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3 The Telescope
extremely difficult to do mechanically without the use of a computer. A camera fixed
to a telescope using an alt-az mount rotates as the mount tracks, which can cause
problems with longer exposures. There is a simple solution, however: the equatorial
mount. If you tilt the mount so that the centre of rotation of the horizontal axis
now points to the celestial pole and the vertical axis is perpendicular to the inclined
horizontal axis, the telescope naturally follows the same arc as stars, and observed
objects do not drift in the direction of the other axis. This orientation is known as
polar alignment, which means that we need to drive only a single axis, which can be
done simply with clockwork. Multiple designs of equatorial mounts exist, one of the
more popular being the German equatorial mount (Fig. 3.6).
Equatorial mounts tend to be heavy, due to the amount of required counterweighting, and they need a larger dome to house them than do alt-az telescopes. They also
suffer from what is known as meridian flip. Meridian flip occurs, not unsurprisingly,
when the telescope transits the meridian. In order to stop the telescope from hitting
the mount, the telescope must be reorientated, or flipped. This means that if you are
performing a series of observations of an object, there will be a break as the object
crosses the median, which is the very best place to acquire an image.
With the introduction of small, cheap, powerful computers and field rotators that
rotate the camera so that it maintains the same position relative to the field, the
Fig. 3.6 Image of a telescope mounted on a German equatorial mount. Image courtesy the University of Hertfordshire
3 The Telescope
extremely difficult to do mechanically without the use of a computer. A camera fixed
to a telescope using an alt-az mount rotates as the mount tracks, which can cause
problems with longer exposures. There is a simple solution, however: the equatorial
mount. If you tilt the mount so that the centre of rotation of the horizontal axis
now points to the celestial pole and the vertical axis is perpendicular to the inclined
horizontal axis, the telescope naturally follows the same arc as stars, and observed
objects do not drift in the direction of the other axis. This orientation is known as
polar alignment, which means that we need to drive only a single axis, which can be
done simply with clockwork. Multiple designs of equatorial mounts exist, one of the
more popular being the German equatorial mount (Fig. 3.6).
Equatorial mounts tend to be heavy, due to the amount of required counterweighting, and they need a larger dome to house them than do alt-az telescopes. They also
suffer from what is known as meridian flip. Meridian flip occurs, not unsurprisingly,
when the telescope transits the meridian. In order to stop the telescope from hitting
the mount, the telescope must be reorientated, or flipped. This means that if you are
performing a series of observations of an object, there will be a break as the object
crosses the median, which is the very best place to acquire an image.
With the introduction of small, cheap, powerful computers and field rotators that
rotate the camera so that it maintains the same position relative to the field, the
Fig. 3.6 Image of a telescope mounted on a German equatorial mount. Image courtesy the University of Hertfordshire