Meade and Celestron f

The Meade and Celestron " f /6.3" reducers (actually x 0.63) are similar if not identical. The focal length is about 240 mm and the intended lens-to-sensor distance (A) is about 110 mm.

If you do the calculation, you'll see that this lens-to-sensor distance should give x 0.54. But in fact it gives a perfect f/6.3 with an 8-inch (20-cm) f /10 Schmidt-Cassegrain telescope. Why? Because the focal length of a Schmidt-Cassegrain changes as you rack the mirrors closer together (Figure 6.2), and Meade and Celestron take this into account. After all, when you focus to accommodate the focal reducer, you are increasing the effective tube length (from secondary to focal plane) nearly 20%.

To complicate matters, there are reliable reports that when Meade first switched the manufacturing of focal reducers from Japan to China, there was a batch of "f /6.3" reducers with a much shorter focal length (about 150 mm). These of course gave much more than the expected amount of compression and produced severe vignetting.

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