Telescope Reviews: Re: Tell me about Focal Reducers / Field Flattener

The speed of the system doesn't really have any affect on the field curvature. This is mostly influenced by the design of the optic.

SCTs have probably the most field curvature of any commercial telescope made. Field curvature simply means that when the center of the field is in focus on the film plane, the outside of the field is out of focus. So the field is "Curved". If you used an inward curving film (like a shallow bowl), this would allow you to have stars sharp at the center and at the edge.

The Focal reducer/field flattner simply adds more strength to the optics as you move further from the center to the edge, so that stars near the outside are focused further back than those at the center.

Most other telescopes suffer from other types of abberations (Coma for mirrors, and you can get coma correctors to compensate) and Astigmatism (fast refractors, and usually the manufacturer will sell a special telecompressor lens designed for a specific telescope design) that closely matches the amount of astigmatism introduced off axis, but places at 90 degrees, so that that they cancel out.

So, fast Newtonian telescope don't have enough field curvature to be noticed over the primary abberations, which are off-axis Coma (Dominant abberation in that kind of system) and Astigmatism.

For refractors, often the field is almost perfectly flat. A telecompressor that is not designed for the system (SCT reducer used in a refractor) will actually CURVE the field. But usually, because these telescopes are smaller and faster, the blur circle remains so small as to be not visible at the detector. Some people report using SCT focal reducers on their small refractors with some success.

Regards.

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Celestron C14, CGE (Big Al)
Astro-Physics 6" f/8 (Buffy)
Televue 101 (No name, but I call it my Widescreen HD Space TV)

The night sky is my mistress. She seduces me away from all other lovers.

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