Astronomy Principles and Practice Fourth Edition.pdf

Visual resolving power 275
Figure 17.3. The upper frame is by a ground-based telescope of a star field. A section of this has been observed
by the HST telescope. The lower right frame is further magnified and, without the effects of atmospheric seeing,
clearly shows the telescope diffraction pattern imposed on the brighter stellar images. The circles indicate the
positions of very faint stars. (By courtesy of STScI.)
empirical criterion gives resolving powers which are typically about 20% better than Rayleigh’s
theoretical criterion. Thus, to a very good approximation, Dawes’ resolving power can be written
as
α = 115
D
where α is in seconds of arc when D is expressed in mm.
In practice, the clarity of separation of two stars depends on many factors including the seeing
conditions, the amount of scattered or background light, the relative brightnesses of the two stars and
the apparent colours of the stars.
For a reflector telescope, the diffraction pattern corresponding to a point object is influenced and
made more complicated by the central obscuration and the spider which supports the secondary mirror.
The effects are sometimes apparent on long exposed photographs of star fields, where the very bright
stars exhibit a cross-like pattern as depicted in figure 17.3.