![]() ![]() As we said, that distance is measured from the reference surface to the imaging sensor. The “Prime Focus” section in the instructions for the Tele Vue-NP101is says to use a 69.85mm (2.75″) spacing for imaging sensor up to 43mm on the diagonal (full-frame) and 67.82mm (2.67″) for larger sensors. ![]() You must look at the instructions for the optic, or the telescope it was designed for, to learn what distance to set.Ĭontinuing with our Imaging System example, the Large Field Corrector (LCL-1069) optimizes the edge of field when imaging through the Tele Vue-NP101is and NP127is telescopes. Traditionally, however, the Back Focus distance is measured from the center of the last lens surface to the focal plane. For Tele Vue Imaging System accessories, we use the flat surface at the base of the male threads on the metalwork as the reference surface since it is easy surface from which to measure. In this blog, we’ll go through an example of how this is done.ġ) Determining the Required Distance from Optical Accessory to Sensorįor optical accessories to work properly, the camera sensor must be located a prescribed distance from the optic. Determine Required Spacers to Reach Back Focus.Determine Which Adapter will Connect to our Imaging System.Determining the Path Length of Accessories.Determining Your Camera’s Inherent Path Length.Determining the Required Distance from Optical Accessory to Sensor.Here at Tele Vue we often get asked how to achieve the proper back focus for imaging with our Imaging System optical accessories. This fully multi-coated, 3-element unit, with 48mm filter thread in the 2″ barrel, inserts directly into 2″ focusers, accepts standard T-rings for interchangeable lens camera bodies or astro-cameras with the correct spacers (55mm from back flange of TRF-2008 to camera sensor ) and T-ring adapter. For example, our Tele Vue-85 APO becomes a 480mm f/5.6 super-telephoto lens and Tele Vue-76 APO converts to a 380mm f/5.0 telephoto for flat-field, fast photography. To function properly, all of these optical accessories require the camera sensor to be at a certain back focus distance from the correcting optic.Īny doublet objective scope with a focal length from 400- to 600mm and 2″ focuser is easily adapted for imaging with the addition of the Tele Vue TRF-2008 0.8x Reducer/Flattener ( mobile site). These can be fixed using correctors specifically designed for your telescope. ![]() Reducers are often combined with flattening correction to create a reducer/flattener.įor large sensors covering greater field angles, aberrations in addition to curvature become apparent toward the edge of the field. This also naturally lowers (speeds up) the photographic f-ratio which in-turn allows shorter imaging times for non-stellar objects. Reducers allow the scope to see a greater field of view by shortening the effective focal length of the system. So, telescopes can have either built-in or accessory flatteners to flatten the image plane for photography. While your eye can compensate for some curvature of field, the flat imaging sensor on your camera can’t. Most telescopes produce curved image surfaces, meaning that the focus at the center of the field is different than at the edge of the field. ![]()
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