Celestro focal reducer agena astro11/21/2023 The C6 XLT can be collimated by adjusting a set of three screws on the secondary mirror. The C6 XLT is, thankfully, largely immune to this problem due to the small size and consequently low mass of its primary mirror and internal parts. Large SCTs tend to suffer from “image shift” or “mirror flop” as the mirror wobbles during focusing or slowly slides during a long photographic exposure. Like most Schmidt-Cassegrains, the C6 XLT focuses by moving the primary mirror back and forth along a sliding rod inside the tube. Using a reducer doesn’t change this-you’re still limited to the same usable field of view size before vignetting becomes a problem. The C6’s baffle tube diameter limits the normal field of view possible with eyepieces, focal reducers, and/or cameras to a ~35mm image circle, which corresponds to about 1.3 degrees across. Thanks to the relatively thin corrector, the C6 tends to cool down to ambient temperatures fairly rapidly compared to a larger SCT or similar-sized Maksutov-Cassegrain, albeit more slowly than most 6” Newtonian reflectors. This is probably a combination of the relative newness of the tooling used to produce the Schmidt corrector plates and the care that goes into their manufacture. Optically, the C6 XLT tends to be quite good-much better than a typical C5 or C8, and on par with the C9.25. The C6 XLT is marginally heavier and bulkier than a C5, is more widely available, and is much more capable-so there’s really no reason to get a C5. The C5’s large central obstruction of around 40%, compared to the C6 at around 35%, leads to lower contrast and performance than specs would indicate, and tends to cost about the same as the C6. Compared to the older, smaller C5, the C6 XLT has more than 50% more light gathering ability and about 25% more resolving power.
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