If b> 0, then RA > 0.5. With b = 0, RA = 0.5.

The folding flat reflector thus has a very high axial obstruction ratio Ra, whereby at least a quarter of the light is obstructed at centre field. If b = 0, the telephoto effect T is still 2 from (2.59), even though the secondary has no optical power. This therefore gives a major reduction in length compared with a simple prime focus. For this reason the solution has been used in special cases such as the astrometric reflector at the U.S. Naval Observatory, where image stability considerations ruled out a powered secondary because of possible image variations from decentering (see Chap. 3).

2.2.6 The scale of astronomical telescopes and the magnification in afocal use of compound telescopes

As the equivalent of a large-scale photographic objective, the modern astronomical telescope is essentially defined by its free diameter D, its effective focal length f and its semi-angular field upr in object space. The parameter magnification of a conventional refractor using an ocular in the afocal mode is replaced by the scale of the telescope image. The scale is usually expressed, because of the traditional use of arcsec as the angular measure of fine structures, in arcsec/mm or its inverse function mm/arcsec. From Eq. (2.31) we have upr = ^' rad = 206 265 ^d arcsec (2.101)

for an object at infinity with up = upr. Setting r/ =1 mm gives the scale S as

and the inverse scale S as

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