Meade f

The Meade "f /3.3" reducer is designed for astronomical CCD cameras and works at x 0.3 to x 0.5 depending on how you use the spacers provided with it. Its focal length is about 85 mm. For the rated magnification of x 0.33, the lens-to-sensor distance is just under 55 mm.

Note that you cannot get x 0.33 with a DSLR. The front of the camera's T-ring is 55 mm from the sensor, and the T-thread adapter provided by Meade takes up approximately another 8 mm without spacers, so A = 63 mm (or a little more) and the magnification is about x 0.25.

Ml F3.3 FR

Figure 6.5. Compression, extreme and moderate. Uncropped 1-minute exposures of the Crab Nebula (M1) with a Nikon D100 at ISO 1600 through an equatorially mounted 12-inch (30-cm) f /10 Schmidt-Cassegrain telescope. Top: With Meade x 0.33 focal reducer working at x 0.25, giving effective f /2.5. Bottom: With Meade x 0.63 focal reducer giving effective f /6.3 or slightly less. (Rick Jackimowicz.)

Figure 6.5. Compression, extreme and moderate. Uncropped 1-minute exposures of the Crab Nebula (M1) with a Nikon D100 at ISO 1600 through an equatorially mounted 12-inch (30-cm) f /10 Schmidt-Cassegrain telescope. Top: With Meade x 0.33 focal reducer working at x 0.25, giving effective f /2.5. Bottom: With Meade x 0.63 focal reducer giving effective f /6.3 or slightly less. (Rick Jackimowicz.)

Aberrations in such an extreme compressor are a serious problem, and optical image quality leaves something to be desired. You will probably find that the lens-to-sensor spacing greatly affects the correction of off-axis aberrations. Still, the focal reducer provides a way to image faint objects under difficult conditions.1

1 I am grateful to Rick Jackimowicz for the focal length measurements and related information, as well as for Figure 6.5.

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