Some of the best images of planets are taken with Schmidt-Cassegrain telescopes, yet people often say that the large central obstruction makes this design ill-suited for planetary observing. Is the obstruction of less consequence for photography than for visual observing?

Do CCDs and digital cameras make the central obstruction less of a problem in lunar and planetary imaging? In other words, does advanced technology make diffraction theory obsolete?

Central obstructions reduce contrast rather than resolution; they make it harder to distinguish two similar shades of gray without affecting the ability to split tight double stars. Modern photoprocessing software can “stretch” contrast to show things that would otherwise be invisible. This can make the harm done by central obstructions

A cross-section of a Schmidt-Cassegrain telescope.

less obvious, but it doesn’t make it go away.

Even without image manipulation, an 8-inch Schmidt-Cassegrain telescope with a 3-inch central obstruction shows low-contrast planetary detail as well as an unobstructed 5-inch telescope does (no, the math isn’t as easy as simple subtraction) — and a 5-inch apochromatic refractor is considered a superb planetary instrument. But despite the obstruction, the 8-inch scope gathers twice as much light as the 5-inch. This gives it a decided advantage for photography, allowing shorter exposures to freeze moments of good seeing and lower (less noisy) gain settings for electronic sensors.

The biggest advantage of Schmidt-Cassegrains for planetary imaging is mechanical rather than optical. Their stubby tubes make them less prone to vibration than equivalent refractors or reflectors on similar mounts.

— Tony Flanders


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