expect from them? Certainly high film speed comes to mind for many budding astrophotographers, yet with the fast lenses used in wide field work, this may not be the best choice. Many of the objects recorded will be small on the frame, so a finer grained and slower film will work out much better. With a lot more light from the faster focal ratios to work with compared to prime focus through the scope astrophotography, sky fog will be more of a problem. Also a higher contrast film is desirable to help differentiate the nebula from the bright sky background. Another desirable characteristic is good red speed. Many of the targets most suitable for the wide field camera are emission nebula. Since their primary color is a deep red from H-Alpha light emission, they require a film that will respond adequately to this wavelength. The best film choice will also include low reciprocity failure - the ability of a film to keep on recording fainter and fainter objects in long exposures. Most films off the shelf are very poor in this regard. Unfortunately, no one film can be said that has all of the aspiring characteristics that are needed. In fact, most color films are totally unsuitable because of low contrast or very low red speed. Fortunately, there are a few decent films to use that will meet most of our criteria. For color slides, Elite 400 is still the best for red sensitivity and high contrast. However it has a fairly high reciprocity failure in exposures longer than 15 minutes. Agfachrome 1000 slide film has been very well received despite its somewhat larger grain. It has good red speed, and a very pleasing neutral colored sky background, even with more light polluted skies. We recommend this film for beginning sky shooters. For color prints, Kodak Pro 400 ppf works fairly well, but many have had surprisingly good results with the Fuji 400 and 800 super G plus films. For B&W users, hypered tech pan - 2415 emulsion is by far the best choice. In lieu of a hyper tank, the Tmax 400 has had good reviews for Milky Way photography, but is poor in the red region of the spectrum. What I hope was accomplished in this lecture was to save the budding astrophotographer a great deal of time and suffering trying to select a suitable lens and film combination for wide field work.