Speckle interferometry works with any telescope design. Because the necessary field-of-view is quite small, no complex optical system is necessary to get good astrometric results. The most important descriptive number of a telescope system is the objective focal length, /obj. If it is not listed on the telescope, an approximate value is easy to calculate. It is the focal ratio multiplied by the objective diameter. In the case where the main telescope is only one part of a more complex optical system, f obj can be calculated from a simple measurement in the image plane of an object of known angular extent. For example, the angular extent of a certain object is known to be 2.3''. After passing through the optical system, the size in the image plane is measured to be 14.5 ^m. So,

^obj 2.3 arc sec 1 rad 1000 um

Note that in the above calculation, the radian "units" have been ignored. Radians are a unitless measure. In order to do optical calculations, all angular measurements need to be converted to the "natural" units, radians.

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