equivalent focal length


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equivalent focal length

n
(General Physics) optics the ratio of the size of an image of a small distant object near the optical axis to the angular distance of the object in radians

equivalent focal length

The distance measured along the optical axis of the lens from the rear nodal point to the plane of best average definition over the entire field used in a camera. See also focal length.
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Specifications of the PAT (All dimensions are in inches) Primary mirror (concave paraboloid) Diameter 8.00 Radius of curvature 64.00 Focal length 32.00 Distance to secondary 29.75 Secondary mirror (convex sphere) Diameter 0.65 Radius of curvature 4.77 Distance inside prime focus 2.25 Distance to Cassegrain focus 39.75 System parameters Equivalent focal length 560.0 Effective focal ratio f/70 Cassegrain amplification 17.5x Image scale 14.5 arcsec/mm For me, the design's biggest attraction is that it allows high-resolution planetary photography without the need for a projection lens of any kind - just two simple reflections.
In either case the equivalent focal length is 96 inches.
The final design has a 29.1-inch equivalent focal length, a decrease in the 36-inch focal length of the mirror itself.

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