effective focal length

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effective focal length

The property of a compound lens, such as an eyepiece, that is analogous to the focal length of a single thin lens and can be used, for example, in the determination of magnifying power. For a two-lens system the effective focal length, f , is given by
1/f = 1/f 1 + 1/f 2d /f 1 f 2

where f 1 and f 2 are the focal lengths of the components and d is their separation.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
References in periodicals archive ?
Our goal was to obtain a lateral resolution of 5 [micro]m, so we accordingly selected an OCT objective lens (LSM02, Thorlabs, NJ, USA) with an effective focal length of 18 mm.
To achieve the often-recommended scale of 2 arcseconds per pixel for deep-sky work, the KAF-8300 requires an effective focal length of only 560 mm.
The AFR-IV has a 0.8x magnification factor, and while its made for several William Optic refractors, with the FLT-98 it yields an effective focal length of 500 mm and an image scale of 2.2 arcseconds per pixel on the ST-8300M.
I used the scope with a 1991-vintage Tele Vue Paracorr, which increases the scope's effective focal length by 15% but also fills the 1.4[degrees]-wide field of an APS-C chip with round stars.
It works purely as a field flattener and does not change the effective focal length or focal ratio of your telescope.
SPECIFICATIONS & MEASUREMENTS * TMB-92 Signature Series Refractor Clear aperture 92 mm Effective focal length 505 mm Focal ratio f/5.5 Weight 9.4 pounds (4.3 kg) Minimum tube length 15 inches (38 cm) (assembled) * All measurements by Sky & Telescope.
To increase the size of the object on the imaging sensor, we need to increase the effective focal length of our telescope, at the cost of a slower focal ratio.
WHAT WE LIKE: Compact and lightweight Superbly flat fields with reducer lenses Versatile photo-visual system WHAT WE DON'T LIKE: Slight chromatic aberration apparent visually Light falloff at image corners Complex and confusing stacks of adapter rings SPECIFICATIONS & MEASUREMENTS * BORG 77EDII Clear aperture 76 mm Effective focal length 331 mm Focal ratio f/4.36 Weight 4.2 pounds (1.9 kg) Minimum tube lenght 16.5 inches (42 cm) BORG 101ED Clear aperture 100 mm Effective focal lenght 414 mm Focal ratio f/4.14 Weight 6.2 pounds (2.5 kg) Minimum tube lenght 20 inches (51 cm) * All measurements by Sky & Telescope; telescopes equipped with f/4 Super Reducer and optional Feather Touch focuser.

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