Astronomy Principles and Practice Fourth Edition.pdf

444 Practical projects
Figure 24.29. An experiment to investigate the resolution of a telescope when viewing an artificial double star.
resolved at full aperture. If possible, provide means for the insertion of red, green and blue colour
filters.
24.8.1 Resolving power
Starting with the diaphragm at full aperture, slowly close it down until it is no longer possible to resolve
the stars as being separate points of light. Measure the diameter of the aperture with the selected
diaphragm at this condition. Repeat the procedure and take the mean from five results. By assuming
λ = 5500 Å, determine the resolving power for the setting of the diaphragm (use equation (16.14)).
Repeat the experiment but this time commence with minimum aperture and slowly enlarge the
diaphragm until the stars are resolved. Measure the diameter of the aperture required to resolve the
stars under this operation.
Take five resolving power measurements with the blue, green and then the red filters in the beam.
According to the effective wavelengths of the filter–eye combination, evaluate the resolving power of
the telescope and see how it changes with wavelength.
Compare the observed values of resolving power with the actual angular separation of the double
star.
These experiments serve to illustrate the difference between the resolving power as established
by diffraction theory, using some chosen criterion, and the actual resolving power as determined with
a telescope in combination with the eye.
In practice, when real double stars are being observed, the resolving power depends on the
observing conditions, on the relative brightnesses of the stars and on their relative colours. The amount
of scattered or background light may also affect the value of the resolving power. Apparent resolution
can sometimes be achieved by partial knowledge of the objects such as the position angle of the line
joining the two stars.
Some people are better than others at being able to resolve star pairs and may be able to resolve
stars which are, in fact, closer than the Rayleigh limit.
24.8.2 Magnifying power
Direct the telescope to the day sky and place a thin piece of paper over the eyepiece. Measure the
diameter of the exit pupil, d, for a range of values of the entrance aperture, D. From a plot of d against
D, determine the gradient. The magnifying power is defined by the ratio D/d (see equation (17.5))
and, hence, the measured gradient provides a value for the magnifying power of the telescope and
eyepiece combination.
Change the eyepiece for one with a different focal length and repeat the experiment.