3.1 Telescopes
25
Fig. 3.1 Diagram showing the optical pathway of a Galilean telescope
Although in attempting to resolve two stars, the Rayleigh criterion (3.5) (which
gives the resolution in radians) is more representative, the simpler Dawes’ limit
is also widely used (D is in millimetres). We have
α = sin
−1 1.22λ
D
(3.5)
and
α ∼ =
115.82
D
.
(3.6)
In reality, most telescopes are not diffraction limited but are limited by atmospheric
turbulence, that is, seeing limited.
The first refracting telescope was made in the early sixteen hundreds in the Netherlands by spectacle makers. It was not until Galileo Galilei designed and constructed
his own a telescope a few years after their first appearance that the telescope began
to be used for astronomy. Galileo’s telescope used a convergent objective lens and a
divergent eyepiece, which, unlike modern instruments, produced an upright image.
1
Galileo’s design was rapidly improved upon by Johannes Kepler, who changed the
divergent eyepiece to a convex one and thereby inverted the image (Fig. 3.1).
The Keplerian telescope allowed the use of increased magnification but failed to
overcome two of the refracting telescope’s principal problems, the first of which is
an engineering problem, due to the fact that the mass of an objective lens increases
disproportionately with its diameter. Not only does this make the telescope unwieldy
to use, but the lens becomes difficult to mount due to the thinness of the glass at
the edge, and it tends to sag under gravity, resulting in altered optical characteristics
with elevation. Additionally, as the glass volume increases, it becomes increasingly
more difficult to keep the lens free of imperfections (Fig. 3.2).
You will recall that a lens’s refractive index is wavelength specific. This being
the case, a lens will bring each colour to a different focus, resulting in coloured
rings around objects, a phenomenon known as chromatic aberration. This can be
overcome in part by using much longer focal lengths. However, it was not until
the introduction of Achromatic, and later Apochromatic, lenses that the problem
was resolved. Achromatic lenses use two types of glass with different refractive
1 Most modern telescopes invert the image.
25
Fig. 3.1 Diagram showing the optical pathway of a Galilean telescope
Although in attempting to resolve two stars, the Rayleigh criterion (3.5) (which
gives the resolution in radians) is more representative, the simpler Dawes’ limit
is also widely used (D is in millimetres). We have
α = sin
−1 1.22λ
D
(3.5)
and
α ∼ =
115.82
D
.
(3.6)
In reality, most telescopes are not diffraction limited but are limited by atmospheric
turbulence, that is, seeing limited.
The first refracting telescope was made in the early sixteen hundreds in the Netherlands by spectacle makers. It was not until Galileo Galilei designed and constructed
his own a telescope a few years after their first appearance that the telescope began
to be used for astronomy. Galileo’s telescope used a convergent objective lens and a
divergent eyepiece, which, unlike modern instruments, produced an upright image.
1
Galileo’s design was rapidly improved upon by Johannes Kepler, who changed the
divergent eyepiece to a convex one and thereby inverted the image (Fig. 3.1).
The Keplerian telescope allowed the use of increased magnification but failed to
overcome two of the refracting telescope’s principal problems, the first of which is
an engineering problem, due to the fact that the mass of an objective lens increases
disproportionately with its diameter. Not only does this make the telescope unwieldy
to use, but the lens becomes difficult to mount due to the thinness of the glass at
the edge, and it tends to sag under gravity, resulting in altered optical characteristics
with elevation. Additionally, as the glass volume increases, it becomes increasingly
more difficult to keep the lens free of imperfections (Fig. 3.2).
You will recall that a lens’s refractive index is wavelength specific. This being
the case, a lens will bring each colour to a different focus, resulting in coloured
rings around objects, a phenomenon known as chromatic aberration. This can be
overcome in part by using much longer focal lengths. However, it was not until
the introduction of Achromatic, and later Apochromatic, lenses that the problem
was resolved. Achromatic lenses use two types of glass with different refractive
1 Most modern telescopes invert the image.