ADVENTURES IN WIDE FIELD ASTROPHOTOGRAPHY By Chris Schur In this first of a series of lecture articles on wide field astrophotography, we will discuss the concepts of wide field work, applications, suitability of lenses and their aberrations so that you can select a good lens from the many inferior ones out there. Introduction. Skyshooting with a wide field camera is astrophotography in its purest form. Here, in a single frame it is possible to capture dozens of deep sky objects, along with their relative positions at a casual glance. Probing the sky with such a system will reveal many fine details that even the best charts lack. Also, we can use wide field photographs as observing tools to help pin down many of the elusive objects which can be challenging visually. Many useful projects can be accomplished with a simple 35mm. camera, such as charting a favorite region for later study, nova patrols, seeking comets and recording their motions in the sky, or perhaps mapping the hundreds of dark nebula along the plane of our galaxy. Also remaining to be discovered is the large scale structure of star clouds, chains of bright emission nebula, and knots of star clusters, along with aurora and meteors. The concept of wide field astrophotography is simple: A small camera with either a wide angle or telephoto lens, is secured to the tube of the telescope in some fashion. While the clock drive in the mounting moves the tube at the same rate as the stars, the cameras field of view will remain fixed, and time exposures can be made. In reality this task is not so simple, however with a little care and patience any determined individual will be able to produce acceptable results. While in theory, any type of camera will suffice for the job, the variety of films and lenses available for 35mm. cameras make them most suitable for the job. This allows us to choose the three most common formats: Wide angle, normal and telephoto. The only other requirements to begin skyshooting are clear skies and a tracking equatorial mount. Applications for Wide Field Astrophotography. Wide angle astrophotography is the first step for most at seeing more of the universe than can be normally seen with the naked eye or telescope. Because of the accumulative effect of light during extended exposures on film, your camera is far more sensitive than the human eye at seeing faint objects and subtle colors in the sky. Often invisible objects such as dark nebula, strongly colored stars, and extended faint emission nebula are clearly revealed. Suitability of lenses. A quality lens can make the difference between an average shot and a superb one. But how do we evaluate a good lens for astrophotography? All too often, many of the name brand lenses offered for 35mm. cameras are unsuitable for skyshooting due to various aberrations or distortions. A lens that may be perfectly suited for terrestrial photography may be useless for sky mapping. The type of subjects you will be shooting will for the most part, determine the type of optics you will need. For example, a large aperture is essential for recording the faintest stellar images such as might be needed in nova patrols or star mapping. However a fast focal ratio is the only choice for recording dim diffuse objects such as emission nebulas, or dark nebulas in the Milky Way. In short, the diameter of the objective lens determines the faintest stars reached, while the focal ratio limits the faintest extended objects recorded. Fortunately, telephotos and wide angle lenses are more suitable for photographing extended objects which are far too large and faint for the telescope to record. No lens system is perfect. All lenses are plagued by various defects, some more important than others which can limit their performance in deep sky photography. Probably the worst distortion is coma. This aberration is usually seen as comet or parachute shaped images near the edges of the field, that gets worse as you go further off axis. Another type of distortion is chromatic aberration, and is caused by the inherent curves in the glass itself. It is characterized by swollen star images in the center of the field, and possibly a blue halo around bright star images. The inability of lenses to bring all colors to a common point of focus is the cause of this condition. Vignetting is the third most common defect in lenses. This can be seen as a falling off in the intensity of the light striking the film near the edges of the frame. This can in extreme cases, cause the photo to look like we are looking out of a porthole! The fourth difficulty with wide field systems, and one of the more difficult to remedy is field curvature. This occurs when the center of the field is at a different focal point than the edges of the frame. Often this may not be distinguishable from the coma in off axis images, but in severe cases, the lens may have to be rejected for use in astrophotography. It is hoped that some insight to widefield work has been successfully conveyed here. In future lectures we will discuss some of the many additional aspects of the purest form of astrophotography.