The Use of Wide-angle Slide Projection Lenses in the Planetarium
Frank Andrews, Richard Hall and Wayne Orchiston
PO Box 2909,
Wellington, New Zealand
The use of ultra wide-angle projection lenses in slide projectors at the Carter Observatory's Golden Bay Planetarium is discussed. The placement of the ultra wide-angle projectors and ways in which they are used in conjunction with standard projectors to produce spectacular and dynamic visual effects is outlined.
Planetariums have an important role in science education and in popularising astronomy (e.g. see Manning 1995, Othman 1991, Sampson 1993, Urke and Laerarhogskule 1993), and the emergence of the thematic "feature planetarium show" is one of the highlights in the twentieth century evolution of the planetarium. Such shows take advantage of the stunning visual images that are now available through the Hubble Space Telescope, and the achievements of such astro-photographic pioneers as Dr. David Malin.
Colourful undistorted astronomical images can have major public appeal, but their impact is often reduced in small to medium-sized planetariums where limitations of dome size and capital budget prohibit the use of expensive wide-angle projection systems found in larger more affluent planetariums. Since more than 62% of all planetariums have dome diameters of 9m or less (Petersen 1997), this means that the majority of the world's planetariums are disadvantaged in this way. This paper reports on the way in which this deficiency was overcome at the Carter Observatory by using a mix of standard slide projectors and slide projectors with relatively inexpensive ultra wide-angle lenses.
Carter Observatory is the gazetted National Observatory of New Zealand and was opened in 1941. It currently has four discrete functions: research, heritage preservation, education, and public astronomy. In 1992 a small visitor centre was added, and this included the relocated Golden Bay Planetarium with its aging ZKP1 Zeiss projector. Since this event there has been a major expansion of the Observatory's education and public astronomy programs (e.g. see Leather et al. 1997; Orchiston and Andrews 1995; Orchiston and Dodd 1996; Orchiston and Hall 1996; Orchiston et al. 1998). This was prompted, in part, by changes to the national schools curriculum where "astronomy" is now a compulsory segment of the Science curriculum, within the strand "Making Sense of Planet Earth and Beyond" (Leather et al., 1998; Science in the New Zealand Curriculum, 1995).
The Ultra Wide-angle Lens System in the Golden Bay Planetarium
When it was first incorporated into the Carter Observatory's operations, the Golden Bay Planetarium employed six standard Kodak carousel projectors, with 85mm f/2.8 S-AV 1000 lenses. These projectors were equi-spaced around the periphery of the 6-m dome (in harmony with a concentric seating configuration), and were controlled by a tape which also carried narration, sound effects and background music.
Visits to IMAX theatres and planetariums in the United States, Europe and Hong Kong, quickly brought home to the authors the advantages of using wide-angle projection systems in a planetarium environment. Film projection systems encountered typically used projection lenses of 40-50mm focal length and focal ratios of 2/2.5 or f/2.8, which produced large impressive images, but such systems were beyond the Observatory's price range and incompatible with our dome size. The challenge was to find an affordable lens of between 24 and 30mm focal length which would provide an acceptable "IMAX-effect" in our small planetarium dome.
Enquiries in the United States and Germany revealed that a small number of 26mm f/2.8 lenses had been manufactured for the Kodak Carousel S-AV 2050 projector, which was the model used at the Carter Observatory, but this line had been discontinued when the projector was superseded. However, the factory in Germany had one remaining lens, and this was purchased.
This lens was then tested in the Golden Bay Planetarium and proved very satisfactory, although slight repositioning of the front condenser lens was necessary in order to obtain even illumination. In addition, only the sharpest slides, made on the finest grain film, produced acceptable results.
The search was then on for a further lens, and a secondhand one was eventually located in the United States.
The Observatory's first planetarium show featuring the new lenses was titled "Venus and Mars: The Doomed Planets", and this was launched on 1995 January 29. The concentric seating arrangement was retained, and projectors with the two ultra wide-angle lenses were positioned so as to project onto the east and west sides of the dome. Meanwhile, five projectors with "standard" lenses were distributed around the dome. Although the planetarium show was a great improvement on earlier shows, and the large images produced by the two new lenses were much appreciated by audiences, the new configuration generated excessive head movement and failed to provide an effective focus of interest for viewers.
The Observatory's next planetarium show, "Journey to the Centre of the Galaxy", was due to be launched on 1996 February 19, and in planning this a unidirectional seating arrangement was decided on. This allowed us to reposition the two slide projectors with the ultra wide-angle lenses so that they would project lap-dissolved images on the northern half of the dome. These projectors were housed in a purpose-built "projection booth", together with three standard projectors whose images could be superimposed on those produced by the ultra wide-angle lenses. The other two standard projectors were positioned on the dome perimeter 45° on either side of the main projector bank, in order to project images onto the eastern and western sides of the dome (near the edges of the ultra wide-angle frames).
By using such techniques as masking, pin registration, lap dissolve and customised art work we were able to produce far more dynamic effects and images than were feasible with the original projection system.
By using a combination of standard and ultra wide-angle 26mm f/2.8 Kodak projector lenses it has been possible to produce captivating and visually stunning planetarium shows at the Carter Observatory that are comparable to shows produced in larger planetariums employing much more expensive wide-angle projection systems.
A relatively inexpensive hybrid projection system like that employed at the Carter Observatory can be replicated with comparative ease at any small to medium-sized planetarium facility with a limited budget, provided the appropriate ultra wide-angle lenses can be sourced.
We are grateful to Mark Petersen for providing relevant data on planetariums.
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Reproduced from the Planetarian, Vol. 27, #1, March 1998. Copyright 1998 International Planetarium Society. For permission to reproduce please contact Executive Editor, Sharon Shanks.