PRIME FOCUS ASTROPHOTOGRAPHY 2 LECTURE In this lecture we will consider the optical system of the scope itself, including aperature stops, collimation and guiding. Optical considerations. There are many advantages of a large telescope: Observatory class light grasp, big and clear images of faint objects, and shorter exposures. However, most of the larger mirrors that are used in amateur scopes are much too fast optically to produce reasonable results over the entire field. To achieve an acceptable performance level using todays higher resolution 35mm. films, these typically faster optical systems must be stopped down to about f/5 or f/6. This will yield sharp and clear images right to the edge or the frame, rather than bloated parachute shapes. Exposures of objects that fill the frame will be blurred, and aesthetically unappealing. We made several aperature stops for our scopes out of masonite, and painted them flat black. The slight increase in exposure is more than compensated by the excellent improvement in sharpness over the entire frame. Another aspect to consider that will greatly affect the the images formed on the film, is collimation. The sharp field at f/5 is just equal to the size of the 35mm frame. However if the collimation is not right on the money, much of your field can appear as blurred streaks. Spend the extra time on fine tuning your collimation, those extra few minutes of work will really pay off in the end result! One last reflection in dealing with the optics of your scope, is the potential for focal shift during the exposure. In typical exposures of thirty to sixty minutes, the outside temperature can drop ten or twenty degrees. In the longer tubes of sizable scopes, the length can change from thermal contraction. This may be enough to shift the focus out of acceptable limits. Aluminum tubes are the worst offenders in this respect, followed by fiberglass or plastic tubes, then steel. The best material to use will have a low thermal expansion coefficient. Such materials include cardboard, wood, or plywood laminates. These should also be well sealed from the(2) moisture, as a damp night can also affect the dimensional stability. Guiding methods. Guiding methods with a large scope are more demanding than with smaller instruments. The lightweight tubes found on these bulky portable instruments are not usually suitable for mounting a separate guidescope. This is because of flexure which can occur relative to the main instrument. Two methods to keep your scope on track is either the Arizona ON/Off axis guider, or the Lumicon Newtonian Easy Guider. These systems are well proven in the field, and produce excellent results. Each system has its advantages and disadvantages, but should fit a wide variety of needs. The Easy Guider uses a small pick off prism at the edge of the cameras field view. This captures a small portion of the sky to select a guide star for tracking purposes. This technique does not require modifying the telescope, but poor guidestar selection, and guidestars exhibiting severe coma distortions from being so close to the edge of the field of a fast optical system are present. Despite this, we believe that it is the best commercially available off axis system, and have obtained many well tracked astrophotos with it. The Arizona On/Off axis system requires the installation of a tiny adjustable mirror in the center of the main diagonal. Here small annulus of light around the shadow of the secondary is used for selecting guidestars. Light from the guidestar is diverted to a 1 1/4 inch focuser on the outside of the tube. This second focuser is set at an angle next to the primary focuser. By tilting the small mirror with thumbscrews, one can select guidestars within a 1 1/2 degree field surrounding the subject in the camera's field. This method allows us to guide on a star contained within the subject, or one anywhere within a two degree circle. As an added bonus, the much longer EFR of this system (about f/10), allows us to guide on perfectly round pinpoint star images over most of the obtainable field. This setup does not require a separate guidescope, is easy to use, and eliminates all differential flexure problems. Regardless of which tracking system you choose to use, an important aspect is the type of reticle eyepiece used, and the color of the reticle illumination. We have found that a high quality 12mm ocular, with a red LED for illumination works the best. A dim guidestar can be much easier to see against a deep red reticle, allowing you to track on substantialy fainter stars. Use this in combination with a 2 - 3x barlow to achieve a magnification of at least five power per inch of focal length in your prime focus system. For example, if you stop down your 13.1 inch f/4.5 to an f/5 to improve sharpness, your focal length is still 59 inches. A 12mm eyepiece, and a 2.5x barlow will yield 312 power. This is 5.3 power per inch, and is within the recommended tolerance for guiding.