26
3 The Telescope
Fig. 3.2 Diagram showing the optical pathway of a Keplerian telescope
indexes bonded together to form a lens with the same refractive index for red and
blue light. Apochromatic lenses use the same concept but have three components, for
red, blue, and green. The problem with mass has never really been resolved, and by
the beginning of the twentieth century, refracting telescopes reached (and possibly
even exceeded) their maximum operational size. Over the years, additional optical
coating has been applied to astronomical lenses, and although this has improved
them, it has not overcome their physical limitations. Of course, telescopes must
be exposed to the elements, although they should not be made to encounter very
extreme weather, and over time, a lens becomes pitted, a coating becomes worn, and
the optical bonding compound used to bond achromatic and apochromatic lenses
loses its optical transparency, at which point the lens will either need maintenance
or replacement.
The concept of using mirrors to magnify an image had been known for at least five
hundred years before their successful application to astronomy by Newton. Mirrors
have a number of advantages over lenses; only the surface of the mirror needs to be
optically smooth, reducing the need to produce optically perfect glass; mirror-based
telescopes do not suffer from chromatic aberration; and the mirror’s position at the
base of the telescope makes it easier to operate. However, mirrors are not entirely
without design problems. Typically, a secondary mirror is needed near the primary
focus, which then partially obstructs the optical path and reduces the efficiency of
the telescope. Also, light is diffracted around the secondary mirror’s supports and
the secondary mirror itself, which causes the spikes and halos seen around some
stars in astronomical images, although admittedly in some cases such artifacts might
have been added digitally for effect. There is also a tendency for light incident on
the edge of the mirror to be brought to a slightly different focus from that of light at
the centre. As a result, objects become smeared out, a problem known as coma. An
additional problem is the alignment of the primary and the secondary mirrors. If this
alignment is not within a very small tolerance (Collimated), the light arrives at the
focus at a slight angle to the focal plane, and consequently, point sources become
elongated.
2 Collimation problems normally worsen over time, but they are easy
to address. Hence recollimation is part of the normal maintenance procedure for a
telescope.
As with lenses, mirrors become increasingly massive as their diameters increase.
Sagging under gravity is prevented by supporting the mirror across its entire rear surface area. However, it appears that the engineering limit for producing astronomical2 They are often described as looking comet-like.
3 The Telescope
Fig. 3.2 Diagram showing the optical pathway of a Keplerian telescope
indexes bonded together to form a lens with the same refractive index for red and
blue light. Apochromatic lenses use the same concept but have three components, for
red, blue, and green. The problem with mass has never really been resolved, and by
the beginning of the twentieth century, refracting telescopes reached (and possibly
even exceeded) their maximum operational size. Over the years, additional optical
coating has been applied to astronomical lenses, and although this has improved
them, it has not overcome their physical limitations. Of course, telescopes must
be exposed to the elements, although they should not be made to encounter very
extreme weather, and over time, a lens becomes pitted, a coating becomes worn, and
the optical bonding compound used to bond achromatic and apochromatic lenses
loses its optical transparency, at which point the lens will either need maintenance
or replacement.
The concept of using mirrors to magnify an image had been known for at least five
hundred years before their successful application to astronomy by Newton. Mirrors
have a number of advantages over lenses; only the surface of the mirror needs to be
optically smooth, reducing the need to produce optically perfect glass; mirror-based
telescopes do not suffer from chromatic aberration; and the mirror’s position at the
base of the telescope makes it easier to operate. However, mirrors are not entirely
without design problems. Typically, a secondary mirror is needed near the primary
focus, which then partially obstructs the optical path and reduces the efficiency of
the telescope. Also, light is diffracted around the secondary mirror’s supports and
the secondary mirror itself, which causes the spikes and halos seen around some
stars in astronomical images, although admittedly in some cases such artifacts might
have been added digitally for effect. There is also a tendency for light incident on
the edge of the mirror to be brought to a slightly different focus from that of light at
the centre. As a result, objects become smeared out, a problem known as coma. An
additional problem is the alignment of the primary and the secondary mirrors. If this
alignment is not within a very small tolerance (Collimated), the light arrives at the
focus at a slight angle to the focal plane, and consequently, point sources become
elongated.
2 Collimation problems normally worsen over time, but they are easy
to address. Hence recollimation is part of the normal maintenance procedure for a
telescope.
As with lenses, mirrors become increasingly massive as their diameters increase.
Sagging under gravity is prevented by supporting the mirror across its entire rear surface area. However, it appears that the engineering limit for producing astronomical2 They are often described as looking comet-like.