ASTROPHOTOGRAPHY WITH THE ARIZONA ON/OFF AXIS GUIDING SYSTEM In this lecture, I will discuss a relatively new concept in guiding a newtonian telescope for astrophotography. It was invented and refined by astrophotographer Pierre Schwaar and the Author to make skyshooting of large newtonians a pleasant experience. These days, amateurs perform astrophotography at remote observing sites where there are darker and more transparent skies than in their own backyards. As a result, the hosts of photographic accessories, and the size of the guide scopes must be kept minimal as to fit into the family sedan. After many years of struggling with flexure prone, and cumbersome external guide scopes, or the difficult to use off axis guiders, we decided it was time to develop a totally new concept in guiding. It had to work with any newtonian optical system, and make guide star location extremely rapid. Our new system also had to have the following characteristics: A) Very low cost with a minimum of components B) Absolutely zero flexure with any component in the optical system C) Be able to locate guide stars within two degrees of subject with pinpoint imagery D) Be able to guide on the subject itself, or any star in the area E) The cameras orientation and focus must independent of the guider F) Must work well with the fastest mirrors, even far off axis G) Easy to install on a newtonian with a minimum of modifications. H) It must be easily removable within seconds to put the scope back in the visual mode Such design goals seem quite formidable, and truly impossible in many regards. However after many years of trial and error, we were able to devise a complete guiding system that is self contained, and met ALL of our criteria! The Design Concept: The basic idea is to install a small first surface mirror in the exact optical center of the secondary mirror, and about 1/3 the size. The size of the smaller mirror is not critical, but must be larger than the converging shadow beam of the secondary and smaller than the secondary itself. For our ten inch f/4.5 newtonian, we used a 1.25 inch minor axis square mirror and for the 16 inch, a 1.3 inch. The idea is to divert a small ring of light from around the secondarys shadow off to the side of the tube for guiding purposes. Using simple geometry, we were able to calculate the percent obscuration, and were astounded to discover just how little an effect the parasitic mirror had on the optical system. For example, by using a small elliptical diagonal at about 1/3 the light cone size at the secondary mirror we were using the central ten inches of our ten inch primary mirror for guiding. The net effect was the equivelent surface area of a four inch guide scope with the remaining surface area equal to that of an eight inch mirror. Since we are not actually reducing the size of the primary mirror, the full resolution is maintained, and the net effect on the image is to make an f/4.5 system give the image brightness of an f/5.1 system. Such a small loss in the image brightness is negligable when considering other aspects in the system. An added advantage of using this system is that the guide scope is really the center part of the main parabolic mirror, which is in reality closer to shperical in curvature. The result is that the off axis images are sharp and clear out to at least one degree off axis. this added advantage allows guiding on either the subject itself, or any convinient star within a two degree circle around the subject! Next time I'll be uploading a gif image to illustrate the optical and mechancal layout of this system.