![]() If the corrected image field isn’t large enough to cover the full sensor, then stars may appear misshapen or elongated towards the corners of the image since it’s not fully corrected. Second, the larger a camera sensor is, the more corrected the image field has to be to cover the entire image. The faster the focal ratio of a telescope (or corrector), the narrower the focal plane is and therefore the more precise focus has to be. Meade ACF series telescopes - 55mm (with add'l T-Adapter)īack Focus for Fast Telescopes and Large Camera Sensorsīack focus spacing becomes even more critical to get right when you’re using a telescope/corrector with a fast focal ratio, or when you’re using a camera with a large camera sensor such as full frame or larger.Celestron 11" & 36cm RASA - 55mm (with included T-Adapter).Celestron 8" RASA - 25mm (with included T-Adapter, incompatible with DSLRs).Celestron EdgeHD series telescopes - 55mm (with add'l T-Adapter).Common Telescopes That Have Correctors Built-in & Their Back Focus Spacing If you use any of the below telescopes, then make sure your back focus spacing is accurate. Optical correctors and reducers can also be built right into the telescope, which is becoming more and more popular for astrograph telescopes that are specifically designed for imaging. The Radian 61's integrated corrector requires 55mm of back focus Accessories That Require Correct Back Focus for Your Telescope It becomes especially noticeable when using large sensors like full frame and/or a fast optical system. This usually results in elongated or misshapen stars, most noticeably towards the corners (off-axis) in the image. camera sensor) at the recommended distance away from the corrector, then the corrector will not work as intended. Both optical correctors and focal reducers are designed to work best when the focal plane is at a set distance away. ![]() When back focus becomes really critical to get right is when you’re using an optical corrector or focal reducer with your telescope or one that is integrated into the design. Measuring Back Focus When Using a Corrector or Reducer However, when you’re using a corrector or reducer, back focus must be a set distance away. When using your telescope stock without any accessories, you can easily reach focus because the telescope’s focuser travel is designed to move around this distance. What is Back Focus, and Why it Matters for Your Telescopeīack focus, in its simplest definition, is the measurement between the last optical component, such as a corrector or reducer, of your telescope and the focal plane. In this article, we’ll go over why back focus spacing is so important to get right, everything to consider for back focus spacing, and how to use adapters and spacers to get your back focus spacing perfect. Figuring out the correct back focus for your telescope can be a daunting task to beginners and experienced astrophotographers alike. ![]()
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