Astronomy Principles and Practice Fourth Edition.pdf

The telescope collector 239
Dolland in 1760 that the effect of chromatic aberration could be reduced considerably by the use of a
compound lens system.
When the photographic process was introduced, its potential as a means of recording many star
images simultaneously was immediately apparent. Optical design was, therefore, directed to the
improvement of the telescope to perform as an astronomical camera. In the first instance, all-lens
systems were used but now modern astronomical cameras may make use of a combination of both
lenses and mirrors in the same instrument.
When the emphasis in astronomical research was directed to making use of telescopes with
the maximum light-gathering power, the mechanical aspects of design dictated that large telescopes
must be reflectors. In fact, large professional refractors are no longer constructed because of the
technological problems that abound.
Thus, the designs of telescopes have been influenced by many factors, some of these being:
1. our knowledge and understanding of image formation,
2. the detector system available for appending to the telescope and
3. the technology of optical materials and methods of preparing optical surfaces.
It is usual practice to have a series of instruments of various purposes for attachment to a given
telescope. As a particular telescope is normally designed for some special function, it is impossible to
have a complete range of instruments which can be efficiently matched to that telescope. For example,
a long-focus refractor is ideally suited for making measurements of the separations of double stars,
either by eye or by photography—a long focus gives a good separation between images and this
allows accurate measurement. However, this same telescope is unsuitable for the usual spectrometric
techniques: any single star image is physically large, again as a result of the long focus, and it is
usually impossible to pass the whole light from this image through the slit of the spectrometer. Any
combination which wastes light in this way is inefficient. Whenever possible, efforts should be made
to use the best telescope–instrument combination for the particular type of observation that is being
undertaken. Because of the wide range of observations which are performed, it is not always possible
to conduct the measurements with maximum efficiency and some compromise is normally made.
Novel telescope designs are emerging which rely on the support of recent technological
developments. Some large telescope mirrors are being made as composite mosaics, with each of
the elements being actively kept in position under computer control to maintain the sharpest image.
Another concept features the idea of large multiple mirrors whose collected light might be combined
by using fibre optic links. After reading the following sections, it will be appreciated that one of the
advantages of such a system is that a large collection aperture can be achieved but with an overall
system of relatively short length, thus reducing the costs of the supporting engineering framework.
16.2 The telescope collector
The principal part of a telescope consists of the collecting aperture, acting as a means of producing
a primary image. This function is depicted in a simplified way in figure 16.1. It will be seen from
this figure that the light entering the collector is arriving in parallel rays. This is general for all
astronomical objects as they can all be considered to be at infinity in comparison with the dimensions
of any telescope.
The quantity of energy which is collected per unit time by the telescope is proportional to the area
of the collecting aperture. The larger the aperture of the telescope, the better it performs as a means of
collecting energy and the more capable it is as a tool for investigating faint sources. As the aperture is
usually circular in form, the light-gathering power is proportional to the square of its diameter, D. Itis
convenient, therefore, to express the size of a telescope in terms of its diameter.
A telescope would be working with perfect transmission efficiency if it were able to concentrate
all the energy collected into the images. However, it is impossible to do this as some of the energy