3.2 Mounts
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limitations of the alt-az mount have now been overcome. Consequently, almost all
modern large telescopes are of the alt-az design, although equatorial mounts are still
common.
Pioneered by amateur astronomer John Dobson, the Dobsonian is one of the simplest astronomical telescopes. In simple terms, it is optically a Newtonian, mounted
in an open boxlike structure, supported by two pegs that extend from the tube and pass
through the box. Because the bulk of the mass of a Newtonian is in the primary mirror,
the centre of gravity tends to be located near the primary—the feature that makes the
Dobsonian possible. The box, which provides the alt-az movement, is mounted on a
flat plate with a single fixing passing through the base of the box at its centre. Typically, there is a low-friction bearing (often Teflon) between the two surfaces. Since
its invention, the Dobsonian has evolved into a wide range of differing, although
fundamentally similar, designs. For example, thee are now open tube Dobsonian and
push-go Dobsonian, which have encoders linked to a tablet PC or smartphone that
allows unpowered GoTo (the user pushes the tube to the target under the direction of
the tablet or smartphone), and there are even powered tracking GoTo Dobsonians.
The big advantage of Dobsonians over other telescopes is their price. In most cases, a
good mount costs as much as the telescope to which it is mounted. The very low cost
(many are still built in home workshops) of this mount has resulted in an increase
in aperture size, giving amateur astronomers access to deep sky obsessing that a few
decades ago would have been out of reach for many. However, for astrophotography,
the Dobsonian struggles to compete with more sophisticated mounts such as the
German equatorial. However, there is a lot to be said for having what effectively is a
huge light bucket for observing the night sky, especially one as light and portable as
the Dobsonian. Australian amateur astronomer and minister Robert Evans discovered a large number of extragalactic supernovae from his backyard using his 31 cm
Dobsonian. This can be attributed mostly to his ability to identify supernovae by eye
but also to the ease of use of the Dobsonian design and to the good weather and clear
skies of Australia.
Perhaps one of the least common telescope mounts that you are still likely to
encounter is the transit telescope. Transit telescopes point only at the meridian (the
line that runs north to south) and consequently can move only in elevation. Transit
telescopes, which tend to have very long focal lengths, were used to determine when
an object crossed the local meridian, and its elevation, which enables the observer
to determine both the local time (and hence their position on Earth) and the location
of the object in the sky. Transit telescopes tend to be found in older observatories,
and new constructions are rare. The monument to the Great Fire of London, near
Pudding Lane, contained, somewhat bizarrely, a transit telescope.
One of the biggest changes in telescope mounts in recent decades is the introduction of GoTo telescopes, and more recently robotic telescopes. GoTo telescopes
feature powered mounts with encoders that feed their orientation back to a computer. If the telescope is portable, it may feature a Global Positioning System (GPS)
receiver to aid in the setup. Effectively, a GoTo telescope always knows where it
is pointing, although some degree of setup is needed, which normally consists in
manually pointing the telescope at three bright stars so the computer can work out
31
limitations of the alt-az mount have now been overcome. Consequently, almost all
modern large telescopes are of the alt-az design, although equatorial mounts are still
common.
Pioneered by amateur astronomer John Dobson, the Dobsonian is one of the simplest astronomical telescopes. In simple terms, it is optically a Newtonian, mounted
in an open boxlike structure, supported by two pegs that extend from the tube and pass
through the box. Because the bulk of the mass of a Newtonian is in the primary mirror,
the centre of gravity tends to be located near the primary—the feature that makes the
Dobsonian possible. The box, which provides the alt-az movement, is mounted on a
flat plate with a single fixing passing through the base of the box at its centre. Typically, there is a low-friction bearing (often Teflon) between the two surfaces. Since
its invention, the Dobsonian has evolved into a wide range of differing, although
fundamentally similar, designs. For example, thee are now open tube Dobsonian and
push-go Dobsonian, which have encoders linked to a tablet PC or smartphone that
allows unpowered GoTo (the user pushes the tube to the target under the direction of
the tablet or smartphone), and there are even powered tracking GoTo Dobsonians.
The big advantage of Dobsonians over other telescopes is their price. In most cases, a
good mount costs as much as the telescope to which it is mounted. The very low cost
(many are still built in home workshops) of this mount has resulted in an increase
in aperture size, giving amateur astronomers access to deep sky obsessing that a few
decades ago would have been out of reach for many. However, for astrophotography,
the Dobsonian struggles to compete with more sophisticated mounts such as the
German equatorial. However, there is a lot to be said for having what effectively is a
huge light bucket for observing the night sky, especially one as light and portable as
the Dobsonian. Australian amateur astronomer and minister Robert Evans discovered a large number of extragalactic supernovae from his backyard using his 31 cm
Dobsonian. This can be attributed mostly to his ability to identify supernovae by eye
but also to the ease of use of the Dobsonian design and to the good weather and clear
skies of Australia.
Perhaps one of the least common telescope mounts that you are still likely to
encounter is the transit telescope. Transit telescopes point only at the meridian (the
line that runs north to south) and consequently can move only in elevation. Transit
telescopes, which tend to have very long focal lengths, were used to determine when
an object crossed the local meridian, and its elevation, which enables the observer
to determine both the local time (and hence their position on Earth) and the location
of the object in the sky. Transit telescopes tend to be found in older observatories,
and new constructions are rare. The monument to the Great Fire of London, near
Pudding Lane, contained, somewhat bizarrely, a transit telescope.
One of the biggest changes in telescope mounts in recent decades is the introduction of GoTo telescopes, and more recently robotic telescopes. GoTo telescopes
feature powered mounts with encoders that feed their orientation back to a computer. If the telescope is portable, it may feature a Global Positioning System (GPS)
receiver to aid in the setup. Effectively, a GoTo telescope always knows where it
is pointing, although some degree of setup is needed, which normally consists in
manually pointing the telescope at three bright stars so the computer can work out