Astronomy Principles and Practice Fourth Edition.pdf

Chapter 20
Modern telescopes and other optical systems
20.1 The new technologies
It might be thought, with the ability to put telescopes such as the Hubble Space Telescope into orbit
above the disturbing effects of the Earth’s atmosphere (see section 19.7), and with the well-known
difficulties experienced in the past in building ground-based telescopes larger than the 6 m Hale
Telescope at Mt Palomar, that the creation of larger ground-based telescopes would not have been
undertaken. This is by no means the case. Two recent developments have enabled new observatories
to be established with telescopes far larger than the Hale Telescope and rivalling the Hubble Space
Telescope in resolving power. The developments are active optics and adaptive optics.
The current quest for providing new telescopes is chiefly the provision of larger diameters
allowing the collection of more and more flux from the very faint objects and the utilization of the
potential angular resolution that such diameters provide. Modern technologies involving new optical
materials, new concepts in achieving large imaging apertures and the introduction of computer support
have revolutionized the approach of producing new large telescopes.
20.1.1 Active optics
The previous generation of large telescopes involved the production of large monolithic primary
mirrors to collect the flux from celestial sources and to produce images for detection. In order for the
image quality to be maintained as the telescope is oriented to different parts of the sky, it was important
to have mirrors with good mechanical stability. To achieve this, the mirror design required a thicknessto-diameter
ratio of the order 1:9. As telescopes grow in size, the requirement of producing thick blanks
from which the mirror is figured causes difficulties in their production. In addition, primary mirrors
with large mass require very heavy engineering to allow the telescope to be readily manoeuvrable.
As the telescope alters its orientation in moving from one target object to another, the mirror tends
to flex under its own weight. Such flexure allows the optical surface to deform with the consequent
deterioration of image quality.
However, by using ‘active’ supports on the underside of the mirror in the form of a distribution
of pistons, it is possible to readjust the shape of the optical surface of the mirror and maintain the
required figure. By continuously monitoring the image quality of some reference object in the field,
the adjustments may be applied continuously by computer control. Because of the inertia of the large
optical system, the response to the feedback is relatively slow (∼ tens of seconds) but easily sufficient
to allow ‘continuous’ adjustment as the telescope tracks a celestial object. Thus, it is now possible
to use lighter mirrors with smaller thickness-to-diameter ratios. Such mirrors are obviously more
likely to suffer flexure but can also be corrected more easily. Many modern monolithic mirrors are
now manufactured with material removed from their underside to form a honeycomb pattern with the
correcting pistons in contact with the rib structure.
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