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Focal Reducer

As with all forms of photography, faster focal ratios (or smaller f-numbers) result in brighter images and therefore faster exposure times. One can make EAA work with any astronomy telescope, but keep in mind, with a focal ratio of f/7 to f/10 or larger, it will take a lot longer to collect the light necessary to capture images of extended deep-sky objects. That’s why some consider it useful to invest in a focal reducer, a small optical component that’s placed just in front of the camera sensor to produce an effective focal ratio of f/3.5 to f/6 or so, fast enough for nearly real-time viewing. However, with the right OTA (remember — that’s Optical Tube Assembly… the telescope portion itself), the need for a focal reducer is substantially lessened. I don’t use a focal reducer because my OTA is already f/2.2 by itself. But I previously had a telescope that was f/10. Way too “slow,” really.

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